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diff --git a/Documentation/admin-guide/bug-hunting.rst b/Documentation/admin-guide/bug-hunting.rst new file mode 100644 index 000000000..95299b08c --- /dev/null +++ b/Documentation/admin-guide/bug-hunting.rst @@ -0,0 +1,378 @@ +Bug hunting +=========== + +Kernel bug reports often come with a stack dump like the one below:: + + ------------[ 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 ]--- + +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 analyzed. + +If the kernel is compiled with ``CONFIG_DEBUG_INFO``, you can enhance the +quality of the stack trace by using file:`scripts/decode_stacktrace.sh`. + +Modules linked in +----------------- + +Modules that are tainted or are being loaded or unloaded are marked with +"(...)", where the taint flags are described in +file:`Documentation/admin-guide/tainted-kernels.rst`, "being loaded" is +annotated with "+", and "being unloaded" is annotated with "-". + + +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, +since ``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 (e.g., ``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/admin-guide/kdump/kdump.rst), + extract the kernel ring buffer from old memory with using dmesg + gdbmacro in Documentation/admin-guide/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 debugger (``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 is 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 +will see the assembler code for the routine shown, but if your kernel has +debug symbols the C code will also be available. (Debug symbols can be enabled +in the kernel hacking menu of the menu configuration.) For example:: + + $ objdump -r -S -l --disassemble net/dccp/ipv4.o + +.. note:: + + You need to be at the top level of the kernel tree for this to pick up + your C files. + +If you don't have access to the source code you can still debug some crash +dumps using the following method (example crash dump output as shown by +Dave Miller):: + + EIP is at +0x14/0x4c0 + ... + Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00 + 00 00 55 57 56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08 + <8b> 83 3c 01 00 00 89 44 24 14 8b 45 28 85 c0 89 44 24 18 0f 85 + + Put the bytes into a "foo.s" file like this: + + .text + .globl foo + foo: + .byte .... /* bytes from Code: part of OOPS dump */ + + Compile it with "gcc -c -o foo.o foo.s" then look at the output of + "objdump --disassemble foo.o". + + Output: + + ip_queue_xmit: + push %ebp + push %edi + push %esi + push %ebx + sub $0xbc, %esp + mov 0xd0(%esp), %ebp ! %ebp = arg0 (skb) + mov 0x8(%ebp), %ebx ! %ebx = skb->sk + mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt + +file:`scripts/decodecode` can be used to automate most of this, depending +on what CPU architecture is being debugged. + +Reporting the bug +----------------- + +Once you find where the bug happened, by inspecting its location, +you could either try to fix it yourself or report it upstream. + +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. + +For example, if you find a bug at the gspca's sonixj.c file, you can get +its maintainers with:: + + $ ./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) + +Please notice that it will point to: + +- The last developers that touched the source code (if this is done inside + a git tree). On the above example, Tejun and Bhaktipriya (in this + specific case, none really involved 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). + +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 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. + +Thanks for your help in making Linux as stable as humanly possible. + +Fixing the bug +-------------- + +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? + +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 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 |