| 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 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 | |
| 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 code you can also debug on some crash dumps | |
| e.g. 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 | |
| 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 conex.c file, you can get | |
| their 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 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). | |
| 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 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 | |
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