| |
| Debugging on Linux for s/390 & z/Architecture |
| by |
| Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com) |
| Copyright (C) 2000-2001 IBM Deutschland Entwicklung GmbH, IBM Corporation |
| Best viewed with fixed width fonts |
| |
| Overview of Document: |
| ===================== |
| This document is intended to give an good overview of how to debug |
| Linux for s/390 & z/Architecture. It isn't intended as a complete reference & not a |
| tutorial on the fundamentals of C & assembly. It doesn't go into |
| 390 IO in any detail. It is intended to complement the documents in the |
| reference section below & any other worthwhile references you get. |
| |
| It is intended like the Enterprise Systems Architecture/390 Reference Summary |
| to be printed out & used as a quick cheat sheet self help style reference when |
| problems occur. |
| |
| Contents |
| ======== |
| Register Set |
| Address Spaces on Intel Linux |
| Address Spaces on Linux for s/390 & z/Architecture |
| The Linux for s/390 & z/Architecture Kernel Task Structure |
| Register Usage & Stackframes on Linux for s/390 & z/Architecture |
| A sample program with comments |
| Compiling programs for debugging on Linux for s/390 & z/Architecture |
| Figuring out gcc compile errors |
| Debugging Tools |
| objdump |
| strace |
| Performance Debugging |
| Debugging under VM |
| s/390 & z/Architecture IO Overview |
| Debugging IO on s/390 & z/Architecture under VM |
| GDB on s/390 & z/Architecture |
| Stack chaining in gdb by hand |
| Examining core dumps |
| ldd |
| Debugging modules |
| The proc file system |
| Starting points for debugging scripting languages etc. |
| Dumptool & Lcrash |
| SysRq |
| References |
| Special Thanks |
| |
| Register Set |
| ============ |
| The current architectures have the following registers. |
| |
| 16 General propose registers, 32 bit on s/390 64 bit on z/Architecture, r0-r15 or gpr0-gpr15 used for arithmetic & addressing. |
| |
| 16 Control registers, 32 bit on s/390 64 bit on z/Architecture, ( cr0-cr15 kernel usage only ) used for memory management, |
| interrupt control,debugging control etc. |
| |
| 16 Access registers ( ar0-ar15 ) 32 bit on s/390 & z/Architecture |
| not used by normal programs but potentially could |
| be used as temporary storage. Their main purpose is their 1 to 1 |
| association with general purpose registers and are used in |
| the kernel for copying data between kernel & user address spaces. |
| Access register 0 ( & access register 1 on z/Architecture ( needs 64 bit |
| pointer ) ) is currently used by the pthread library as a pointer to |
| the current running threads private area. |
| |
| 16 64 bit floating point registers (fp0-fp15 ) IEEE & HFP floating |
| point format compliant on G5 upwards & a Floating point control reg (FPC) |
| 4 64 bit registers (fp0,fp2,fp4 & fp6) HFP only on older machines. |
| Note: |
| Linux (currently) always uses IEEE & emulates G5 IEEE format on older machines, |
| ( provided the kernel is configured for this ). |
| |
| |
| The PSW is the most important register on the machine it |
| is 64 bit on s/390 & 128 bit on z/Architecture & serves the roles of |
| a program counter (pc), condition code register,memory space designator. |
| In IBM standard notation I am counting bit 0 as the MSB. |
| It has several advantages over a normal program counter |
| in that you can change address translation & program counter |
| in a single instruction. To change address translation, |
| e.g. switching address translation off requires that you |
| have a logical=physical mapping for the address you are |
| currently running at. |
| |
| Bit Value |
| s/390 z/Architecture |
| 0 0 Reserved ( must be 0 ) otherwise specification exception occurs. |
| |
| 1 1 Program Event Recording 1 PER enabled, |
| PER is used to facilitate debugging e.g. single stepping. |
| |
| 2-4 2-4 Reserved ( must be 0 ). |
| |
| 5 5 Dynamic address translation 1=DAT on. |
| |
| 6 6 Input/Output interrupt Mask |
| |
| 7 7 External interrupt Mask used primarily for interprocessor signalling & |
| clock interrupts. |
| |
| 8-11 8-11 PSW Key used for complex memory protection mechanism not used under linux |
| |
| 12 12 1 on s/390 0 on z/Architecture |
| |
| 13 13 Machine Check Mask 1=enable machine check interrupts |
| |
| 14 14 Wait State set this to 1 to stop the processor except for interrupts & give |
| time to other LPARS used in CPU idle in the kernel to increase overall |
| usage of processor resources. |
| |
| 15 15 Problem state ( if set to 1 certain instructions are disabled ) |
| all linux user programs run with this bit 1 |
| ( useful info for debugging under VM ). |
| |
| 16-17 16-17 Address Space Control |
| |
| 00 Primary Space Mode when DAT on |
| The linux kernel currently runs in this mode, CR1 is affiliated with |
| this mode & points to the primary segment table origin etc. |
| |
| 01 Access register mode this mode is used in functions to |
| copy data between kernel & user space. |
| |
| 10 Secondary space mode not used in linux however CR7 the |
| register affiliated with this mode is & this & normally |
| CR13=CR7 to allow us to copy data between kernel & user space. |
| We do this as follows: |
| We set ar2 to 0 to designate its |
| affiliated gpr ( gpr2 )to point to primary=kernel space. |
| We set ar4 to 1 to designate its |
| affiliated gpr ( gpr4 ) to point to secondary=home=user space |
| & then essentially do a memcopy(gpr2,gpr4,size) to |
| copy data between the address spaces, the reason we use home space for the |
| kernel & don't keep secondary space free is that code will not run in |
| secondary space. |
| |
| 11 Home Space Mode all user programs run in this mode. |
| it is affiliated with CR13. |
| |
| 18-19 18-19 Condition codes (CC) |
| |
| 20 20 Fixed point overflow mask if 1=FPU exceptions for this event |
| occur ( normally 0 ) |
| |
| 21 21 Decimal overflow mask if 1=FPU exceptions for this event occur |
| ( normally 0 ) |
| |
| 22 22 Exponent underflow mask if 1=FPU exceptions for this event occur |
| ( normally 0 ) |
| |
| 23 23 Significance Mask if 1=FPU exceptions for this event occur |
| ( normally 0 ) |
| |
| 24-31 24-30 Reserved Must be 0. |
| |
| 31 Extended Addressing Mode |
| 32 Basic Addressing Mode |
| Used to set addressing mode |
| PSW 31 PSW 32 |
| 0 0 24 bit |
| 0 1 31 bit |
| 1 1 64 bit |
| |
| 32 1=31 bit addressing mode 0=24 bit addressing mode (for backward |
| compatibility), linux always runs with this bit set to 1 |
| |
| 33-64 Instruction address. |
| 33-63 Reserved must be 0 |
| 64-127 Address |
| In 24 bits mode bits 64-103=0 bits 104-127 Address |
| In 31 bits mode bits 64-96=0 bits 97-127 Address |
| Note: unlike 31 bit mode on s/390 bit 96 must be zero |
| when loading the address with LPSWE otherwise a |
| specification exception occurs, LPSW is fully backward |
| compatible. |
| |
| |
| Prefix Page(s) |
| -------------- |
| This per cpu memory area is too intimately tied to the processor not to mention. |
| It exists between the real addresses 0-4096 on s/390 & 0-8192 z/Architecture & is exchanged |
| with a 1 page on s/390 or 2 pages on z/Architecture in absolute storage by the set |
| prefix instruction in linux'es startup. |
| This page is mapped to a different prefix for each processor in an SMP configuration |
| ( assuming the os designer is sane of course :-) ). |
| Bytes 0-512 ( 200 hex ) on s/390 & 0-512,4096-4544,4604-5119 currently on z/Architecture |
| are used by the processor itself for holding such information as exception indications & |
| entry points for exceptions. |
| Bytes after 0xc00 hex are used by linux for per processor globals on s/390 & z/Architecture |
| ( there is a gap on z/Architecture too currently between 0xc00 & 1000 which linux uses ). |
| The closest thing to this on traditional architectures is the interrupt |
| vector table. This is a good thing & does simplify some of the kernel coding |
| however it means that we now cannot catch stray NULL pointers in the |
| kernel without hard coded checks. |
| |
| |
| |
| Address Spaces on Intel Linux |
| ============================= |
| |
| The traditional Intel Linux is approximately mapped as follows forgive |
| the ascii art. |
| 0xFFFFFFFF 4GB Himem ***************** |
| * * |
| * Kernel Space * |
| * * |
| ***************** **************** |
| User Space Himem (typically 0xC0000000 3GB )* User Stack * * * |
| ***************** * * |
| * Shared Libs * * Next Process * |
| ***************** * to * |
| * * <== * Run * <== |
| * User Program * * * |
| * Data BSS * * * |
| * Text * * * |
| * Sections * * * |
| 0x00000000 ***************** **************** |
| |
| Now it is easy to see that on Intel it is quite easy to recognise a kernel address |
| as being one greater than user space himem ( in this case 0xC0000000). |
| & addresses of less than this are the ones in the current running program on this |
| processor ( if an smp box ). |
| If using the virtual machine ( VM ) as a debugger it is quite difficult to |
| know which user process is running as the address space you are looking at |
| could be from any process in the run queue. |
| |
| The limitation of Intels addressing technique is that the linux |
| kernel uses a very simple real address to virtual addressing technique |
| of Real Address=Virtual Address-User Space Himem. |
| This means that on Intel the kernel linux can typically only address |
| Himem=0xFFFFFFFF-0xC0000000=1GB & this is all the RAM these machines |
| can typically use. |
| They can lower User Himem to 2GB or lower & thus be |
| able to use 2GB of RAM however this shrinks the maximum size |
| of User Space from 3GB to 2GB they have a no win limit of 4GB unless |
| they go to 64 Bit. |
| |
| |
| On 390 our limitations & strengths make us slightly different. |
| For backward compatibility we are only allowed use 31 bits (2GB) |
| of our 32 bit addresses, however, we use entirely separate address |
| spaces for the user & kernel. |
| |
| This means we can support 2GB of non Extended RAM on s/390, & more |
| with the Extended memory management swap device & |
| currently 4TB of physical memory currently on z/Architecture. |
| |
| |
| Address Spaces on Linux for s/390 & z/Architecture |
| ================================================== |
| |
| Our addressing scheme is as follows |
| |
| |
| Himem 0x7fffffff 2GB on s/390 ***************** **************** |
| currently 0x3ffffffffff (2^42)-1 * User Stack * * * |
| on z/Architecture. ***************** * * |
| * Shared Libs * * * |
| ***************** * * |
| * * * Kernel * |
| * User Program * * * |
| * Data BSS * * * |
| * Text * * * |
| * Sections * * * |
| 0x00000000 ***************** **************** |
| |
| This also means that we need to look at the PSW problem state bit |
| or the addressing mode to decide whether we are looking at |
| user or kernel space. |
| |
| Virtual Addresses on s/390 & z/Architecture |
| =========================================== |
| |
| A virtual address on s/390 is made up of 3 parts |
| The SX ( segment index, roughly corresponding to the PGD & PMD in linux terminology ) |
| being bits 1-11. |
| The PX ( page index, corresponding to the page table entry (pte) in linux terminology ) |
| being bits 12-19. |
| The remaining bits BX (the byte index are the offset in the page ) |
| i.e. bits 20 to 31. |
| |
| On z/Architecture in linux we currently make up an address from 4 parts. |
| The region index bits (RX) 0-32 we currently use bits 22-32 |
| The segment index (SX) being bits 33-43 |
| The page index (PX) being bits 44-51 |
| The byte index (BX) being bits 52-63 |
| |
| Notes: |
| 1) s/390 has no PMD so the PMD is really the PGD also. |
| A lot of this stuff is defined in pgtable.h. |
| |
| 2) Also seeing as s/390's page indexes are only 1k in size |
| (bits 12-19 x 4 bytes per pte ) we use 1 ( page 4k ) |
| to make the best use of memory by updating 4 segment indices |
| entries each time we mess with a PMD & use offsets |
| 0,1024,2048 & 3072 in this page as for our segment indexes. |
| On z/Architecture our page indexes are now 2k in size |
| ( bits 12-19 x 8 bytes per pte ) we do a similar trick |
| but only mess with 2 segment indices each time we mess with |
| a PMD. |
| |
| 3) As z/Architecture supports upto a massive 5-level page table lookup we |
| can only use 3 currently on Linux ( as this is all the generic kernel |
| currently supports ) however this may change in future |
| this allows us to access ( according to my sums ) |
| 4TB of virtual storage per process i.e. |
| 4096*512(PTES)*1024(PMDS)*2048(PGD) = 4398046511104 bytes, |
| enough for another 2 or 3 of years I think :-). |
| to do this we use a region-third-table designation type in |
| our address space control registers. |
| |
| |
| The Linux for s/390 & z/Architecture Kernel Task Structure |
| ========================================================== |
| Each process/thread under Linux for S390 has its own kernel task_struct |
| defined in linux/include/linux/sched.h |
| The S390 on initialisation & resuming of a process on a cpu sets |
| the __LC_KERNEL_STACK variable in the spare prefix area for this cpu |
| ( which we use for per processor globals). |
| |
| The kernel stack pointer is intimately tied with the task stucture for |
| each processor as follows. |
| |
| s/390 |
| ************************ |
| * 1 page kernel stack * |
| * ( 4K ) * |
| ************************ |
| * 1 page task_struct * |
| * ( 4K ) * |
| 8K aligned ************************ |
| |
| z/Architecture |
| ************************ |
| * 2 page kernel stack * |
| * ( 8K ) * |
| ************************ |
| * 2 page task_struct * |
| * ( 8K ) * |
| 16K aligned ************************ |
| |
| What this means is that we don't need to dedicate any register or global variable |
| to point to the current running process & can retrieve it with the following |
| very simple construct for s/390 & one very similar for z/Architecture. |
| |
| static inline struct task_struct * get_current(void) |
| { |
| struct task_struct *current; |
| __asm__("lhi %0,-8192\n\t" |
| "nr %0,15" |
| : "=r" (current) ); |
| return current; |
| } |
| |
| i.e. just anding the current kernel stack pointer with the mask -8192. |
| Thankfully because Linux doesn't have support for nested IO interrupts |
| & our devices have large buffers can survive interrupts being shut for |
| short amounts of time we don't need a separate stack for interrupts. |
| |
| |
| |
| |
| Register Usage & Stackframes on Linux for s/390 & z/Architecture |
| ================================================================= |
| Overview: |
| --------- |
| This is the code that gcc produces at the top & the bottom of |
| each function, it usually is fairly consistent & similar from |
| function to function & if you know its layout you can probalby |
| make some headway in finding the ultimate cause of a problem |
| after a crash without a source level debugger. |
| |
| Note: To follow stackframes requires a knowledge of C or Pascal & |
| limited knowledge of one assembly language. |
| |
| It should be noted that there are some differences between the |
| s/390 & z/Architecture stack layouts as the z/Architecture stack layout didn't have |
| to maintain compatibility with older linkage formats. |
| |
| Glossary: |
| --------- |
| alloca: |
| This is a built in compiler function for runtime allocation |
| of extra space on the callers stack which is obviously freed |
| up on function exit ( e.g. the caller may choose to allocate nothing |
| of a buffer of 4k if required for temporary purposes ), it generates |
| very efficient code ( a few cycles ) when compared to alternatives |
| like malloc. |
| |
| automatics: These are local variables on the stack, |
| i.e they aren't in registers & they aren't static. |
| |
| back-chain: |
| This is a pointer to the stack pointer before entering a |
| framed functions ( see frameless function ) prologue got by |
| dereferencing the address of the current stack pointer, |
| i.e. got by accessing the 32 bit value at the stack pointers |
| current location. |
| |
| base-pointer: |
| This is a pointer to the back of the literal pool which |
| is an area just behind each procedure used to store constants |
| in each function. |
| |
| call-clobbered: The caller probably needs to save these registers if there |
| is something of value in them, on the stack or elsewhere before making a |
| call to another procedure so that it can restore it later. |
| |
| epilogue: |
| The code generated by the compiler to return to the caller. |
| |
| frameless-function |
| A frameless function in Linux for s390 & z/Architecture is one which doesn't |
| need more than the register save area ( 96 bytes on s/390, 160 on z/Architecture ) |
| given to it by the caller. |
| A frameless function never: |
| 1) Sets up a back chain. |
| 2) Calls alloca. |
| 3) Calls other normal functions |
| 4) Has automatics. |
| |
| GOT-pointer: |
| This is a pointer to the global-offset-table in ELF |
| ( Executable Linkable Format, Linux'es most common executable format ), |
| all globals & shared library objects are found using this pointer. |
| |
| lazy-binding |
| ELF shared libraries are typically only loaded when routines in the shared |
| library are actually first called at runtime. This is lazy binding. |
| |
| procedure-linkage-table |
| This is a table found from the GOT which contains pointers to routines |
| in other shared libraries which can't be called to by easier means. |
| |
| prologue: |
| The code generated by the compiler to set up the stack frame. |
| |
| outgoing-args: |
| This is extra area allocated on the stack of the calling function if the |
| parameters for the callee's cannot all be put in registers, the same |
| area can be reused by each function the caller calls. |
| |
| routine-descriptor: |
| A COFF executable format based concept of a procedure reference |
| actually being 8 bytes or more as opposed to a simple pointer to the routine. |
| This is typically defined as follows |
| Routine Descriptor offset 0=Pointer to Function |
| Routine Descriptor offset 4=Pointer to Table of Contents |
| The table of contents/TOC is roughly equivalent to a GOT pointer. |
| & it means that shared libraries etc. can be shared between several |
| environments each with their own TOC. |
| |
| |
| static-chain: This is used in nested functions a concept adopted from pascal |
| by gcc not used in ansi C or C++ ( although quite useful ), basically it |
| is a pointer used to reference local variables of enclosing functions. |
| You might come across this stuff once or twice in your lifetime. |
| |
| e.g. |
| The function below should return 11 though gcc may get upset & toss warnings |
| about unused variables. |
| int FunctionA(int a) |
| { |
| int b; |
| FunctionC(int c) |
| { |
| b=c+1; |
| } |
| FunctionC(10); |
| return(b); |
| } |
| |
| |
| s/390 & z/Architecture Register usage |
| ===================================== |
| r0 used by syscalls/assembly call-clobbered |
| r1 used by syscalls/assembly call-clobbered |
| r2 argument 0 / return value 0 call-clobbered |
| r3 argument 1 / return value 1 (if long long) call-clobbered |
| r4 argument 2 call-clobbered |
| r5 argument 3 call-clobbered |
| r6 argument 5 saved |
| r7 pointer-to arguments 5 to ... saved |
| r8 this & that saved |
| r9 this & that saved |
| r10 static-chain ( if nested function ) saved |
| r11 frame-pointer ( if function used alloca ) saved |
| r12 got-pointer saved |
| r13 base-pointer saved |
| r14 return-address saved |
| r15 stack-pointer saved |
| |
| f0 argument 0 / return value ( float/double ) call-clobbered |
| f2 argument 1 call-clobbered |
| f4 z/Architecture argument 2 saved |
| f6 z/Architecture argument 3 saved |
| The remaining floating points |
| f1,f3,f5 f7-f15 are call-clobbered. |
| |
| Notes: |
| ------ |
| 1) The only requirement is that registers which are used |
| by the callee are saved, e.g. the compiler is perfectly |
| capible of using r11 for purposes other than a frame a |
| frame pointer if a frame pointer is not needed. |
| 2) In functions with variable arguments e.g. printf the calling procedure |
| is identical to one without variable arguments & the same number of |
| parameters. However, the prologue of this function is somewhat more |
| hairy owing to it having to move these parameters to the stack to |
| get va_start, va_arg & va_end to work. |
| 3) Access registers are currently unused by gcc but are used in |
| the kernel. Possibilities exist to use them at the moment for |
| temporary storage but it isn't recommended. |
| 4) Only 4 of the floating point registers are used for |
| parameter passing as older machines such as G3 only have only 4 |
| & it keeps the stack frame compatible with other compilers. |
| However with IEEE floating point emulation under linux on the |
| older machines you are free to use the other 12. |
| 5) A long long or double parameter cannot be have the |
| first 4 bytes in a register & the second four bytes in the |
| outgoing args area. It must be purely in the outgoing args |
| area if crossing this boundary. |
| 6) Floating point parameters are mixed with outgoing args |
| on the outgoing args area in the order the are passed in as parameters. |
| 7) Floating point arguments 2 & 3 are saved in the outgoing args area for |
| z/Architecture |
| |
| |
| Stack Frame Layout |
| ------------------ |
| s/390 z/Architecture |
| 0 0 back chain ( a 0 here signifies end of back chain ) |
| 4 8 eos ( end of stack, not used on Linux for S390 used in other linkage formats ) |
| 8 16 glue used in other s/390 linkage formats for saved routine descriptors etc. |
| 12 24 glue used in other s/390 linkage formats for saved routine descriptors etc. |
| 16 32 scratch area |
| 20 40 scratch area |
| 24 48 saved r6 of caller function |
| 28 56 saved r7 of caller function |
| 32 64 saved r8 of caller function |
| 36 72 saved r9 of caller function |
| 40 80 saved r10 of caller function |
| 44 88 saved r11 of caller function |
| 48 96 saved r12 of caller function |
| 52 104 saved r13 of caller function |
| 56 112 saved r14 of caller function |
| 60 120 saved r15 of caller function |
| 64 128 saved f4 of caller function |
| 72 132 saved f6 of caller function |
| 80 undefined |
| 96 160 outgoing args passed from caller to callee |
| 96+x 160+x possible stack alignment ( 8 bytes desirable ) |
| 96+x+y 160+x+y alloca space of caller ( if used ) |
| 96+x+y+z 160+x+y+z automatics of caller ( if used ) |
| 0 back-chain |
| |
| A sample program with comments. |
| =============================== |
| |
| Comments on the function test |
| ----------------------------- |
| 1) It didn't need to set up a pointer to the constant pool gpr13 as it isn't used |
| ( :-( ). |
| 2) This is a frameless function & no stack is bought. |
| 3) The compiler was clever enough to recognise that it could return the |
| value in r2 as well as use it for the passed in parameter ( :-) ). |
| 4) The basr ( branch relative & save ) trick works as follows the instruction |
| has a special case with r0,r0 with some instruction operands is understood as |
| the literal value 0, some risc architectures also do this ). So now |
| we are branching to the next address & the address new program counter is |
| in r13,so now we subtract the size of the function prologue we have executed |
| + the size of the literal pool to get to the top of the literal pool |
| 0040037c int test(int b) |
| { # Function prologue below |
| 40037c: 90 de f0 34 stm %r13,%r14,52(%r15) # Save registers r13 & r14 |
| 400380: 0d d0 basr %r13,%r0 # Set up pointer to constant pool using |
| 400382: a7 da ff fa ahi %r13,-6 # basr trick |
| return(5+b); |
| # Huge main program |
| 400386: a7 2a 00 05 ahi %r2,5 # add 5 to r2 |
| |
| # Function epilogue below |
| 40038a: 98 de f0 34 lm %r13,%r14,52(%r15) # restore registers r13 & 14 |
| 40038e: 07 fe br %r14 # return |
| } |
| |
| Comments on the function main |
| ----------------------------- |
| 1) The compiler did this function optimally ( 8-) ) |
| |
| Literal pool for main. |
| 400390: ff ff ff ec .long 0xffffffec |
| main(int argc,char *argv[]) |
| { # Function prologue below |
| 400394: 90 bf f0 2c stm %r11,%r15,44(%r15) # Save necessary registers |
| 400398: 18 0f lr %r0,%r15 # copy stack pointer to r0 |
| 40039a: a7 fa ff a0 ahi %r15,-96 # Make area for callee saving |
| 40039e: 0d d0 basr %r13,%r0 # Set up r13 to point to |
| 4003a0: a7 da ff f0 ahi %r13,-16 # literal pool |
| 4003a4: 50 00 f0 00 st %r0,0(%r15) # Save backchain |
| |
| return(test(5)); # Main Program Below |
| 4003a8: 58 e0 d0 00 l %r14,0(%r13) # load relative address of test from |
| # literal pool |
| 4003ac: a7 28 00 05 lhi %r2,5 # Set first parameter to 5 |
| 4003b0: 4d ee d0 00 bas %r14,0(%r14,%r13) # jump to test setting r14 as return |
| # address using branch & save instruction. |
| |
| # Function Epilogue below |
| 4003b4: 98 bf f0 8c lm %r11,%r15,140(%r15)# Restore necessary registers. |
| 4003b8: 07 fe br %r14 # return to do program exit |
| } |
| |
| |
| Compiler updates |
| ---------------- |
| |
| main(int argc,char *argv[]) |
| { |
| 4004fc: 90 7f f0 1c stm %r7,%r15,28(%r15) |
| 400500: a7 d5 00 04 bras %r13,400508 <main+0xc> |
| 400504: 00 40 04 f4 .long 0x004004f4 |
| # compiler now puts constant pool in code to so it saves an instruction |
| 400508: 18 0f lr %r0,%r15 |
| 40050a: a7 fa ff a0 ahi %r15,-96 |
| 40050e: 50 00 f0 00 st %r0,0(%r15) |
| return(test(5)); |
| 400512: 58 10 d0 00 l %r1,0(%r13) |
| 400516: a7 28 00 05 lhi %r2,5 |
| 40051a: 0d e1 basr %r14,%r1 |
| # compiler adds 1 extra instruction to epilogue this is done to |
| # avoid processor pipeline stalls owing to data dependencies on g5 & |
| # above as register 14 in the old code was needed directly after being loaded |
| # by the lm %r11,%r15,140(%r15) for the br %14. |
| 40051c: 58 40 f0 98 l %r4,152(%r15) |
| 400520: 98 7f f0 7c lm %r7,%r15,124(%r15) |
| 400524: 07 f4 br %r4 |
| } |
| |
| |
| Hartmut ( our compiler developer ) also has been threatening to take out the |
| stack backchain in optimised code as this also causes pipeline stalls, you |
| have been warned. |
| |
| 64 bit z/Architecture code disassembly |
| -------------------------------------- |
| |
| If you understand the stuff above you'll understand the stuff |
| below too so I'll avoid repeating myself & just say that |
| some of the instructions have g's on the end of them to indicate |
| they are 64 bit & the stack offsets are a bigger, |
| the only other difference you'll find between 32 & 64 bit is that |
| we now use f4 & f6 for floating point arguments on 64 bit. |
| 00000000800005b0 <test>: |
| int test(int b) |
| { |
| return(5+b); |
| 800005b0: a7 2a 00 05 ahi %r2,5 |
| 800005b4: b9 14 00 22 lgfr %r2,%r2 # downcast to integer |
| 800005b8: 07 fe br %r14 |
| 800005ba: 07 07 bcr 0,%r7 |
| |
| |
| } |
| |
| 00000000800005bc <main>: |
| main(int argc,char *argv[]) |
| { |
| 800005bc: eb bf f0 58 00 24 stmg %r11,%r15,88(%r15) |
| 800005c2: b9 04 00 1f lgr %r1,%r15 |
| 800005c6: a7 fb ff 60 aghi %r15,-160 |
| 800005ca: e3 10 f0 00 00 24 stg %r1,0(%r15) |
| return(test(5)); |
| 800005d0: a7 29 00 05 lghi %r2,5 |
| # brasl allows jumps > 64k & is overkill here bras would do fune |
| 800005d4: c0 e5 ff ff ff ee brasl %r14,800005b0 <test> |
| 800005da: e3 40 f1 10 00 04 lg %r4,272(%r15) |
| 800005e0: eb bf f0 f8 00 04 lmg %r11,%r15,248(%r15) |
| 800005e6: 07 f4 br %r4 |
| } |
| |
| |
| |
| Compiling programs for debugging on Linux for s/390 & z/Architecture |
| ==================================================================== |
| -gdwarf-2 now works it should be considered the default debugging |
| format for s/390 & z/Architecture as it is more reliable for debugging |
| shared libraries, normal -g debugging works much better now |
| Thanks to the IBM java compiler developers bug reports. |
| |
| This is typically done adding/appending the flags -g or -gdwarf-2 to the |
| CFLAGS & LDFLAGS variables Makefile of the program concerned. |
| |
| If using gdb & you would like accurate displays of registers & |
| stack traces compile without optimisation i.e make sure |
| that there is no -O2 or similar on the CFLAGS line of the Makefile & |
| the emitted gcc commands, obviously this will produce worse code |
| ( not advisable for shipment ) but it is an aid to the debugging process. |
| |
| This aids debugging because the compiler will copy parameters passed in |
| in registers onto the stack so backtracing & looking at passed in |
| parameters will work, however some larger programs which use inline functions |
| will not compile without optimisation. |
| |
| Debugging with optimisation has since much improved after fixing |
| some bugs, please make sure you are using gdb-5.0 or later developed |
| after Nov'2000. |
| |
| Figuring out gcc compile errors |
| =============================== |
| If you are getting a lot of syntax errors compiling a program & the problem |
| isn't blatantly obvious from the source. |
| It often helps to just preprocess the file, this is done with the -E |
| option in gcc. |
| What this does is that it runs through the very first phase of compilation |
| ( compilation in gcc is done in several stages & gcc calls many programs to |
| achieve its end result ) with the -E option gcc just calls the gcc preprocessor (cpp). |
| The c preprocessor does the following, it joins all the files #included together |
| recursively ( #include files can #include other files ) & also the c file you wish to compile. |
| It puts a fully qualified path of the #included files in a comment & it |
| does macro expansion. |
| This is useful for debugging because |
| 1) You can double check whether the files you expect to be included are the ones |
| that are being included ( e.g. double check that you aren't going to the i386 asm directory ). |
| 2) Check that macro definitions aren't clashing with typedefs, |
| 3) Check that definitions aren't being used before they are being included. |
| 4) Helps put the line emitting the error under the microscope if it contains macros. |
| |
| For convenience the Linux kernel's makefile will do preprocessing automatically for you |
| by suffixing the file you want built with .i ( instead of .o ) |
| |
| e.g. |
| from the linux directory type |
| make arch/s390/kernel/signal.i |
| this will build |
| |
| s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer |
| -fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -E arch/s390/kernel/signal.c |
| > arch/s390/kernel/signal.i |
| |
| Now look at signal.i you should see something like. |
| |
| |
| # 1 "/home1/barrow/linux/include/asm/types.h" 1 |
| typedef unsigned short umode_t; |
| typedef __signed__ char __s8; |
| typedef unsigned char __u8; |
| typedef __signed__ short __s16; |
| typedef unsigned short __u16; |
| |
| If instead you are getting errors further down e.g. |
| unknown instruction:2515 "move.l" or better still unknown instruction:2515 |
| "Fixme not implemented yet, call Martin" you are probably are attempting to compile some code |
| meant for another architecture or code that is simply not implemented, with a fixme statement |
| stuck into the inline assembly code so that the author of the file now knows he has work to do. |
| To look at the assembly emitted by gcc just before it is about to call gas ( the gnu assembler ) |
| use the -S option. |
| Again for your convenience the Linux kernel's Makefile will hold your hand & |
| do all this donkey work for you also by building the file with the .s suffix. |
| e.g. |
| from the Linux directory type |
| make arch/s390/kernel/signal.s |
| |
| s390-gcc -D__KERNEL__ -I/home1/barrow/linux/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer |
| -fno-strict-aliasing -D__SMP__ -pipe -fno-strength-reduce -S arch/s390/kernel/signal.c |
| -o arch/s390/kernel/signal.s |
| |
| |
| This will output something like, ( please note the constant pool & the useful comments |
| in the prologue to give you a hand at interpreting it ). |
| |
| .LC54: |
| .string "misaligned (__u16 *) in __xchg\n" |
| .LC57: |
| .string "misaligned (__u32 *) in __xchg\n" |
| .L$PG1: # Pool sys_sigsuspend |
| .LC192: |
| .long -262401 |
| .LC193: |
| .long -1 |
| .LC194: |
| .long schedule-.L$PG1 |
| .LC195: |
| .long do_signal-.L$PG1 |
| .align 4 |
| .globl sys_sigsuspend |
| .type sys_sigsuspend,@function |
| sys_sigsuspend: |
| # leaf function 0 |
| # automatics 16 |
| # outgoing args 0 |
| # need frame pointer 0 |
| # call alloca 0 |
| # has varargs 0 |
| # incoming args (stack) 0 |
| # function length 168 |
| STM 8,15,32(15) |
| LR 0,15 |
| AHI 15,-112 |
| BASR 13,0 |
| .L$CO1: AHI 13,.L$PG1-.L$CO1 |
| ST 0,0(15) |
| LR 8,2 |
| N 5,.LC192-.L$PG1(13) |
| |
| Adding -g to the above output makes the output even more useful |
| e.g. typing |
| make CC:="s390-gcc -g" kernel/sched.s |
| |
| which compiles. |
| s390-gcc -g -D__KERNEL__ -I/home/barrow/linux-2.3/include -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer -fno-strict-aliasing -pipe -fno-strength-reduce -S kernel/sched.c -o kernel/sched.s |
| |
| also outputs stabs ( debugger ) info, from this info you can find out the |
| offsets & sizes of various elements in structures. |
| e.g. the stab for the structure |
| struct rlimit { |
| unsigned long rlim_cur; |
| unsigned long rlim_max; |
| }; |
| is |
| .stabs "rlimit:T(151,2)=s8rlim_cur:(0,5),0,32;rlim_max:(0,5),32,32;;",128,0,0,0 |
| from this stab you can see that |
| rlimit_cur starts at bit offset 0 & is 32 bits in size |
| rlimit_max starts at bit offset 32 & is 32 bits in size. |
| |
| |
| Debugging Tools: |
| ================ |
| |
| objdump |
| ======= |
| This is a tool with many options the most useful being ( if compiled with -g). |
| objdump --source <victim program or object file> > <victims debug listing > |
| |
| |
| The whole kernel can be compiled like this ( Doing this will make a 17MB kernel |
| & a 200 MB listing ) however you have to strip it before building the image |
| using the strip command to make it a more reasonable size to boot it. |
| |
| A source/assembly mixed dump of the kernel can be done with the line |
| objdump --source vmlinux > vmlinux.lst |
| Also, if the file isn't compiled -g, this will output as much debugging information |
| as it can (e.g. function names). This is very slow as it spends lots |
| of time searching for debugging info. The following self explanatory line should be used |
| instead if the code isn't compiled -g, as it is much faster: |
| objdump --disassemble-all --syms vmlinux > vmlinux.lst |
| |
| As hard drive space is valuble most of us use the following approach. |
| 1) Look at the emitted psw on the console to find the crash address in the kernel. |
| 2) Look at the file System.map ( in the linux directory ) produced when building |
| the kernel to find the closest address less than the current PSW to find the |
| offending function. |
| 3) use grep or similar to search the source tree looking for the source file |
| with this function if you don't know where it is. |
| 4) rebuild this object file with -g on, as an example suppose the file was |
| ( /arch/s390/kernel/signal.o ) |
| 5) Assuming the file with the erroneous function is signal.c Move to the base of the |
| Linux source tree. |
| 6) rm /arch/s390/kernel/signal.o |
| 7) make /arch/s390/kernel/signal.o |
| 8) watch the gcc command line emitted |
| 9) type it in again or alternatively cut & paste it on the console adding the -g option. |
| 10) objdump --source arch/s390/kernel/signal.o > signal.lst |
| This will output the source & the assembly intermixed, as the snippet below shows |
| This will unfortunately output addresses which aren't the same |
| as the kernel ones you should be able to get around the mental arithmetic |
| by playing with the --adjust-vma parameter to objdump. |
| |
| |
| |
| |
| static inline void spin_lock(spinlock_t *lp) |
| { |
| a0: 18 34 lr %r3,%r4 |
| a2: a7 3a 03 bc ahi %r3,956 |
| __asm__ __volatile(" lhi 1,-1\n" |
| a6: a7 18 ff ff lhi %r1,-1 |
| aa: 1f 00 slr %r0,%r0 |
| ac: ba 01 30 00 cs %r0,%r1,0(%r3) |
| b0: a7 44 ff fd jm aa <sys_sigsuspend+0x2e> |
| saveset = current->blocked; |
| b4: d2 07 f0 68 mvc 104(8,%r15),972(%r4) |
| b8: 43 cc |
| return (set->sig[0] & mask) != 0; |
| } |
| |
| 6) If debugging under VM go down to that section in the document for more info. |
| |
| |
| I now have a tool which takes the pain out of --adjust-vma |
| & you are able to do something like |
| make /arch/s390/kernel/traps.lst |
| & it automatically generates the correctly relocated entries for |
| the text segment in traps.lst. |
| This tool is now standard in linux distro's in scripts/makelst |
| |
| strace: |
| ------- |
| Q. What is it ? |
| A. It is a tool for intercepting calls to the kernel & logging them |
| to a file & on the screen. |
| |
| Q. What use is it ? |
| A. You can used it to find out what files a particular program opens. |
| |
| |
| |
| Example 1 |
| --------- |
| If you wanted to know does ping work but didn't have the source |
| strace ping -c 1 127.0.0.1 |
| & then look at the man pages for each of the syscalls below, |
| ( In fact this is sometimes easier than looking at some spagetti |
| source which conditionally compiles for several architectures ). |
| Not everything that it throws out needs to make sense immediately. |
| |
| Just looking quickly you can see that it is making up a RAW socket |
| for the ICMP protocol. |
| Doing an alarm(10) for a 10 second timeout |
| & doing a gettimeofday call before & after each read to see |
| how long the replies took, & writing some text to stdout so the user |
| has an idea what is going on. |
| |
| socket(PF_INET, SOCK_RAW, IPPROTO_ICMP) = 3 |
| getuid() = 0 |
| setuid(0) = 0 |
| stat("/usr/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory) |
| stat("/usr/share/locale/libc/C", 0xbffff134) = -1 ENOENT (No such file or directory) |
| stat("/usr/local/share/locale/C/libc.cat", 0xbffff134) = -1 ENOENT (No such file or directory) |
| getpid() = 353 |
| setsockopt(3, SOL_SOCKET, SO_BROADCAST, [1], 4) = 0 |
| setsockopt(3, SOL_SOCKET, SO_RCVBUF, [49152], 4) = 0 |
| fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 1), ...}) = 0 |
| mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40008000 |
| ioctl(1, TCGETS, {B9600 opost isig icanon echo ...}) = 0 |
| write(1, "PING 127.0.0.1 (127.0.0.1): 56 d"..., 42PING 127.0.0.1 (127.0.0.1): 56 data bytes |
| ) = 42 |
| sigaction(SIGINT, {0x8049ba0, [], SA_RESTART}, {SIG_DFL}) = 0 |
| sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {SIG_DFL}) = 0 |
| gettimeofday({948904719, 138951}, NULL) = 0 |
| sendto(3, "\10\0D\201a\1\0\0\17#\2178\307\36"..., 64, 0, {sin_family=AF_INET, |
| sin_port=htons(0), sin_addr=inet_addr("127.0.0.1")}, 16) = 64 |
| sigaction(SIGALRM, {0x8049600, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0 |
| sigaction(SIGALRM, {0x8049ba0, [], SA_RESTART}, {0x8049600, [], SA_RESTART}) = 0 |
| alarm(10) = 0 |
| recvfrom(3, "E\0\0T\0005\0\0@\1|r\177\0\0\1\177"..., 192, 0, |
| {sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84 |
| gettimeofday({948904719, 160224}, NULL) = 0 |
| recvfrom(3, "E\0\0T\0006\0\0\377\1\275p\177\0"..., 192, 0, |
| {sin_family=AF_INET, sin_port=htons(50882), sin_addr=inet_addr("127.0.0.1")}, [16]) = 84 |
| gettimeofday({948904719, 166952}, NULL) = 0 |
| write(1, "64 bytes from 127.0.0.1: icmp_se"..., |
| 5764 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=28.0 ms |
| |
| Example 2 |
| --------- |
| strace passwd 2>&1 | grep open |
| produces the following output |
| open("/etc/ld.so.cache", O_RDONLY) = 3 |
| open("/opt/kde/lib/libc.so.5", O_RDONLY) = -1 ENOENT (No such file or directory) |
| open("/lib/libc.so.5", O_RDONLY) = 3 |
| open("/dev", O_RDONLY) = 3 |
| open("/var/run/utmp", O_RDONLY) = 3 |
| open("/etc/passwd", O_RDONLY) = 3 |
| open("/etc/shadow", O_RDONLY) = 3 |
| open("/etc/login.defs", O_RDONLY) = 4 |
| open("/dev/tty", O_RDONLY) = 4 |
| |
| The 2>&1 is done to redirect stderr to stdout & grep is then filtering this input |
| through the pipe for each line containing the string open. |
| |
| |
| Example 3 |
| --------- |
| Getting sophistocated |
| telnetd crashes on & I don't know why |
| Steps |
| ----- |
| 1) Replace the following line in /etc/inetd.conf |
| telnet stream tcp nowait root /usr/sbin/in.telnetd -h |
| with |
| telnet stream tcp nowait root /blah |
| |
| 2) Create the file /blah with the following contents to start tracing telnetd |
| #!/bin/bash |
| /usr/bin/strace -o/t1 -f /usr/sbin/in.telnetd -h |
| 3) chmod 700 /blah to make it executable only to root |
| 4) |
| killall -HUP inetd |
| or ps aux | grep inetd |
| get inetd's process id |
| & kill -HUP inetd to restart it. |
| |
| Important options |
| ----------------- |
| -o is used to tell strace to output to a file in our case t1 in the root directory |
| -f is to follow children i.e. |
| e.g in our case above telnetd will start the login process & subsequently a shell like bash. |
| You will be able to tell which is which from the process ID's listed on the left hand side |
| of the strace output. |
| -p<pid> will tell strace to attach to a running process, yup this can be done provided |
| it isn't being traced or debugged already & you have enough privileges, |
| the reason 2 processes cannot trace or debug the same program is that strace |
| becomes the parent process of the one being debugged & processes ( unlike people ) |
| can have only one parent. |
| |
| |
| However the file /t1 will get big quite quickly |
| to test it telnet 127.0.0.1 |
| |
| now look at what files in.telnetd execve'd |
| 413 execve("/usr/sbin/in.telnetd", ["/usr/sbin/in.telnetd", "-h"], [/* 17 vars */]) = 0 |
| 414 execve("/bin/login", ["/bin/login", "-h", "localhost", "-p"], [/* 2 vars */]) = 0 |
| |
| Whey it worked!. |
| |
| |
| Other hints: |
| ------------ |
| If the program is not very interactive ( i.e. not much keyboard input ) |
| & is crashing in one architecture but not in another you can do |
| an strace of both programs under as identical a scenario as you can |
| on both architectures outputting to a file then. |
| do a diff of the two traces using the diff program |
| i.e. |
| diff output1 output2 |
| & maybe you'll be able to see where the call paths differed, this |
| is possibly near the cause of the crash. |
| |
| More info |
| --------- |
| Look at man pages for strace & the various syscalls |
| e.g. man strace, man alarm, man socket. |
| |
| |
| Performance Debugging |
| ===================== |
| gcc is capible of compiling in profiling code just add the -p option |
| to the CFLAGS, this obviously affects program size & performance. |
| This can be used by the gprof gnu profiling tool or the |
| gcov the gnu code coverage tool ( code coverage is a means of testing |
| code quality by checking if all the code in an executable in exercised by |
| a tester ). |
| |
| |
| Using top to find out where processes are sleeping in the kernel |
| ---------------------------------------------------------------- |
| To do this copy the System.map from the root directory where |
| the linux kernel was built to the /boot directory on your |
| linux machine. |
| Start top |
| Now type fU<return> |
| You should see a new field called WCHAN which |
| tells you where each process is sleeping here is a typical output. |
| |
| 6:59pm up 41 min, 1 user, load average: 0.00, 0.00, 0.00 |
| 28 processes: 27 sleeping, 1 running, 0 zombie, 0 stopped |
| CPU states: 0.0% user, 0.1% system, 0.0% nice, 99.8% idle |
| Mem: 254900K av, 45976K used, 208924K free, 0K shrd, 28636K buff |
| Swap: 0K av, 0K used, 0K free 8620K cached |
| |
| PID USER PRI NI SIZE RSS SHARE WCHAN STAT LIB %CPU %MEM TIME COMMAND |
| 750 root 12 0 848 848 700 do_select S 0 0.1 0.3 0:00 in.telnetd |
| 767 root 16 0 1140 1140 964 R 0 0.1 0.4 0:00 top |
| 1 root 8 0 212 212 180 do_select S 0 0.0 0.0 0:00 init |
| 2 root 9 0 0 0 0 down_inte SW 0 0.0 0.0 0:00 kmcheck |
| |
| The time command |
| ---------------- |
| Another related command is the time command which gives you an indication |
| of where a process is spending the majority of its time. |
| e.g. |
| time ping -c 5 nc |
| outputs |
| real 0m4.054s |
| user 0m0.010s |
| sys 0m0.010s |
| |
| Debugging under VM |
| ================== |
| |
| Notes |
| ----- |
| Addresses & values in the VM debugger are always hex never decimal |
| Address ranges are of the format <HexValue1>-<HexValue2> or <HexValue1>.<HexValue2> |
| e.g. The address range 0x2000 to 0x3000 can be described described as |
| 2000-3000 or 2000.1000 |
| |
| The VM Debugger is case insensitive. |
| |
| VM's strengths are usually other debuggers weaknesses you can get at any resource |
| no matter how sensitive e.g. memory management resources,change address translation |
| in the PSW. For kernel hacking you will reap dividends if you get good at it. |
| |
| The VM Debugger displays operators but not operands, probably because some |
| of it was written when memory was expensive & the programmer was probably proud that |
| it fitted into 2k of memory & the programmers & didn't want to shock hardcore VM'ers by |
| changing the interface :-), also the debugger displays useful information on the same line & |
| the author of the code probably felt that it was a good idea not to go over |
| the 80 columns on the screen. |
| |
| As some of you are probably in a panic now this isn't as unintuitive as it may seem |
| as the 390 instructions are easy to decode mentally & you can make a good guess at a lot |
| of them as all the operands are nibble ( half byte aligned ) & if you have an objdump listing |
| also it is quite easy to follow, if you don't have an objdump listing keep a copy of |
| the s/390 Reference Summary & look at between pages 2 & 7 or alternatively the |
| s/390 principles of operation. |
| e.g. even I can guess that |
| 0001AFF8' LR 180F CC 0 |
| is a ( load register ) lr r0,r15 |
| |
| Also it is very easy to tell the length of a 390 instruction from the 2 most significant |
| bits in the instruction ( not that this info is really useful except if you are trying to |
| make sense of a hexdump of code ). |
| Here is a table |
| Bits Instruction Length |
| ------------------------------------------ |
| 00 2 Bytes |
| 01 4 Bytes |
| 10 4 Bytes |
| 11 6 Bytes |
| |
| |
| |
| |
| The debugger also displays other useful info on the same line such as the |
| addresses being operated on destination addresses of branches & condition codes. |
| e.g. |
| 00019736' AHI A7DAFF0E CC 1 |
| 000198BA' BRC A7840004 -> 000198C2' CC 0 |
| 000198CE' STM 900EF068 >> 0FA95E78 CC 2 |
| |
| |
| |
| Useful VM debugger commands |
| --------------------------- |
| |
| I suppose I'd better mention this before I start |
| to list the current active traces do |
| Q TR |
| there can be a maximum of 255 of these per set |
| ( more about trace sets later ). |
| To stop traces issue a |
| TR END. |
| To delete a particular breakpoint issue |
| TR DEL <breakpoint number> |
| |
| The PA1 key drops to CP mode so you can issue debugger commands, |
| Doing alt c (on my 3270 console at least ) clears the screen. |
| hitting b <enter> comes back to the running operating system |
| from cp mode ( in our case linux ). |
| It is typically useful to add shortcuts to your profile.exec file |
| if you have one ( this is roughly equivalent to autoexec.bat in DOS ). |
| file here are a few from mine. |
| /* this gives me command history on issuing f12 */ |
| set pf12 retrieve |
| /* this continues */ |
| set pf8 imm b |
| /* goes to trace set a */ |
| set pf1 imm tr goto a |
| /* goes to trace set b */ |
| set pf2 imm tr goto b |
| /* goes to trace set c */ |
| set pf3 imm tr goto c |
| |
| |
| |
| Instruction Tracing |
| ------------------- |
| Setting a simple breakpoint |
| TR I PSWA <address> |
| To debug a particular function try |
| TR I R <function address range> |
| TR I on its own will single step. |
| TR I DATA <MNEMONIC> <OPTIONAL RANGE> will trace for particular mnemonics |
| e.g. |
| TR I DATA 4D R 0197BC.4000 |
| will trace for BAS'es ( opcode 4D ) in the range 0197BC.4000 |
| if you were inclined you could add traces for all branch instructions & |
| suffix them with the run prefix so you would have a backtrace on screen |
| when a program crashes. |
| TR BR <INTO OR FROM> will trace branches into or out of an address. |
| e.g. |
| TR BR INTO 0 is often quite useful if a program is getting awkward & deciding |
| to branch to 0 & crashing as this will stop at the address before in jumps to 0. |
| TR I R <address range> RUN cmd d g |
| single steps a range of addresses but stays running & |
| displays the gprs on each step. |
| |
| |
| |
| Displaying & modifying Registers |
| -------------------------------- |
| D G will display all the gprs |
| Adding a extra G to all the commands is necessary to access the full 64 bit |
| content in VM on z/Architecture obviously this isn't required for access registers |
| as these are still 32 bit. |
| e.g. DGG instead of DG |
| D X will display all the control registers |
| D AR will display all the access registers |
| D AR4-7 will display access registers 4 to 7 |
| CPU ALL D G will display the GRPS of all CPUS in the configuration |
| D PSW will display the current PSW |
| st PSW 2000 will put the value 2000 into the PSW & |
| cause crash your machine. |
| D PREFIX displays the prefix offset |
| |
| |
| Displaying Memory |
| ----------------- |
| To display memory mapped using the current PSW's mapping try |
| D <range> |
| To make VM display a message each time it hits a particular address & continue try |
| D I<range> will disassemble/display a range of instructions. |
| ST addr 32 bit word will store a 32 bit aligned address |
| D T<range> will display the EBCDIC in an address ( if you are that way inclined ) |
| D R<range> will display real addresses ( without DAT ) but with prefixing. |
| There are other complex options to display if you need to get at say home space |
| but are in primary space the easiest thing to do is to temporarily |
| modify the PSW to the other addressing mode, display the stuff & then |
| restore it. |
| |
| |
| |
| Hints |
| ----- |
| If you want to issue a debugger command without halting your virtual machine with the |
| PA1 key try prefixing the command with #CP e.g. |
| #cp tr i pswa 2000 |
| also suffixing most debugger commands with RUN will cause them not |
| to stop just display the mnemonic at the current instruction on the console. |
| If you have several breakpoints you want to put into your program & |
| you get fed up of cross referencing with System.map |
| you can do the following trick for several symbols. |
| grep do_signal System.map |
| which emits the following among other things |
| 0001f4e0 T do_signal |
| now you can do |
| |
| TR I PSWA 0001f4e0 cmd msg * do_signal |
| This sends a message to your own console each time do_signal is entered. |
| ( As an aside I wrote a perl script once which automatically generated a REXX |
| script with breakpoints on every kernel procedure, this isn't a good idea |
| because there are thousands of these routines & VM can only set 255 breakpoints |
| at a time so you nearly had to spend as long pruning the file down as you would |
| entering the msg's by hand ),however, the trick might be useful for a single object file. |
| On linux'es 3270 emulator x3270 there is a very useful option under the file ment |
| Save Screens In File this is very good of keeping a copy of traces. |
| |
| From CMS help <command name> will give you online help on a particular command. |
| e.g. |
| HELP DISPLAY |
| |
| Also CP has a file called profile.exec which automatically gets called |
| on startup of CMS ( like autoexec.bat ), keeping on a DOS analogy session |
| CP has a feature similar to doskey, it may be useful for you to |
| use profile.exec to define some keystrokes. |
| e.g. |
| SET PF9 IMM B |
| This does a single step in VM on pressing F8. |
| SET PF10 ^ |
| This sets up the ^ key. |
| which can be used for ^c (ctrl-c),^z (ctrl-z) which can't be typed directly into some 3270 consoles. |
| SET PF11 ^- |
| This types the starting keystrokes for a sysrq see SysRq below. |
| SET PF12 RETRIEVE |
| This retrieves command history on pressing F12. |
| |
| |
| Sometimes in VM the display is set up to scroll automatically this |
| can be very annoying if there are messages you wish to look at |
| to stop this do |
| TERM MORE 255 255 |
| This will nearly stop automatic screen updates, however it will |
| cause a denial of service if lots of messages go to the 3270 console, |
| so it would be foolish to use this as the default on a production machine. |
| |
| |
| Tracing particular processes |
| ---------------------------- |
| The kernel's text segment is intentionally at an address in memory that it will |
| very seldom collide with text segments of user programs ( thanks Martin ), |
| this simplifies debugging the kernel. |
| However it is quite common for user processes to have addresses which collide |
| this can make debugging a particular process under VM painful under normal |
| circumstances as the process may change when doing a |
| TR I R <address range>. |
| Thankfully after reading VM's online help I figured out how to debug |
| I particular process. |
| |
| Your first problem is to find the STD ( segment table designation ) |
| of the program you wish to debug. |
| There are several ways you can do this here are a few |
| 1) objdump --syms <program to be debugged> | grep main |
| To get the address of main in the program. |
| tr i pswa <address of main> |
| Start the program, if VM drops to CP on what looks like the entry |
| point of the main function this is most likely the process you wish to debug. |
| Now do a D X13 or D XG13 on z/Architecture. |
| On 31 bit the STD is bits 1-19 ( the STO segment table origin ) |
| & 25-31 ( the STL segment table length ) of CR13. |
| now type |
| TR I R STD <CR13's value> 0.7fffffff |
| e.g. |
| TR I R STD 8F32E1FF 0.7fffffff |
| Another very useful variation is |
| TR STORE INTO STD <CR13's value> <address range> |
| for finding out when a particular variable changes. |
| |
| An alternative way of finding the STD of a currently running process |
| is to do the following, ( this method is more complex but |
| could be quite convenient if you aren't updating the kernel much & |
| so your kernel structures will stay constant for a reasonable period of |
| time ). |
| |
| grep task /proc/<pid>/status |
| from this you should see something like |
| task: 0f160000 ksp: 0f161de8 pt_regs: 0f161f68 |
| This now gives you a pointer to the task structure. |
| Now make CC:="s390-gcc -g" kernel/sched.s |
| To get the task_struct stabinfo. |
| ( task_struct is defined in include/linux/sched.h ). |
| Now we want to look at |
| task->active_mm->pgd |
| on my machine the active_mm in the task structure stab is |
| active_mm:(4,12),672,32 |
| its offset is 672/8=84=0x54 |
| the pgd member in the mm_struct stab is |
| pgd:(4,6)=*(29,5),96,32 |
| so its offset is 96/8=12=0xc |
| |
| so we'll |
| hexdump -s 0xf160054 /dev/mem | more |
| i.e. task_struct+active_mm offset |
| to look at the active_mm member |
| f160054 0fee cc60 0019 e334 0000 0000 0000 0011 |
| hexdump -s 0x0feecc6c /dev/mem | more |
| i.e. active_mm+pgd offset |
| feecc6c 0f2c 0000 0000 0001 0000 0001 0000 0010 |
| we get something like |
| now do |
| TR I R STD <pgd|0x7f> 0.7fffffff |
| i.e. the 0x7f is added because the pgd only |
| gives the page table origin & we need to set the low bits |
| to the maximum possible segment table length. |
| TR I R STD 0f2c007f 0.7fffffff |
| on z/Architecture you'll probably need to do |
| TR I R STD <pgd|0x7> 0.ffffffffffffffff |
| to set the TableType to 0x1 & the Table length to 3. |
| |
| |
| |
| Tracing Program Exceptions |
| -------------------------- |
| If you get a crash which says something like |
| illegal operation or specification exception followed by a register dump |
| You can restart linux & trace these using the tr prog <range or value> trace option. |
| |
| |
| |
| The most common ones you will normally be tracing for is |
| 1=operation exception |
| 2=privileged operation exception |
| 4=protection exception |
| 5=addressing exception |
| 6=specification exception |
| 10=segment translation exception |
| 11=page translation exception |
| |
| The full list of these is on page 22 of the current s/390 Reference Summary. |
| e.g. |
| tr prog 10 will trace segment translation exceptions. |
| tr prog on its own will trace all program interruption codes. |
| |
| Trace Sets |
| ---------- |
| On starting VM you are initially in the INITIAL trace set. |
| You can do a Q TR to verify this. |
| If you have a complex tracing situation where you wish to wait for instance |
| till a driver is open before you start tracing IO, but know in your |
| heart that you are going to have to make several runs through the code till you |
| have a clue whats going on. |
| |
| What you can do is |
| TR I PSWA <Driver open address> |
| hit b to continue till breakpoint |
| reach the breakpoint |
| now do your |
| TR GOTO B |
| TR IO 7c08-7c09 inst int run |
| or whatever the IO channels you wish to trace are & hit b |
| |
| To got back to the initial trace set do |
| TR GOTO INITIAL |
| & the TR I PSWA <Driver open address> will be the only active breakpoint again. |
| |
| |
| Tracing linux syscalls under VM |
| ------------------------------- |
| Syscalls are implemented on Linux for S390 by the Supervisor call instruction (SVC) there 256 |
| possibilities of these as the instruction is made up of a 0xA opcode & the second byte being |
| the syscall number. They are traced using the simple command. |
| TR SVC <Optional value or range> |
| the syscalls are defined in linux/include/asm-s390/unistd.h |
| e.g. to trace all file opens just do |
| TR SVC 5 ( as this is the syscall number of open ) |
| |
| |
| SMP Specific commands |
| --------------------- |
| To find out how many cpus you have |
| Q CPUS displays all the CPU's available to your virtual machine |
| To find the cpu that the current cpu VM debugger commands are being directed at do |
| Q CPU to change the current cpu cpu VM debugger commands are being directed at do |
| CPU <desired cpu no> |
| |
| On a SMP guest issue a command to all CPUs try prefixing the command with cpu all. |
| To issue a command to a particular cpu try cpu <cpu number> e.g. |
| CPU 01 TR I R 2000.3000 |
| If you are running on a guest with several cpus & you have a IO related problem |
| & cannot follow the flow of code but you know it isnt smp related. |
| from the bash prompt issue |
| shutdown -h now or halt. |
| do a Q CPUS to find out how many cpus you have |
| detach each one of them from cp except cpu 0 |
| by issuing a |
| DETACH CPU 01-(number of cpus in configuration) |
| & boot linux again. |
| TR SIGP will trace inter processor signal processor instructions. |
| DEFINE CPU 01-(number in configuration) |
| will get your guests cpus back. |
| |
| |
| Help for displaying ascii textstrings |
| ------------------------------------- |
| On the very latest VM Nucleus'es VM can now display ascii |
| ( thanks Neale for the hint ) by doing |
| D TX<lowaddr>.<len> |
| e.g. |
| D TX0.100 |
| |
| Alternatively |
| ============= |
| Under older VM debuggers ( I love EBDIC too ) you can use this little program I wrote which |
| will convert a command line of hex digits to ascii text which can be compiled under linux & |
| you can copy the hex digits from your x3270 terminal to your xterm if you are debugging |
| from a linuxbox. |
| |
| This is quite useful when looking at a parameter passed in as a text string |
| under VM ( unless you are good at decoding ASCII in your head ). |
| |
| e.g. consider tracing an open syscall |
| TR SVC 5 |
| We have stopped at a breakpoint |
| 000151B0' SVC 0A05 -> 0001909A' CC 0 |
| |
| D 20.8 to check the SVC old psw in the prefix area & see was it from userspace |
| ( for the layout of the prefix area consult P18 of the s/390 390 Reference Summary |
| if you have it available ). |
| V00000020 070C2000 800151B2 |
| The problem state bit wasn't set & it's also too early in the boot sequence |
| for it to be a userspace SVC if it was we would have to temporarily switch the |
| psw to user space addressing so we could get at the first parameter of the open in |
| gpr2. |
| Next do a |
| D G2 |
| GPR 2 = 00014CB4 |
| Now display what gpr2 is pointing to |
| D 00014CB4.20 |
| V00014CB4 2F646576 2F636F6E 736F6C65 00001BF5 |
| V00014CC4 FC00014C B4001001 E0001000 B8070707 |
| Now copy the text till the first 00 hex ( which is the end of the string |
| to an xterm & do hex2ascii on it. |
| hex2ascii 2F646576 2F636F6E 736F6C65 00 |
| outputs |
| Decoded Hex:=/ d e v / c o n s o l e 0x00 |
| We were opening the console device, |
| |
| You can compile the code below yourself for practice :-), |
| /* |
| * hex2ascii.c |
| * a useful little tool for converting a hexadecimal command line to ascii |
| * |
| * Author(s): Denis Joseph Barrow (djbarrow@de.ibm.com,barrow_dj@yahoo.com) |
| * (C) 2000 IBM Deutschland Entwicklung GmbH, IBM Corporation. |
| */ |
| #include <stdio.h> |
| |
| int main(int argc,char *argv[]) |
| { |
| int cnt1,cnt2,len,toggle=0; |
| int startcnt=1; |
| unsigned char c,hex; |
| |
| if(argc>1&&(strcmp(argv[1],"-a")==0)) |
| startcnt=2; |
| printf("Decoded Hex:="); |
| for(cnt1=startcnt;cnt1<argc;cnt1++) |
| { |
| len=strlen(argv[cnt1]); |
| for(cnt2=0;cnt2<len;cnt2++) |
| { |
| c=argv[cnt1][cnt2]; |
| if(c>='0'&&c<='9') |
| c=c-'0'; |
| if(c>='A'&&c<='F') |
| c=c-'A'+10; |
| if(c>='a'&&c<='f') |
| c=c-'a'+10; |
| switch(toggle) |
| { |
| case 0: |
| hex=c<<4; |
| toggle=1; |
| break; |
| case 1: |
| hex+=c; |
| if(hex<32||hex>127) |
| { |
| if(startcnt==1) |
| printf("0x%02X ",(int)hex); |
| else |
| printf("."); |
| } |
| else |
| { |
| printf("%c",hex); |
| if(startcnt==1) |
| printf(" "); |
| } |
| toggle=0; |
| break; |
| } |
| } |
| } |
| printf("\n"); |
| } |
| |
| |
| |
| |
| Stack tracing under VM |
| ---------------------- |
| A basic backtrace |
| ----------------- |
| |
| Here are the tricks I use 9 out of 10 times it works pretty well, |
| |
| When your backchain reaches a dead end |
| -------------------------------------- |
| This can happen when an exception happens in the kernel & the kernel is entered twice |
| if you reach the NULL pointer at the end of the back chain you should be |
| able to sniff further back if you follow the following tricks. |
| 1) A kernel address should be easy to recognise since it is in |
| primary space & the problem state bit isn't set & also |
| The Hi bit of the address is set. |
| 2) Another backchain should also be easy to recognise since it is an |
| address pointing to another address approximately 100 bytes or 0x70 hex |
| behind the current stackpointer. |
| |
| |
| Here is some practice. |
| boot the kernel & hit PA1 at some random time |
| d g to display the gprs, this should display something like |
| GPR 0 = 00000001 00156018 0014359C 00000000 |
| GPR 4 = 00000001 001B8888 000003E0 00000000 |
| GPR 8 = 00100080 00100084 00000000 000FE000 |
| GPR 12 = 00010400 8001B2DC 8001B36A 000FFED8 |
| Note that GPR14 is a return address but as we are real men we are going to |
| trace the stack. |
| display 0x40 bytes after the stack pointer. |
| |
| V000FFED8 000FFF38 8001B838 80014C8E 000FFF38 |
| V000FFEE8 00000000 00000000 000003E0 00000000 |
| V000FFEF8 00100080 00100084 00000000 000FE000 |
| V000FFF08 00010400 8001B2DC 8001B36A 000FFED8 |
| |
| |
| Ah now look at whats in sp+56 (sp+0x38) this is 8001B36A our saved r14 if |
| you look above at our stackframe & also agrees with GPR14. |
| |
| now backchain |
| d 000FFF38.40 |
| we now are taking the contents of SP to get our first backchain. |
| |
| V000FFF38 000FFFA0 00000000 00014995 00147094 |
| V000FFF48 00147090 001470A0 000003E0 00000000 |
| V000FFF58 00100080 00100084 00000000 001BF1D0 |
| V000FFF68 00010400 800149BA 80014CA6 000FFF38 |
| |
| This displays a 2nd return address of 80014CA6 |
| |
| now do d 000FFFA0.40 for our 3rd backchain |
| |
| V000FFFA0 04B52002 0001107F 00000000 00000000 |
| V000FFFB0 00000000 00000000 FF000000 0001107F |
| V000FFFC0 00000000 00000000 00000000 00000000 |
| V000FFFD0 00010400 80010802 8001085A 000FFFA0 |
| |
| |
| our 3rd return address is 8001085A |
| |
| as the 04B52002 looks suspiciously like rubbish it is fair to assume that the kernel entry routines |
| for the sake of optimisation dont set up a backchain. |
| |
| now look at System.map to see if the addresses make any sense. |
| |
| grep -i 0001b3 System.map |
| outputs among other things |
| 0001b304 T cpu_idle |
| so 8001B36A |
| is cpu_idle+0x66 ( quiet the cpu is asleep, don't wake it ) |
| |
| |
| grep -i 00014 System.map |
| produces among other things |
| 00014a78 T start_kernel |
| so 0014CA6 is start_kernel+some hex number I can't add in my head. |
| |
| grep -i 00108 System.map |
| this produces |
| 00010800 T _stext |
| so 8001085A is _stext+0x5a |
| |
| Congrats you've done your first backchain. |
| |
| |
| |
| s/390 & z/Architecture IO Overview |
| ================================== |
| |
| I am not going to give a course in 390 IO architecture as this would take me quite a |
| while & I'm no expert. Instead I'll give a 390 IO architecture summary for Dummies if you have |
| the s/390 principles of operation available read this instead. If nothing else you may find a few |
| useful keywords in here & be able to use them on a web search engine like altavista to find |
| more useful information. |
| |
| Unlike other bus architectures modern 390 systems do their IO using mostly |
| fibre optics & devices such as tapes & disks can be shared between several mainframes, |
| also S390 can support upto 65536 devices while a high end PC based system might be choking |
| with around 64. Here is some of the common IO terminology |
| |
| Subchannel: |
| This is the logical number most IO commands use to talk to an IO device there can be upto |
| 0x10000 (65536) of these in a configuration typically there is a few hundred. Under VM |
| for simplicity they are allocated contiguously, however on the native hardware they are not |
| they typically stay consistent between boots provided no new hardware is inserted or removed. |
| Under Linux for 390 we use these as IRQ's & also when issuing an IO command (CLEAR SUBCHANNEL, |
| HALT SUBCHANNEL,MODIFY SUBCHANNEL,RESUME SUBCHANNEL,START SUBCHANNEL,STORE SUBCHANNEL & |
| TEST SUBCHANNEL ) we use this as the ID of the device we wish to talk to, the most |
| important of these instructions are START SUBCHANNEL ( to start IO ), TEST SUBCHANNEL ( to check |
| whether the IO completed successfully ), & HALT SUBCHANNEL ( to kill IO ), a subchannel |
| can have up to 8 channel paths to a device this offers redunancy if one is not available. |
| |
| |
| Device Number: |
| This number remains static & Is closely tied to the hardware, there are 65536 of these |
| also they are made up of a CHPID ( Channel Path ID, the most significant 8 bits ) |
| & another lsb 8 bits. These remain static even if more devices are inserted or removed |
| from the hardware, there is a 1 to 1 mapping between Subchannels & Device Numbers provided |
| devices arent inserted or removed. |
| |
| Channel Control Words: |
| CCWS are linked lists of instructions initially pointed to by an operation request block (ORB), |
| which is initially given to Start Subchannel (SSCH) command along with the subchannel number |
| for the IO subsystem to process while the CPU continues executing normal code. |
| These come in two flavours, Format 0 ( 24 bit for backward ) |
| compatibility & Format 1 ( 31 bit ). These are typically used to issue read & write |
| ( & many other instructions ) they consist of a length field & an absolute address field. |
| For each IO typically get 1 or 2 interrupts one for channel end ( primary status ) when the |
| channel is idle & the second for device end ( secondary status ) sometimes you get both |
| concurrently, you check how the IO went on by issuing a TEST SUBCHANNEL at each interrupt, |
| from which you receive an Interruption response block (IRB). If you get channel & device end |
| status in the IRB without channel checks etc. your IO probably went okay. If you didn't you |
| probably need a doctor to examine the IRB & extended status word etc. |
| If an error occurs, more sophistocated control units have a facitity known as |
| concurrent sense this means that if an error occurs Extended sense information will |
| be presented in the Extended status word in the IRB if not you have to issue a |
| subsequent SENSE CCW command after the test subchannel. |
| |
| |
| TPI( Test pending interrupt) can also be used for polled IO but in multitasking multiprocessor |
| systems it isn't recommended except for checking special cases ( i.e. non looping checks for |
| pending IO etc. ). |
| |
| Store Subchannel & Modify Subchannel can be used to examine & modify operating characteristics |
| of a subchannel ( e.g. channel paths ). |
| |
| Other IO related Terms: |
| Sysplex: S390's Clustering Technology |
| QDIO: S390's new high speed IO architecture to support devices such as gigabit ethernet, |
| this architecture is also designed to be forward compatible with up & coming 64 bit machines. |
| |
| |
| General Concepts |
| |
| Input Output Processors (IOP's) are responsible for communicating between |
| the mainframe CPU's & the channel & relieve the mainframe CPU's from the |
| burden of communicating with IO devices directly, this allows the CPU's to |
| concentrate on data processing. |
| |
| IOP's can use one or more links ( known as channel paths ) to talk to each |
| IO device. It first checks for path availability & chooses an available one, |
| then starts ( & sometimes terminates IO ). |
| There are two types of channel path ESCON & the Paralell IO interface. |
| |
| IO devices are attached to control units, control units provide the |
| logic to interface the channel paths & channel path IO protocols to |
| the IO devices, they can be integrated with the devices or housed separately |
| & often talk to several similar devices ( typical examples would be raid |
| controllers or a control unit which connects to 1000 3270 terminals ). |
| |
| |
| +---------------------------------------------------------------+ |
| | +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ | |
| | | CPU | | CPU | | CPU | | CPU | | Main | | Expanded | | |
| | | | | | | | | | | Memory | | Storage | | |
| | +-----+ +-----+ +-----+ +-----+ +----------+ +----------+ | |
| |---------------------------------------------------------------+ |
| | IOP | IOP | IOP | |
| |--------------------------------------------------------------- |
| | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | C | |
| ---------------------------------------------------------------- |
| || || |
| || Bus & Tag Channel Path || ESCON |
| || ====================== || Channel |
| || || || || Path |
| +----------+ +----------+ +----------+ |
| | | | | | | |
| | CU | | CU | | CU | |
| | | | | | | |
| +----------+ +----------+ +----------+ |
| | | | | | |
| +----------+ +----------+ +----------+ +----------+ +----------+ |
| |I/O Device| |I/O Device| |I/O Device| |I/O Device| |I/O Device| |
| +----------+ +----------+ +----------+ +----------+ +----------+ |
| CPU = Central Processing Unit |
| C = Channel |
| IOP = IP Processor |
| CU = Control Unit |
| |
| The 390 IO systems come in 2 flavours the current 390 machines support both |
| |
| The Older 360 & 370 Interface,sometimes called the paralell I/O interface, |
| sometimes called Bus-and Tag & sometimes Original Equipment Manufacturers |
| Interface (OEMI). |
| |
| This byte wide paralell channel path/bus has parity & data on the "Bus" cable |
| & control lines on the "Tag" cable. These can operate in byte multiplex mode for |
| sharing between several slow devices or burst mode & monopolize the channel for the |
| whole burst. Upto 256 devices can be addressed on one of these cables. These cables are |
| about one inch in diameter. The maximum unextended length supported by these cables is |
| 125 Meters but this can be extended up to 2km with a fibre optic channel extended |
| such as a 3044. The maximum burst speed supported is 4.5 megabytes per second however |
| some really old processors support only transfer rates of 3.0, 2.0 & 1.0 MB/sec. |
| One of these paths can be daisy chained to up to 8 control units. |
| |
| |
| ESCON if fibre optic it is also called FICON |
| Was introduced by IBM in 1990. Has 2 fibre optic cables & uses either leds or lasers |
| for communication at a signaling rate of upto 200 megabits/sec. As 10bits are transferred |
| for every 8 bits info this drops to 160 megabits/sec & to 18.6 Megabytes/sec once |
| control info & CRC are added. ESCON only operates in burst mode. |
| |
| ESCONs typical max cable length is 3km for the led version & 20km for the laser version |
| known as XDF ( extended distance facility ). This can be further extended by using an |
| ESCON director which triples the above mentioned ranges. Unlike Bus & Tag as ESCON is |
| serial it uses a packet switching architecture the standard Bus & Tag control protocol |
| is however present within the packets. Upto 256 devices can be attached to each control |
| unit that uses one of these interfaces. |
| |
| Common 390 Devices include: |
| Network adapters typically OSA2,3172's,2116's & OSA-E gigabit ethernet adapters, |
| Consoles 3270 & 3215 ( a teletype emulated under linux for a line mode console ). |
| DASD's direct access storage devices ( otherwise known as hard disks ). |
| Tape Drives. |
| CTC ( Channel to Channel Adapters ), |
| ESCON or Paralell Cables used as a very high speed serial link |
| between 2 machines. We use 2 cables under linux to do a bi-directional serial link. |
| |
| |
| Debugging IO on s/390 & z/Architecture under VM |
| =============================================== |
| |
| Now we are ready to go on with IO tracing commands under VM |
| |
| A few self explanatory queries: |
| Q OSA |
| Q CTC |
| Q DISK ( This command is CMS specific ) |
| Q DASD |
| |
| |
| |
| |
| |
| |
| Q OSA on my machine returns |
| OSA 7C08 ON OSA 7C08 SUBCHANNEL = 0000 |
| OSA 7C09 ON OSA 7C09 SUBCHANNEL = 0001 |
| OSA 7C14 ON OSA 7C14 SUBCHANNEL = 0002 |
| OSA 7C15 ON OSA 7C15 SUBCHANNEL = 0003 |
| |
| If you have a guest with certain priviliges you may be able to see devices |
| which don't belong to you to avoid this do add the option V. |
| e.g. |
| Q V OSA |
| |
| Now using the device numbers returned by this command we will |
| Trace the io starting up on the first device 7c08 & 7c09 |
| In our simplest case we can trace the |
| start subchannels |
| like TR SSCH 7C08-7C09 |
| or the halt subchannels |
| or TR HSCH 7C08-7C09 |
| MSCH's ,STSCH's I think you can guess the rest |
| |
| Ingo's favourite trick is tracing all the IO's & CCWS & spooling them into the reader of another |
| VM guest so he can ftp the logfile back to his own machine.I'll do a small bit of this & give you |
| a look at the output. |
| |
| 1) Spool stdout to VM reader |
| SP PRT TO (another vm guest ) or * for the local vm guest |
| 2) Fill the reader with the trace |
| TR IO 7c08-7c09 INST INT CCW PRT RUN |
| 3) Start up linux |
| i 00c |
| 4) Finish the trace |
| TR END |
| 5) close the reader |
| C PRT |
| 6) list reader contents |
| RDRLIST |
| 7) copy it to linux4's minidisk |
| RECEIVE / LOG TXT A1 ( replace |
| 8) |
| filel & press F11 to look at it |
| You should see someting like. |
| |
| 00020942' SSCH B2334000 0048813C CC 0 SCH 0000 DEV 7C08 |
| CPA 000FFDF0 PARM 00E2C9C4 KEY 0 FPI C0 LPM 80 |
| CCW 000FFDF0 E4200100 00487FE8 0000 E4240100 ........ |
| IDAL 43D8AFE8 |
| IDAL 0FB76000 |
| 00020B0A' I/O DEV 7C08 -> 000197BC' SCH 0000 PARM 00E2C9C4 |
| 00021628' TSCH B2354000 >> 00488164 CC 0 SCH 0000 DEV 7C08 |
| CCWA 000FFDF8 DEV STS 0C SCH STS 00 CNT 00EC |
| KEY 0 FPI C0 CC 0 CTLS 4007 |
| 00022238' STSCH B2344000 >> 00488108 CC 0 SCH 0000 DEV 7C08 |
| |
| If you don't like messing up your readed ( because you possibly booted from it ) |
| you can alternatively spool it to another readers guest. |
| |
| |
| Other common VM device related commands |
| --------------------------------------------- |
| These commands are listed only because they have |
| been of use to me in the past & may be of use to |
| you too. For more complete info on each of the commands |
| use type HELP <command> from CMS. |
| detaching devices |
| DET <devno range> |
| ATT <devno range> <guest> |
| attach a device to guest * for your own guest |
| READY <devno> cause VM to issue a fake interrupt. |
| |
| The VARY command is normally only available to VM administrators. |
| VARY ON PATH <path> TO <devno range> |
| VARY OFF PATH <PATH> FROM <devno range> |
| This is used to switch on or off channel paths to devices. |
| |
| Q CHPID <channel path ID> |
| This displays state of devices using this channel path |
| D SCHIB <subchannel> |
| This displays the subchannel information SCHIB block for the device. |
| this I believe is also only available to administrators. |
| DEFINE CTC <devno> |
| defines a virtual CTC channel to channel connection |
| 2 need to be defined on each guest for the CTC driver to use. |
| COUPLE devno userid remote devno |
| Joins a local virtual device to a remote virtual device |
| ( commonly used for the CTC driver ). |
| |
| Building a VM ramdisk under CMS which linux can use |
| def vfb-<blocksize> <subchannel> <number blocks> |
| blocksize is commonly 4096 for linux. |
| Formatting it |
| format <subchannel> <driver letter e.g. x> (blksize <blocksize> |
| |
| Sharing a disk between multiple guests |
| LINK userid devno1 devno2 mode password |
| |
| |
| |
| GDB on S390 |
| =========== |
| N.B. if compiling for debugging gdb works better without optimisation |
| ( see Compiling programs for debugging ) |
| |
| invocation |
| ---------- |
| gdb <victim program> <optional corefile> |
| |
| Online help |
| ----------- |
| help: gives help on commands |
| e.g. |
| help |
| help display |
| Note gdb's online help is very good use it. |
| |
| |
| Assembly |
| -------- |
| info registers: displays registers other than floating point. |
| info all-registers: displays floating points as well. |
| disassemble: disassembles |
| e.g. |
| disassemble without parameters will disassemble the current function |
| disassemble $pc $pc+10 |
| |
| Viewing & modifying variables |
| ----------------------------- |
| print or p: displays variable or register |
| e.g. p/x $sp will display the stack pointer |
| |
| display: prints variable or register each time program stops |
| e.g. |
| display/x $pc will display the program counter |
| display argc |
| |
| undisplay : undo's display's |
| |
| info breakpoints: shows all current breakpoints |
| |
| info stack: shows stack back trace ( if this doesn't work too well, I'll show you the |
| stacktrace by hand below ). |
| |
| info locals: displays local variables. |
| |
| info args: display current procedure arguments. |
| |
| set args: will set argc & argv each time the victim program is invoked. |
| |
| set <variable>=value |
| set argc=100 |
| set $pc=0 |
| |
| |
| |
| Modifying execution |
| ------------------- |
| step: steps n lines of sourcecode |
| step steps 1 line. |
| step 100 steps 100 lines of code. |
| |
| next: like step except this will not step into subroutines |
| |
| stepi: steps a single machine code instruction. |
| e.g. stepi 100 |
| |
| nexti: steps a single machine code instruction but will not step into subroutines. |
| |
| finish: will run until exit of the current routine |
| |
| run: (re)starts a program |
| |
| cont: continues a program |
| |
| quit: exits gdb. |
| |
| |
| breakpoints |
| ------------ |
| |
| break |
| sets a breakpoint |
| e.g. |
| |
| break main |
| |
| break *$pc |
| |
| break *0x400618 |
| |
| heres a really useful one for large programs |
| rbr |
| Set a breakpoint for all functions matching REGEXP |
| e.g. |
| rbr 390 |
| will set a breakpoint with all functions with 390 in their name. |
| |
| info breakpoints |
| lists all breakpoints |
| |
| delete: delete breakpoint by number or delete them all |
| e.g. |
| delete 1 will delete the first breakpoint |
| delete will delete them all |
| |
| watch: This will set a watchpoint ( usually hardware assisted ), |
| This will watch a variable till it changes |
| e.g. |
| watch cnt, will watch the variable cnt till it changes. |
| As an aside unfortunately gdb's, architecture independent watchpoint code |
| is inconsistent & not very good, watchpoints usually work but not always. |
| |
| info watchpoints: Display currently active watchpoints |
| |
| condition: ( another useful one ) |
| Specify breakpoint number N to break only if COND is true. |
| Usage is `condition N COND', where N is an integer and COND is an |
| expression to be evaluated whenever breakpoint N is reached. |
| |
| |
| |
| User defined functions/macros |
| ----------------------------- |
| define: ( Note this is very very useful,simple & powerful ) |
| usage define <name> <list of commands> end |
| |
| examples which you should consider putting into .gdbinit in your home directory |
| define d |
| stepi |
| disassemble $pc $pc+10 |
| end |
| |
| define e |
| nexti |
| disassemble $pc $pc+10 |
| end |
| |
| |
| Other hard to classify stuff |
| ---------------------------- |
| signal n: |
| sends the victim program a signal. |
| e.g. signal 3 will send a SIGQUIT. |
| |
| info signals: |
| what gdb does when the victim receives certain signals. |
| |
| list: |
| e.g. |
| list lists current function source |
| list 1,10 list first 10 lines of current file. |
| list test.c:1,10 |
| |
| |
| directory: |
| Adds directories to be searched for source if gdb cannot find the source. |
| (note it is a bit sensititive about slashes) |
| e.g. To add the root of the filesystem to the searchpath do |
| directory // |
| |
| |
| call <function> |
| This calls a function in the victim program, this is pretty powerful |
| e.g. |
| (gdb) call printf("hello world") |
| outputs: |
| $1 = 11 |
| |
| You might now be thinking that the line above didn't work, something extra had to be done. |
| (gdb) call fflush(stdout) |
| hello world$2 = 0 |
| As an aside the debugger also calls malloc & free under the hood |
| to make space for the "hello world" string. |
| |
| |
| |
| hints |
| ----- |
| 1) command completion works just like bash |
| ( if you are a bad typist like me this really helps ) |
| e.g. hit br <TAB> & cursor up & down :-). |
| |
| 2) if you have a debugging problem that takes a few steps to recreate |
| put the steps into a file called .gdbinit in your current working directory |
| if you have defined a few extra useful user defined commands put these in |
| your home directory & they will be read each time gdb is launched. |
| |
| A typical .gdbinit file might be. |
| break main |
| run |
| break runtime_exception |
| cont |
| |
| |
| stack chaining in gdb by hand |
| ----------------------------- |
| This is done using a the same trick described for VM |
| p/x (*($sp+56))&0x7fffffff get the first backchain. |
| |
| For z/Architecture |
| Replace 56 with 112 & ignore the &0x7fffffff |
| in the macros below & do nasty casts to longs like the following |
| as gdb unfortunately deals with printed arguments as ints which |
| messes up everything. |
| i.e. here is a 3rd backchain dereference |
| p/x *(long *)(***(long ***)$sp+112) |
| |
| |
| this outputs |
| $5 = 0x528f18 |
| on my machine. |
| Now you can use |
| info symbol (*($sp+56))&0x7fffffff |
| you might see something like. |
| rl_getc + 36 in section .text telling you what is located at address 0x528f18 |
| Now do. |
| p/x (*(*$sp+56))&0x7fffffff |
| This outputs |
| $6 = 0x528ed0 |
| Now do. |
| info symbol (*(*$sp+56))&0x7fffffff |
| rl_read_key + 180 in section .text |
| now do |
| p/x (*(**$sp+56))&0x7fffffff |
| & so on. |
| |
| Disassembling instructions without debug info |
| --------------------------------------------- |
| gdb typically complains if there is a lack of debugging |
| symbols in the disassemble command with |
| "No function contains specified address." To get around |
| this do |
| x/<number lines to disassemble>xi <address> |
| e.g. |
| x/20xi 0x400730 |
| |
| |
| |
| Note: Remember gdb has history just like bash you don't need to retype the |
| whole line just use the up & down arrows. |
| |
| |
| |
| For more info |
| ------------- |
| From your linuxbox do |
| man gdb or info gdb. |
| |
| core dumps |
| ---------- |
| What a core dump ?, |
| A core dump is a file generated by the kernel ( if allowed ) which contains the registers, |
| & all active pages of the program which has crashed. |
| From this file gdb will allow you to look at the registers & stack trace & memory of the |
| program as if it just crashed on your system, it is usually called core & created in the |
| current working directory. |
| This is very useful in that a customer can mail a core dump to a technical support department |
| & the technical support department can reconstruct what happened. |
| Provided the have an identical copy of this program with debugging symbols compiled in & |
| the source base of this build is available. |
| In short it is far more useful than something like a crash log could ever hope to be. |
| |
| In theory all that is missing to restart a core dumped program is a kernel patch which |
| will do the following. |
| 1) Make a new kernel task structure |
| 2) Reload all the dumped pages back into the kernel's memory management structures. |
| 3) Do the required clock fixups |
| 4) Get all files & network connections for the process back into an identical state ( really difficult ). |
| 5) A few more difficult things I haven't thought of. |
| |
| |
| |
| Why have I never seen one ?. |
| Probably because you haven't used the command |
| ulimit -c unlimited in bash |
| to allow core dumps, now do |
| ulimit -a |
| to verify that the limit was accepted. |
| |
| A sample core dump |
| To create this I'm going to do |
| ulimit -c unlimited |
| gdb |
| to launch gdb (my victim app. ) now be bad & do the following from another |
| telnet/xterm session to the same machine |
| ps -aux | grep gdb |
| kill -SIGSEGV <gdb's pid> |
| or alternatively use killall -SIGSEGV gdb if you have the killall command. |
| Now look at the core dump. |
| ./gdb ./gdb core |
| Displays the following |
| GNU gdb 4.18 |
| Copyright 1998 Free Software Foundation, Inc. |
| GDB is free software, covered by the GNU General Public License, and you are |
| welcome to change it and/or distribute copies of it under certain conditions. |
| Type "show copying" to see the conditions. |
| There is absolutely no warranty for GDB. Type "show warranty" for details. |
| This GDB was configured as "s390-ibm-linux"... |
| Core was generated by `./gdb'. |
| Program terminated with signal 11, Segmentation fault. |
| Reading symbols from /usr/lib/libncurses.so.4...done. |
| Reading symbols from /lib/libm.so.6...done. |
| Reading symbols from /lib/libc.so.6...done. |
| Reading symbols from /lib/ld-linux.so.2...done. |
| #0 0x40126d1a in read () from /lib/libc.so.6 |
| Setting up the environment for debugging gdb. |
| Breakpoint 1 at 0x4dc6f8: file utils.c, line 471. |
| Breakpoint 2 at 0x4d87a4: file top.c, line 2609. |
| (top-gdb) info stack |
| #0 0x40126d1a in read () from /lib/libc.so.6 |
| #1 0x528f26 in rl_getc (stream=0x7ffffde8) at input.c:402 |
| #2 0x528ed0 in rl_read_key () at input.c:381 |
| #3 0x5167e6 in readline_internal_char () at readline.c:454 |
| #4 0x5168ee in readline_internal_charloop () at readline.c:507 |
| #5 0x51692c in readline_internal () at readline.c:521 |
| #6 0x5164fe in readline (prompt=0x7ffff810 "\177ÿøx\177ÿ÷Ø\177ÿøxÀ") |
| at readline.c:349 |
| #7 0x4d7a8a in command_line_input (prrompt=0x564420 "(gdb) ", repeat=1, |
| annotation_suffix=0x4d6b44 "prompt") at top.c:2091 |
| #8 0x4d6cf0 in command_loop () at top.c:1345 |
| #9 0x4e25bc in main (argc=1, argv=0x7ffffdf4) at main.c:635 |
| |
| |
| LDD |
| === |
| This is a program which lists the shared libraries which a library needs, |
| Note you also get the relocations of the shared library text segments which |
| help when using objdump --source. |
| e.g. |
| ldd ./gdb |
| outputs |
| libncurses.so.4 => /usr/lib/libncurses.so.4 (0x40018000) |
| libm.so.6 => /lib/libm.so.6 (0x4005e000) |
| libc.so.6 => /lib/libc.so.6 (0x40084000) |
| /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000) |
| |
| |
| Debugging shared libraries |
| ========================== |
| Most programs use shared libraries, however it can be very painful |
| when you single step instruction into a function like printf for the |
| first time & you end up in functions like _dl_runtime_resolve this is |
| the ld.so doing lazy binding, lazy binding is a concept in ELF where |
| shared library functions are not loaded into memory unless they are |
| actually used, great for saving memory but a pain to debug. |
| To get around this either relink the program -static or exit gdb type |
| export LD_BIND_NOW=true this will stop lazy binding & restart the gdb'ing |
| the program in question. |
| |
| |
| |
| Debugging modules |
| ================= |
| As modules are dynamically loaded into the kernel their address can be |
| anywhere to get around this use the -m option with insmod to emit a load |
| map which can be piped into a file if required. |
| |
| The proc file system |
| ==================== |
| What is it ?. |
| It is a filesystem created by the kernel with files which are created on demand |
| by the kernel if read, or can be used to modify kernel parameters, |
| it is a powerful concept. |
| |
| e.g. |
| |
| cat /proc/sys/net/ipv4/ip_forward |
| On my machine outputs |
| 0 |
| telling me ip_forwarding is not on to switch it on I can do |
| echo 1 > /proc/sys/net/ipv4/ip_forward |
| cat it again |
| cat /proc/sys/net/ipv4/ip_forward |
| On my machine now outputs |
| 1 |
| IP forwarding is on. |
| There is a lot of useful info in here best found by going in & having a look around, |
| so I'll take you through some entries I consider important. |
| |
| All the processes running on the machine have there own entry defined by |
| /proc/<pid> |
| So lets have a look at the init process |
| cd /proc/1 |
| |
| cat cmdline |
| emits |
| init [2] |
| |
| cd /proc/1/fd |
| This contains numerical entries of all the open files, |
| some of these you can cat e.g. stdout (2) |
| |
| cat /proc/29/maps |
| on my machine emits |
| |
| 00400000-00478000 r-xp 00000000 5f:00 4103 /bin/bash |
| 00478000-0047e000 rw-p 00077000 5f:00 4103 /bin/bash |
| 0047e000-00492000 rwxp 00000000 00:00 0 |
| 40000000-40015000 r-xp 00000000 5f:00 14382 /lib/ld-2.1.2.so |
| 40015000-40016000 rw-p 00014000 5f:00 14382 /lib/ld-2.1.2.so |
| 40016000-40017000 rwxp 00000000 00:00 0 |
| 40017000-40018000 rw-p 00000000 00:00 0 |
| 40018000-4001b000 r-xp 00000000 5f:00 14435 /lib/libtermcap.so.2.0.8 |
| 4001b000-4001c000 rw-p 00002000 5f:00 14435 /lib/libtermcap.so.2.0.8 |
| 4001c000-4010d000 r-xp 00000000 5f:00 14387 /lib/libc-2.1.2.so |
| 4010d000-40111000 rw-p 000f0000 5f:00 14387 /lib/libc-2.1.2.so |
| 40111000-40114000 rw-p 00000000 00:00 0 |
| 40114000-4011e000 r-xp 00000000 5f:00 14408 /lib/libnss_files-2.1.2.so |
| 4011e000-4011f000 rw-p 00009000 5f:00 14408 /lib/libnss_files-2.1.2.so |
| 7fffd000-80000000 rwxp ffffe000 00:00 0 |
| |
| |
| Showing us the shared libraries init uses where they are in memory |
| & memory access permissions for each virtual memory area. |
| |
| /proc/1/cwd is a softlink to the current working directory. |
| /proc/1/root is the root of the filesystem for this process. |
| |
| /proc/1/mem is the current running processes memory which you |
| can read & write to like a file. |
| strace uses this sometimes as it is a bit faster than the |
| rather inefficient ptrace interface for peeking at DATA. |
| |
| |
| cat status |
| |
| Name: init |
| State: S (sleeping) |
| Pid: 1 |
| PPid: 0 |
| Uid: 0 0 0 0 |
| Gid: 0 0 0 0 |
| Groups: |
| VmSize: 408 kB |
| VmLck: 0 kB |
| VmRSS: 208 kB |
| VmData: 24 kB |
| VmStk: 8 kB |
| VmExe: 368 kB |
| VmLib: 0 kB |
| SigPnd: 0000000000000000 |
| SigBlk: 0000000000000000 |
| SigIgn: 7fffffffd7f0d8fc |
| SigCgt: 00000000280b2603 |
| CapInh: 00000000fffffeff |
| CapPrm: 00000000ffffffff |
| CapEff: 00000000fffffeff |
| |
| User PSW: 070de000 80414146 |
| task: 004b6000 tss: 004b62d8 ksp: 004b7ca8 pt_regs: 004b7f68 |
| User GPRS: |
| 00000400 00000000 0000000b 7ffffa90 |
| 00000000 00000000 00000000 0045d9f4 |
| 0045cafc 7ffffa90 7fffff18 0045cb08 |
| 00010400 804039e8 80403af8 7ffff8b0 |
| User ACRS: |
| 00000000 00000000 00000000 00000000 |
| 00000001 00000000 00000000 00000000 |
| 00000000 00000000 00000000 00000000 |
| 00000000 00000000 00000000 00000000 |
| Kernel BackChain CallChain BackChain CallChain |
| 004b7ca8 8002bd0c 004b7d18 8002b92c |
| 004b7db8 8005cd50 004b7e38 8005d12a |
| 004b7f08 80019114 |
| Showing among other things memory usage & status of some signals & |
| the processes'es registers from the kernel task_structure |
| as well as a backchain which may be useful if a process crashes |
| in the kernel for some unknown reason. |
| |
| Some driver debugging techniques |
| ================================ |
| debug feature |
| ------------- |
| Some of our drivers now support a "debug feature" in |
| /proc/s390dbf see s390dbf.txt in the linux/Documentation directory |
| for more info. |
| e.g. |
| to switch on the lcs "debug feature" |
| echo 5 > /proc/s390dbf/lcs/level |
| & then after the error occurred. |
| cat /proc/s390dbf/lcs/sprintf >/logfile |
| the logfile now contains some information which may help |
| tech support resolve a problem in the field. |
| |
| |
| |
| high level debugging network drivers |
| ------------------------------------ |
| ifconfig is a quite useful command |
| it gives the current state of network drivers. |
| |
| If you suspect your network device driver is dead |
| one way to check is type |
| ifconfig <network device> |
| e.g. tr0 |
| You should see something like |
| tr0 Link encap:16/4 Mbps Token Ring (New) HWaddr 00:04:AC:20:8E:48 |
| inet addr:9.164.185.132 Bcast:9.164.191.255 Mask:255.255.224.0 |
| UP BROADCAST RUNNING MULTICAST MTU:2000 Metric:1 |
| RX packets:246134 errors:0 dropped:0 overruns:0 frame:0 |
| TX packets:5 errors:0 dropped:0 overruns:0 carrier:0 |
| collisions:0 txqueuelen:100 |
| |
| if the device doesn't say up |
| try |
| /etc/rc.d/init.d/network start |
| ( this starts the network stack & hopefully calls ifconfig tr0 up ). |
| ifconfig looks at the output of /proc/net/dev & presents it in a more presentable form |
| Now ping the device from a machine in the same subnet. |
| if the RX packets count & TX packets counts don't increment you probably |
| have problems. |
| next |
| cat /proc/net/arp |
| Do you see any hardware addresses in the cache if not you may have problems. |
| Next try |
| ping -c 5 <broadcast_addr> i.e. the Bcast field above in the output of |
| ifconfig. Do you see any replies from machines other than the local machine |
| if not you may have problems. also if the TX packets count in ifconfig |
| hasn't incremented either you have serious problems in your driver |
| (e.g. the txbusy field of the network device being stuck on ) |
| or you may have multiple network devices connected. |
| |
| |
| chandev |
| ------- |
| There is a new device layer for channel devices, some |
| drivers e.g. lcs are registered with this layer. |
| If the device uses the channel device layer you'll be |
| able to find what interrupts it uses & the current state |
| of the device. |
| See the manpage chandev.8 &type cat /proc/chandev for more info. |
| |
| |
| |
| Starting points for debugging scripting languages etc. |
| ====================================================== |
| |
| bash/sh |
| |
| bash -x <scriptname> |
| e.g. bash -x /usr/bin/bashbug |
| displays the following lines as it executes them. |
| + MACHINE=i586 |
| + OS=linux-gnu |
| + CC=gcc |
| + CFLAGS= -DPROGRAM='bash' -DHOSTTYPE='i586' -DOSTYPE='linux-gnu' -DMACHTYPE='i586-pc-linux-gnu' -DSHELL -DHAVE_CONFIG_H -I. -I. -I./lib -O2 -pipe |
| + RELEASE=2.01 |
| + PATCHLEVEL=1 |
| + RELSTATUS=release |
| + MACHTYPE=i586-pc-linux-gnu |
| |
| perl -d <scriptname> runs the perlscript in a fully interactive debugger |
| <like gdb>. |
| Type 'h' in the debugger for help. |
| |
| for debugging java type |
| jdb <filename> another fully interactive gdb style debugger. |
| & type ? in the debugger for help. |
| |
| |
| |
| Dumptool & Lcrash ( lkcd ) |
| ========================== |
| Michael Holzheu & others here at IBM have a fairly mature port of |
| SGI's lcrash tool which allows one to look at kernel structures in a |
| running kernel. |
| |
| It also complements a tool called dumptool which dumps all the kernel's |
| memory pages & registers to either a tape or a disk. |
| This can be used by tech support or an ambitious end user do |
| post mortem debugging of a machine like gdb core dumps. |
| |
| Going into how to use this tool in detail will be explained |
| in other documentation supplied by IBM with the patches & the |
| lcrash homepage http://oss.sgi.com/projects/lkcd/ & the lcrash manpage. |
| |
| How they work |
| ------------- |
| Lcrash is a perfectly normal program,however, it requires 2 |
| additional files, Kerntypes which is built using a patch to the |
| linux kernel sources in the linux root directory & the System.map. |
| |
| Kerntypes is an an objectfile whose sole purpose in life |
| is to provide stabs debug info to lcrash, to do this |
| Kerntypes is built from kerntypes.c which just includes the most commonly |
| referenced header files used when debugging, lcrash can then read the |
| .stabs section of this file. |
| |
| Debugging a live system it uses /dev/mem |
| alternatively for post mortem debugging it uses the data |
| collected by dumptool. |
| |
| |
| |
| SysRq |
| ===== |
| This is now supported by linux for s/390 & z/Architecture. |
| To enable it do compile the kernel with |
| Kernel Hacking -> Magic SysRq Key Enabled |
| echo "1" > /proc/sys/kernel/sysrq |
| also type |
| echo "8" >/proc/sys/kernel/printk |
| To make printk output go to console. |
| On 390 all commands are prefixed with |
| ^- |
| e.g. |
| ^-t will show tasks. |
| ^-? or some unknown command will display help. |
| The sysrq key reading is very picky ( I have to type the keys in an |
| xterm session & paste them into the x3270 console ) |
| & it may be wise to predefine the keys as described in the VM hints above |
| |
| This is particularly useful for syncing disks unmounting & rebooting |
| if the machine gets partially hung. |
| |
| Read Documentation/sysrq.txt for more info |
| |
| References: |
| =========== |
| Enterprise Systems Architecture Reference Summary |
| Enterprise Systems Architecture Principles of Operation |
| Hartmut Penners s390 stack frame sheet. |
| IBM Mainframe Channel Attachment a technology brief from a CISCO webpage |
| Various bits of man & info pages of Linux. |
| Linux & GDB source. |
| Various info & man pages. |
| CMS Help on tracing commands. |
| Linux for s/390 Elf Application Binary Interface |
| Linux for z/Series Elf Application Binary Interface ( Both Highly Recommended ) |
| z/Architecture Principles of Operation SA22-7832-00 |
| Enterprise Systems Architecture/390 Reference Summary SA22-7209-01 & the |
| Enterprise Systems Architecture/390 Principles of Operation SA22-7201-05 |
| |
| Special Thanks |
| ============== |
| Special thanks to Neale Ferguson who maintains a much |
| prettier HTML version of this page at |
| http://penguinvm.princeton.edu/notes.html#Debug390 |
| Bob Grainger Stefan Bader & others for reporting bugs |