admin-guide: merge oops-tracing with bug-hunting
Now that oops-tracing.rst has only information about
stack dumps found on OOPS, and bug-hunting.rst has only
information about how to identify the source code line
associated with a stack dump, let's merge them and
improve the information inside it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/admin-guide/bug-hunting.rst b/Documentation/admin-guide/bug-hunting.rst
index d245d467..08c4b13 100644
--- a/Documentation/admin-guide/bug-hunting.rst
+++ b/Documentation/admin-guide/bug-hunting.rst
@@ -1,16 +1,190 @@
Bug hunting
-+++++++++++
+===========
-Last updated: 28 October 2016
+Kernel bug reports often come with a stack dump like the one below::
-Fixing the bug
-==============
+ ------------[ cut here ]------------
+ WARNING: CPU: 1 PID: 28102 at kernel/module.c:1108 module_put+0x57/0x70
+ Modules linked in: dvb_usb_gp8psk(-) dvb_usb dvb_core nvidia_drm(PO) nvidia_modeset(PO) snd_hda_codec_hdmi snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_timer snd soundcore nvidia(PO) [last unloaded: rc_core]
+ CPU: 1 PID: 28102 Comm: rmmod Tainted: P WC O 4.8.4-build.1 #1
+ Hardware name: MSI MS-7309/MS-7309, BIOS V1.12 02/23/2009
+ 00000000 c12ba080 00000000 00000000 c103ed6a c1616014 00000001 00006dc6
+ c1615862 00000454 c109e8a7 c109e8a7 00000009 ffffffff 00000000 f13f6a10
+ f5f5a600 c103ee33 00000009 00000000 00000000 c109e8a7 f80ca4d0 c109f617
+ Call Trace:
+ [<c12ba080>] ? dump_stack+0x44/0x64
+ [<c103ed6a>] ? __warn+0xfa/0x120
+ [<c109e8a7>] ? module_put+0x57/0x70
+ [<c109e8a7>] ? module_put+0x57/0x70
+ [<c103ee33>] ? warn_slowpath_null+0x23/0x30
+ [<c109e8a7>] ? module_put+0x57/0x70
+ [<f80ca4d0>] ? gp8psk_fe_set_frontend+0x460/0x460 [dvb_usb_gp8psk]
+ [<c109f617>] ? symbol_put_addr+0x27/0x50
+ [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
+ [<f80bb3bf>] ? dvb_usb_exit+0x2f/0xd0 [dvb_usb]
+ [<c13d03bc>] ? usb_disable_endpoint+0x7c/0xb0
+ [<f80bb48a>] ? dvb_usb_device_exit+0x2a/0x50 [dvb_usb]
+ [<c13d2882>] ? usb_unbind_interface+0x62/0x250
+ [<c136b514>] ? __pm_runtime_idle+0x44/0x70
+ [<c13620d8>] ? __device_release_driver+0x78/0x120
+ [<c1362907>] ? driver_detach+0x87/0x90
+ [<c1361c48>] ? bus_remove_driver+0x38/0x90
+ [<c13d1c18>] ? usb_deregister+0x58/0xb0
+ [<c109fbb0>] ? SyS_delete_module+0x130/0x1f0
+ [<c1055654>] ? task_work_run+0x64/0x80
+ [<c1000fa5>] ? exit_to_usermode_loop+0x85/0x90
+ [<c10013f0>] ? do_fast_syscall_32+0x80/0x130
+ [<c1549f43>] ? sysenter_past_esp+0x40/0x6a
+ ---[ end trace 6ebc60ef3981792f ]---
-Nobody is going to tell you how to fix bugs. Seriously. You need to work it
-out. But below are some hints on how to use the tools.
+Such stack traces provide enough information to identify the line inside the
+Kernel's source code where the bug happened. Depending on the severity of
+the issue, it may also contain the word **Oops**, as on this one::
+
+ BUG: unable to handle kernel NULL pointer dereference at (null)
+ IP: [<c06969d4>] iret_exc+0x7d0/0xa59
+ *pdpt = 000000002258a001 *pde = 0000000000000000
+ Oops: 0002 [#1] PREEMPT SMP
+ ...
+
+Despite being an **Oops** or some other sort of stack trace, the offended
+line is usually required to identify and handle the bug. Along this chapter,
+we'll refer to "Oops" for all kinds of stack traces that need to be analized.
+
+.. note::
+
+ ``ksymoops`` is useless on 2.6 or upper. Please use the Oops in its original
+ format (from ``dmesg``, etc). Ignore any references in this or other docs to
+ "decoding the Oops" or "running it through ksymoops".
+ If you post an Oops from 2.6+ that has been run through ``ksymoops``,
+ people will just tell you to repost it.
+
+Where is the Oops message is located?
+-------------------------------------
+
+Normally the Oops text is read from the kernel buffers by klogd and
+handed to ``syslogd`` which writes it to a syslog file, typically
+``/var/log/messages`` (depends on ``/etc/syslog.conf``). On systems with
+systemd, it may also be stored by the ``journald`` daemon, and accessed
+by running ``journalctl`` command.
+
+Sometimes ``klogd`` dies, in which case you can run ``dmesg > file`` to
+read the data from the kernel buffers and save it. Or you can
+``cat /proc/kmsg > file``, however you have to break in to stop the transfer,
+``kmsg`` is a "never ending file".
+
+If the machine has crashed so badly that you cannot enter commands or
+the disk is not available then you have three options:
+
+(1) Hand copy the text from the screen and type it in after the machine
+ has restarted. Messy but it is the only option if you have not
+ planned for a crash. Alternatively, you can take a picture of
+ the screen with a digital camera - not nice, but better than
+ nothing. If the messages scroll off the top of the console, you
+ may find that booting with a higher resolution (eg, ``vga=791``)
+ will allow you to read more of the text. (Caveat: This needs ``vesafb``,
+ so won't help for 'early' oopses)
+
+(2) Boot with a serial console (see
+ :ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
+ run a null modem to a second machine and capture the output there
+ using your favourite communication program. Minicom works well.
+
+(3) Use Kdump (see Documentation/kdump/kdump.txt),
+ extract the kernel ring buffer from old memory with using dmesg
+ gdbmacro in Documentation/kdump/gdbmacros.txt.
+
+Finding the bug's location
+--------------------------
+
+Reporting a bug works best if you point the location of the bug at the
+Kernel source file. There are two methods for doing that. Usually, using
+``gdb`` is easier, but the Kernel should be pre-compiled with debug info.
+
+gdb
+^^^
+
+The GNU debug (``gdb``) is the best way to figure out the exact file and line
+number of the OOPS from the ``vmlinux`` file.
+
+The usage of gdb works best on a kernel compiled with ``CONFIG_DEBUG_INFO``.
+This can be set by running::
+
+ $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
+
+On a kernel compiled with ``CONFIG_DEBUG_INFO``, you can simply copy the
+EIP value from the OOPS::
+
+ EIP: 0060:[<c021e50e>] Not tainted VLI
+
+And use GDB to translate that to human-readable form::
+
+ $ gdb vmlinux
+ (gdb) l *0xc021e50e
+
+If you don't have ``CONFIG_DEBUG_INFO`` enabled, you use the function
+offset from the OOPS::
+
+ EIP is at vt_ioctl+0xda8/0x1482
+
+And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled::
+
+ $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
+ $ make vmlinux
+ $ gdb vmlinux
+ (gdb) l *vt_ioctl+0xda8
+ 0x1888 is in vt_ioctl (drivers/tty/vt/vt_ioctl.c:293).
+ 288 {
+ 289 struct vc_data *vc = NULL;
+ 290 int ret = 0;
+ 291
+ 292 console_lock();
+ 293 if (VT_BUSY(vc_num))
+ 294 ret = -EBUSY;
+ 295 else if (vc_num)
+ 296 vc = vc_deallocate(vc_num);
+ 297 console_unlock();
+
+or, if you want to be more verbose::
+
+ (gdb) p vt_ioctl
+ $1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl>
+ (gdb) l *0xae0+0xda8
+
+You could, instead, use the object file::
+
+ $ make drivers/tty/
+ $ gdb drivers/tty/vt/vt_ioctl.o
+ (gdb) l *vt_ioctl+0xda8
+
+If you have a call trace, such as::
+
+ Call Trace:
+ [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
+ [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
+ [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
+ ...
+
+this shows the problem likely in the :jbd: module. You can load that module
+in gdb and list the relevant code::
+
+ $ gdb fs/jbd/jbd.ko
+ (gdb) l *log_wait_commit+0xa3
+
+.. note::
+
+ You can also do the same for any function call at the stack trace,
+ like this one::
+
+ [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
+
+ The position where the above call happened can be seen with::
+
+ $ gdb drivers/media/usb/dvb-usb/dvb-usb.o
+ (gdb) l *dvb_usb_adapter_frontend_exit+0x3a
objdump
--------
+^^^^^^^
To debug a kernel, use objdump and look for the hex offset from the crash
output to find the valid line of code/assembler. Without debug symbols, you
@@ -56,86 +230,140 @@
mov 0x8(%ebp), %ebx ! %ebx = skb->sk
mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
-gdb
----
+Reporting the bug
+-----------------
-In addition, you can use GDB to figure out the exact file and line
-number of the OOPS from the ``vmlinux`` file.
+Once you find where the bug happened, by inspecting its location,
+you could either try to fix it yourself or report it upstream.
-The usage of gdb requires a kernel compiled with ``CONFIG_DEBUG_INFO``.
-This can be set by running::
+In order to report it upstream, you should identify the mailing list
+used for the development of the affected code. This can be done by using
+the ``get_maintainer.pl`` script.
- $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
+For example, if you find a bug at the gspca's conex.c file, you can get
+their maintainers with::
-On a kernel compiled with ``CONFIG_DEBUG_INFO``, you can simply copy the
-EIP value from the OOPS::
+ $ ./scripts/get_maintainer.pl -f drivers/media/usb/gspca/sonixj.c
+ Hans Verkuil <hverkuil@xs4all.nl> (odd fixer:GSPCA USB WEBCAM DRIVER,commit_signer:1/1=100%)
+ Mauro Carvalho Chehab <mchehab@kernel.org> (maintainer:MEDIA INPUT INFRASTRUCTURE (V4L/DVB),commit_signer:1/1=100%)
+ Tejun Heo <tj@kernel.org> (commit_signer:1/1=100%)
+ Bhaktipriya Shridhar <bhaktipriya96@gmail.com> (commit_signer:1/1=100%,authored:1/1=100%,added_lines:4/4=100%,removed_lines:9/9=100%)
+ linux-media@vger.kernel.org (open list:GSPCA USB WEBCAM DRIVER)
+ linux-kernel@vger.kernel.org (open list)
- EIP: 0060:[<c021e50e>] Not tainted VLI
+Please notice that it will point to:
-And use GDB to translate that to human-readable form::
+- The last developers that touched on the source code. On the above example,
+ Tejun and Bhaktipriya (in this specific case, none really envolved on the
+ development of this file);
+- The driver maintainer (Hans Verkuil);
+- The subsystem maintainer (Mauro Carvalho Chehab)
+- The driver and/or subsystem mailing list (linux-media@vger.kernel.org);
+- the Linux Kernel mailing list (linux-kernel@vger.kernel.org).
- $ gdb vmlinux
- (gdb) l *0xc021e50e
+Usually, the fastest way to have your bug fixed is to report it to mailing
+list used for the development of the code (linux-media ML) copying the driver maintainer (Hans).
-If you don't have ``CONFIG_DEBUG_INFO`` enabled, you use the function
-offset from the OOPS::
+If you are totally stumped as to whom to send the report, and
+``get_maintainer.pl`` didn't provide you anything useful, send it to
+linux-kernel@vger.kernel.org.
- EIP is at vt_ioctl+0xda8/0x1482
+Thanks for your help in making Linux as stable as humanly possible.
-And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled::
+Fixing the bug
+--------------
- $ make vmlinux
- $ gdb vmlinux
- (gdb) l *vt_ioctl+0xda8
- 0x1888 is in vt_ioctl (drivers/tty/vt/vt_ioctl.c:293).
- 288 {
- 289 struct vc_data *vc = NULL;
- 290 int ret = 0;
- 291
- 292 console_lock();
- 293 if (VT_BUSY(vc_num))
- 294 ret = -EBUSY;
- 295 else if (vc_num)
- 296 vc = vc_deallocate(vc_num);
- 297 console_unlock();
+If you know programming, you could help us by not only reporting the bug,
+but also providing us with a solution. After all open source is about
+sharing what you do and don't you want to be recognised for your genius?
-or, if you want to be more verbose::
-
- (gdb) p vt_ioctl
- $1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl>
- (gdb) l *0xae0+0xda8
-
-You could, instead, use the object file::
-
- $ make drivers/tty/
- $ gdb drivers/tty/vt/vt_ioctl.o
- (gdb) l *vt_ioctl+0xda8
-
-If you have a call trace, such as::
-
- Call Trace:
- [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
- [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
- [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
- ...
-
-this shows the problem likely in the :jbd: module. You can load that module
-in gdb and list the relevant code::
-
- $ gdb fs/jbd/jbd.ko
- (gdb) l *log_wait_commit+0xa3
-
-Another very useful option of the Kernel Hacking section in menuconfig is
-Debug memory allocations. This will help you see whether data has been
-initialised and not set before use etc. To see the values that get assigned
-with this look at ``mm/slab.c`` and search for ``POISON_INUSE``. When using
-this an Oops will often show the poisoned data instead of zero which is the
-default.
-
-Once you have worked out a fix please submit it upstream. After all open
-source is about sharing what you do and don't you want to be recognised for
-your genius?
+If you decide to take this way, once you have worked out a fix please submit
+it upstream.
Please do read
ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though
to help your code get accepted.
+
+
+---------------------------------------------------------------------------
+
+Notes on Oops tracing with ``klogd``
+------------------------------------
+
+In order to help Linus and the other kernel developers there has been
+substantial support incorporated into ``klogd`` for processing protection
+faults. In order to have full support for address resolution at least
+version 1.3-pl3 of the ``sysklogd`` package should be used.
+
+When a protection fault occurs the ``klogd`` daemon automatically
+translates important addresses in the kernel log messages to their
+symbolic equivalents. This translated kernel message is then
+forwarded through whatever reporting mechanism ``klogd`` is using. The
+protection fault message can be simply cut out of the message files
+and forwarded to the kernel developers.
+
+Two types of address resolution are performed by ``klogd``. The first is
+static translation and the second is dynamic translation. Static
+translation uses the System.map file in much the same manner that
+ksymoops does. In order to do static translation the ``klogd`` daemon
+must be able to find a system map file at daemon initialization time.
+See the klogd man page for information on how ``klogd`` searches for map
+files.
+
+Dynamic address translation is important when kernel loadable modules
+are being used. Since memory for kernel modules is allocated from the
+kernel's dynamic memory pools there are no fixed locations for either
+the start of the module or for functions and symbols in the module.
+
+The kernel supports system calls which allow a program to determine
+which modules are loaded and their location in memory. Using these
+system calls the klogd daemon builds a symbol table which can be used
+to debug a protection fault which occurs in a loadable kernel module.
+
+At the very minimum klogd will provide the name of the module which
+generated the protection fault. There may be additional symbolic
+information available if the developer of the loadable module chose to
+export symbol information from the module.
+
+Since the kernel module environment can be dynamic there must be a
+mechanism for notifying the ``klogd`` daemon when a change in module
+environment occurs. There are command line options available which
+allow klogd to signal the currently executing daemon that symbol
+information should be refreshed. See the ``klogd`` manual page for more
+information.
+
+A patch is included with the sysklogd distribution which modifies the
+``modules-2.0.0`` package to automatically signal klogd whenever a module
+is loaded or unloaded. Applying this patch provides essentially
+seamless support for debugging protection faults which occur with
+kernel loadable modules.
+
+The following is an example of a protection fault in a loadable module
+processed by ``klogd``::
+
+ Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
+ Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
+ Aug 29 09:51:01 blizard kernel: *pde = 00000000
+ Aug 29 09:51:01 blizard kernel: Oops: 0002
+ Aug 29 09:51:01 blizard kernel: CPU: 0
+ Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
+ Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
+ Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
+ Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
+ Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
+ Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
+ Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
+ Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
+ Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
+ Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
+ Aug 29 09:51:01 blizard kernel: Code: c7 00 05 00 00 00 eb 08 90 90 90 90 90 90 90 90 89 ec 5d c3
+
+---------------------------------------------------------------------------
+
+::
+
+ Dr. G.W. Wettstein Oncology Research Div. Computing Facility
+ Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com
+ 820 4th St. N.
+ Fargo, ND 58122
+ Phone: 701-234-7556
