Compile-time stack metadata validation
======================================


Overview
--------

The kernel CONFIG_STACK_VALIDATION option enables a host tool named
objtool which runs at compile time.  It has a "check" subcommand which
analyzes every .o file and ensures the validity of its stack metadata.
It enforces a set of rules on asm code and C inline assembly code so
that stack traces can be reliable.

Currently it only checks frame pointer usage, but there are plans to add
CFI validation for C files and CFI generation for asm files.

For each function, it recursively follows all possible code paths and
validates the correct frame pointer state at each instruction.

It also follows code paths involving special sections, like
.altinstructions, __jump_table, and __ex_table, which can add
alternative execution paths to a given instruction (or set of
instructions).  Similarly, it knows how to follow switch statements, for
which gcc sometimes uses jump tables.


Why do we need stack metadata validation?
-----------------------------------------

Here are some of the benefits of validating stack metadata:

a) More reliable stack traces for frame pointer enabled kernels

   Frame pointers are used for debugging purposes.  They allow runtime
   code and debug tools to be able to walk the stack to determine the
   chain of function call sites that led to the currently executing
   code.

   For some architectures, frame pointers are enabled by
   CONFIG_FRAME_POINTER.  For some other architectures they may be
   required by the ABI (sometimes referred to as "backchain pointers").

   For C code, gcc automatically generates instructions for setting up
   frame pointers when the -fno-omit-frame-pointer option is used.

   But for asm code, the frame setup instructions have to be written by
   hand, which most people don't do.  So the end result is that
   CONFIG_FRAME_POINTER is honored for C code but not for most asm code.

   For stack traces based on frame pointers to be reliable, all
   functions which call other functions must first create a stack frame
   and update the frame pointer.  If a first function doesn't properly
   create a stack frame before calling a second function, the *caller*
   of the first function will be skipped on the stack trace.

   For example, consider the following example backtrace with frame
   pointers enabled:

     [<ffffffff81812584>] dump_stack+0x4b/0x63
     [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
     [<ffffffff8127f568>] seq_read+0x108/0x3e0
     [<ffffffff812cce62>] proc_reg_read+0x42/0x70
     [<ffffffff81256197>] __vfs_read+0x37/0x100
     [<ffffffff81256b16>] vfs_read+0x86/0x130
     [<ffffffff81257898>] SyS_read+0x58/0xd0
     [<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76

   It correctly shows that the caller of cmdline_proc_show() is
   seq_read().

   If we remove the frame pointer logic from cmdline_proc_show() by
   replacing the frame pointer related instructions with nops, here's
   what it looks like instead:

     [<ffffffff81812584>] dump_stack+0x4b/0x63
     [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
     [<ffffffff812cce62>] proc_reg_read+0x42/0x70
     [<ffffffff81256197>] __vfs_read+0x37/0x100
     [<ffffffff81256b16>] vfs_read+0x86/0x130
     [<ffffffff81257898>] SyS_read+0x58/0xd0
     [<ffffffff8181c1f2>] entry_SYSCALL_64_fastpath+0x12/0x76

   Notice that cmdline_proc_show()'s caller, seq_read(), has been
   skipped.  Instead the stack trace seems to show that
   cmdline_proc_show() was called by proc_reg_read().

   The benefit of objtool here is that because it ensures that *all*
   functions honor CONFIG_FRAME_POINTER, no functions will ever[*] be
   skipped on a stack trace.

   [*] unless an interrupt or exception has occurred at the very
       beginning of a function before the stack frame has been created,
       or at the very end of the function after the stack frame has been
       destroyed.  This is an inherent limitation of frame pointers.

b) 100% reliable stack traces for DWARF enabled kernels

   (NOTE: This is not yet implemented)

   As an alternative to frame pointers, DWARF Call Frame Information
   (CFI) metadata can be used to walk the stack.  Unlike frame pointers,
   CFI metadata is out of band.  So it doesn't affect runtime
   performance and it can be reliable even when interrupts or exceptions
   are involved.

   For C code, gcc automatically generates DWARF CFI metadata.  But for
   asm code, generating CFI is a tedious manual approach which requires
   manually placed .cfi assembler macros to be scattered throughout the
   code.  It's clumsy and very easy to get wrong, and it makes the real
   code harder to read.

   Stacktool will improve this situation in several ways.  For code
   which already has CFI annotations, it will validate them.  For code
   which doesn't have CFI annotations, it will generate them.  So an
   architecture can opt to strip out all the manual .cfi annotations
   from their asm code and have objtool generate them instead.

   We might also add a runtime stack validation debug option where we
   periodically walk the stack from schedule() and/or an NMI to ensure
   that the stack metadata is sane and that we reach the bottom of the
   stack.

   So the benefit of objtool here will be that external tooling should
   always show perfect stack traces.  And the same will be true for
   kernel warning/oops traces if the architecture has a runtime DWARF
   unwinder.

c) Higher live patching compatibility rate

   Livepatch has an optional "consistency model", which is needed for
   more complex patches.  In order for the consistency model to work,
   stack traces need to be reliable (or an unreliable condition needs to
   be detectable).  Objtool makes that possible.

   For more details, see the livepatch documentation in the Linux kernel
   source tree at Documentation/livepatch/livepatch.txt.

Rules
-----

To achieve the validation, objtool enforces the following rules:

1. Each callable function must be annotated as such with the ELF
   function type.  In asm code, this is typically done using the
   ENTRY/ENDPROC macros.  If objtool finds a return instruction
   outside of a function, it flags an error since that usually indicates
   callable code which should be annotated accordingly.

   This rule is needed so that objtool can properly identify each
   callable function in order to analyze its stack metadata.

2. Conversely, each section of code which is *not* callable should *not*
   be annotated as an ELF function.  The ENDPROC macro shouldn't be used
   in this case.

   This rule is needed so that objtool can ignore non-callable code.
   Such code doesn't have to follow any of the other rules.

3. Each callable function which calls another function must have the
   correct frame pointer logic, if required by CONFIG_FRAME_POINTER or
   the architecture's back chain rules.  This can by done in asm code
   with the FRAME_BEGIN/FRAME_END macros.

   This rule ensures that frame pointer based stack traces will work as
   designed.  If function A doesn't create a stack frame before calling
   function B, the _caller_ of function A will be skipped on the stack
   trace.

4. Dynamic jumps and jumps to undefined symbols are only allowed if:

   a) the jump is part of a switch statement; or

   b) the jump matches sibling call semantics and the frame pointer has
      the same value it had on function entry.

   This rule is needed so that objtool can reliably analyze all of a
   function's code paths.  If a function jumps to code in another file,
   and it's not a sibling call, objtool has no way to follow the jump
   because it only analyzes a single file at a time.

5. A callable function may not execute kernel entry/exit instructions.
   The only code which needs such instructions is kernel entry code,
   which shouldn't be be in callable functions anyway.

   This rule is just a sanity check to ensure that callable functions
   return normally.


Objtool warnings
----------------

For asm files, if you're getting an error which doesn't make sense,
first make sure that the affected code follows the above rules.

For C files, the common culprits are inline asm statements and calls to
"noreturn" functions.  See below for more details.

Another possible cause for errors in C code is if the Makefile removes
-fno-omit-frame-pointer or adds -fomit-frame-pointer to the gcc options.

Here are some examples of common warnings reported by objtool, what
they mean, and suggestions for how to fix them.


1. file.o: warning: objtool: func()+0x128: call without frame pointer save/setup

   The func() function made a function call without first saving and/or
   updating the frame pointer, and CONFIG_FRAME_POINTER is enabled.

   If the error is for an asm file, and func() is indeed a callable
   function, add proper frame pointer logic using the FRAME_BEGIN and
   FRAME_END macros.  Otherwise, if it's not a callable function, remove
   its ELF function annotation by changing ENDPROC to END, and instead
   use the manual CFI hint macros in asm/undwarf.h.

   If it's a GCC-compiled .c file, the error may be because the function
   uses an inline asm() statement which has a "call" instruction.  An
   asm() statement with a call instruction must declare the use of the
   stack pointer in its output operand.  For example, on x86_64:

     register void *__sp asm("rsp");
     asm volatile("call func" : "+r" (__sp));

   Otherwise the stack frame may not get created before the call.


2. file.o: warning: objtool: .text+0x53: unreachable instruction

   Objtool couldn't find a code path to reach the instruction.

   If the error is for an asm file, and the instruction is inside (or
   reachable from) a callable function, the function should be annotated
   with the ENTRY/ENDPROC macros (ENDPROC is the important one).
   Otherwise, the code should probably be annotated with the CFI hint
   macros in asm/undwarf.h so objtool and the unwinder can know the
   stack state associated with the code.

   If you're 100% sure the code won't affect stack traces, or if you're
   a just a bad person, you can tell objtool to ignore it.  See the
   "Adding exceptions" section below.

   If it's not actually in a callable function (e.g. kernel entry code),
   change ENDPROC to END.


4. file.o: warning: objtool: func(): can't find starting instruction
   or
   file.o: warning: objtool: func()+0x11dd: can't decode instruction

   Does the file have data in a text section?  If so, that can confuse
   objtool's instruction decoder.  Move the data to a more appropriate
   section like .data or .rodata.


5. file.o: warning: objtool: func()+0x6: unsupported instruction in callable function

   This is a kernel entry/exit instruction like sysenter or iret.  Such
   instructions aren't allowed in a callable function, and are most
   likely part of the kernel entry code.  They should usually not have
   the callable function annotation (ENDPROC) and should always be
   annotated with the CFI hint macros in asm/undwarf.h.


6. file.o: warning: objtool: func()+0x26: sibling call from callable instruction with modified stack frame

   This is a dynamic jump or a jump to an undefined symbol.  Objtool
   assumed it's a sibling call and detected that the frame pointer
   wasn't first restored to its original state.

   If it's not really a sibling call, you may need to move the
   destination code to the local file.

   If the instruction is not actually in a callable function (e.g.
   kernel entry code), change ENDPROC to END and annotate manually with
   the CFI hint macros in asm/undwarf.h.


7. file: warning: objtool: func()+0x5c: stack state mismatch

   The instruction's frame pointer state is inconsistent, depending on
   which execution path was taken to reach the instruction.

   Make sure that, when CONFIG_FRAME_POINTER is enabled, the function
   pushes and sets up the frame pointer (for x86_64, this means rbp) at
   the beginning of the function and pops it at the end of the function.
   Also make sure that no other code in the function touches the frame
   pointer.

   Another possibility is that the code has some asm or inline asm which
   does some unusual things to the stack or the frame pointer.  In such
   cases it's probably appropriate to use the CFI hint macros in
   asm/undwarf.h.


8. file.o: warning: objtool: funcA() falls through to next function funcB()

   This means that funcA() doesn't end with a return instruction or an
   unconditional jump, and that objtool has determined that the function
   can fall through into the next function.  There could be different
   reasons for this:

   1) funcA()'s last instruction is a call to a "noreturn" function like
      panic().  In this case the noreturn function needs to be added to
      objtool's hard-coded global_noreturns array.  Feel free to bug the
      objtool maintainer, or you can submit a patch.

   2) funcA() uses the unreachable() annotation in a section of code
      that is actually reachable.

   3) If funcA() calls an inline function, the object code for funcA()
      might be corrupt due to a gcc bug.  For more details, see:
      https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646


If the error doesn't seem to make sense, it could be a bug in objtool.
Feel free to ask the objtool maintainer for help.


Adding exceptions
-----------------

If you _really_ need objtool to ignore something, and are 100% sure
that it won't affect kernel stack traces, you can tell objtool to
ignore it:

- To skip validation of a function, use the STACK_FRAME_NON_STANDARD
  macro.

- To skip validation of a file, add

    OBJECT_FILES_NON_STANDARD_filename.o := n

  to the Makefile.

- To skip validation of a directory, add

    OBJECT_FILES_NON_STANDARD := y

  to the Makefile.