370 lines
15 KiB
ReStructuredText
370 lines
15 KiB
ReStructuredText
Bug hunting
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===========
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Kernel bug reports often come with a stack dump like the one below::
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------------[ cut here ]------------
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WARNING: CPU: 1 PID: 28102 at kernel/module.c:1108 module_put+0x57/0x70
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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]
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CPU: 1 PID: 28102 Comm: rmmod Tainted: P WC O 4.8.4-build.1 #1
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Hardware name: MSI MS-7309/MS-7309, BIOS V1.12 02/23/2009
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00000000 c12ba080 00000000 00000000 c103ed6a c1616014 00000001 00006dc6
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c1615862 00000454 c109e8a7 c109e8a7 00000009 ffffffff 00000000 f13f6a10
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f5f5a600 c103ee33 00000009 00000000 00000000 c109e8a7 f80ca4d0 c109f617
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Call Trace:
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[<c12ba080>] ? dump_stack+0x44/0x64
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[<c103ed6a>] ? __warn+0xfa/0x120
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[<c109e8a7>] ? module_put+0x57/0x70
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[<c109e8a7>] ? module_put+0x57/0x70
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[<c103ee33>] ? warn_slowpath_null+0x23/0x30
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[<c109e8a7>] ? module_put+0x57/0x70
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[<f80ca4d0>] ? gp8psk_fe_set_frontend+0x460/0x460 [dvb_usb_gp8psk]
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[<c109f617>] ? symbol_put_addr+0x27/0x50
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[<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
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[<f80bb3bf>] ? dvb_usb_exit+0x2f/0xd0 [dvb_usb]
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[<c13d03bc>] ? usb_disable_endpoint+0x7c/0xb0
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[<f80bb48a>] ? dvb_usb_device_exit+0x2a/0x50 [dvb_usb]
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[<c13d2882>] ? usb_unbind_interface+0x62/0x250
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[<c136b514>] ? __pm_runtime_idle+0x44/0x70
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[<c13620d8>] ? __device_release_driver+0x78/0x120
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[<c1362907>] ? driver_detach+0x87/0x90
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[<c1361c48>] ? bus_remove_driver+0x38/0x90
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[<c13d1c18>] ? usb_deregister+0x58/0xb0
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[<c109fbb0>] ? SyS_delete_module+0x130/0x1f0
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[<c1055654>] ? task_work_run+0x64/0x80
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[<c1000fa5>] ? exit_to_usermode_loop+0x85/0x90
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[<c10013f0>] ? do_fast_syscall_32+0x80/0x130
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[<c1549f43>] ? sysenter_past_esp+0x40/0x6a
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---[ end trace 6ebc60ef3981792f ]---
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Such stack traces provide enough information to identify the line inside the
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Kernel's source code where the bug happened. Depending on the severity of
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the issue, it may also contain the word **Oops**, as on this one::
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BUG: unable to handle kernel NULL pointer dereference at (null)
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IP: [<c06969d4>] iret_exc+0x7d0/0xa59
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*pdpt = 000000002258a001 *pde = 0000000000000000
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Oops: 0002 [#1] PREEMPT SMP
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...
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Despite being an **Oops** or some other sort of stack trace, the offended
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line is usually required to identify and handle the bug. Along this chapter,
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we'll refer to "Oops" for all kinds of stack traces that need to be analized.
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.. note::
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``ksymoops`` is useless on 2.6 or upper. Please use the Oops in its original
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format (from ``dmesg``, etc). Ignore any references in this or other docs to
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"decoding the Oops" or "running it through ksymoops".
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If you post an Oops from 2.6+ that has been run through ``ksymoops``,
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people will just tell you to repost it.
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Where is the Oops message is located?
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-------------------------------------
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Normally the Oops text is read from the kernel buffers by klogd and
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handed to ``syslogd`` which writes it to a syslog file, typically
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``/var/log/messages`` (depends on ``/etc/syslog.conf``). On systems with
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systemd, it may also be stored by the ``journald`` daemon, and accessed
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by running ``journalctl`` command.
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Sometimes ``klogd`` dies, in which case you can run ``dmesg > file`` to
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read the data from the kernel buffers and save it. Or you can
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``cat /proc/kmsg > file``, however you have to break in to stop the transfer,
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``kmsg`` is a "never ending file".
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If the machine has crashed so badly that you cannot enter commands or
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the disk is not available then you have three options:
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(1) Hand copy the text from the screen and type it in after the machine
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has restarted. Messy but it is the only option if you have not
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planned for a crash. Alternatively, you can take a picture of
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the screen with a digital camera - not nice, but better than
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nothing. If the messages scroll off the top of the console, you
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may find that booting with a higher resolution (eg, ``vga=791``)
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will allow you to read more of the text. (Caveat: This needs ``vesafb``,
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so won't help for 'early' oopses)
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(2) Boot with a serial console (see
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:ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
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run a null modem to a second machine and capture the output there
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using your favourite communication program. Minicom works well.
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(3) Use Kdump (see Documentation/kdump/kdump.txt),
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extract the kernel ring buffer from old memory with using dmesg
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gdbmacro in Documentation/kdump/gdbmacros.txt.
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Finding the bug's location
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--------------------------
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Reporting a bug works best if you point the location of the bug at the
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Kernel source file. There are two methods for doing that. Usually, using
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``gdb`` is easier, but the Kernel should be pre-compiled with debug info.
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gdb
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^^^
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The GNU debug (``gdb``) is the best way to figure out the exact file and line
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number of the OOPS from the ``vmlinux`` file.
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The usage of gdb works best on a kernel compiled with ``CONFIG_DEBUG_INFO``.
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This can be set by running::
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$ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
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On a kernel compiled with ``CONFIG_DEBUG_INFO``, you can simply copy the
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EIP value from the OOPS::
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EIP: 0060:[<c021e50e>] Not tainted VLI
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And use GDB to translate that to human-readable form::
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$ gdb vmlinux
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(gdb) l *0xc021e50e
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If you don't have ``CONFIG_DEBUG_INFO`` enabled, you use the function
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offset from the OOPS::
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EIP is at vt_ioctl+0xda8/0x1482
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And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled::
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$ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
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$ make vmlinux
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$ gdb vmlinux
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(gdb) l *vt_ioctl+0xda8
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0x1888 is in vt_ioctl (drivers/tty/vt/vt_ioctl.c:293).
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288 {
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289 struct vc_data *vc = NULL;
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290 int ret = 0;
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291
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292 console_lock();
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293 if (VT_BUSY(vc_num))
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294 ret = -EBUSY;
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295 else if (vc_num)
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296 vc = vc_deallocate(vc_num);
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297 console_unlock();
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or, if you want to be more verbose::
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(gdb) p vt_ioctl
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$1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl>
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(gdb) l *0xae0+0xda8
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You could, instead, use the object file::
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$ make drivers/tty/
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$ gdb drivers/tty/vt/vt_ioctl.o
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(gdb) l *vt_ioctl+0xda8
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If you have a call trace, such as::
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Call Trace:
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[<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
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[<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
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[<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
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...
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this shows the problem likely in the :jbd: module. You can load that module
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in gdb and list the relevant code::
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$ gdb fs/jbd/jbd.ko
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(gdb) l *log_wait_commit+0xa3
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.. note::
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You can also do the same for any function call at the stack trace,
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like this one::
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[<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
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The position where the above call happened can be seen with::
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$ gdb drivers/media/usb/dvb-usb/dvb-usb.o
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(gdb) l *dvb_usb_adapter_frontend_exit+0x3a
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objdump
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^^^^^^^
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To debug a kernel, use objdump and look for the hex offset from the crash
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output to find the valid line of code/assembler. Without debug symbols, you
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will see the assembler code for the routine shown, but if your kernel has
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debug symbols the C code will also be available. (Debug symbols can be enabled
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in the kernel hacking menu of the menu configuration.) For example::
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$ objdump -r -S -l --disassemble net/dccp/ipv4.o
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.. note::
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You need to be at the top level of the kernel tree for this to pick up
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your C files.
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If you don't have access to the code you can also debug on some crash dumps
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e.g. crash dump output as shown by Dave Miller::
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EIP is at +0x14/0x4c0
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...
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Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
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00 00 55 57 56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
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<8b> 83 3c 01 00 00 89 44 24 14 8b 45 28 85 c0 89 44 24 18 0f 85
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Put the bytes into a "foo.s" file like this:
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.text
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.globl foo
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foo:
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.byte .... /* bytes from Code: part of OOPS dump */
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Compile it with "gcc -c -o foo.o foo.s" then look at the output of
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"objdump --disassemble foo.o".
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Output:
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ip_queue_xmit:
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push %ebp
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push %edi
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push %esi
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push %ebx
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sub $0xbc, %esp
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mov 0xd0(%esp), %ebp ! %ebp = arg0 (skb)
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mov 0x8(%ebp), %ebx ! %ebx = skb->sk
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mov 0x13c(%ebx), %eax ! %eax = inet_sk(sk)->opt
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Reporting the bug
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-----------------
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Once you find where the bug happened, by inspecting its location,
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you could either try to fix it yourself or report it upstream.
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In order to report it upstream, you should identify the mailing list
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used for the development of the affected code. This can be done by using
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the ``get_maintainer.pl`` script.
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For example, if you find a bug at the gspca's sonixj.c file, you can get
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their maintainers with::
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$ ./scripts/get_maintainer.pl -f drivers/media/usb/gspca/sonixj.c
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Hans Verkuil <hverkuil@xs4all.nl> (odd fixer:GSPCA USB WEBCAM DRIVER,commit_signer:1/1=100%)
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Mauro Carvalho Chehab <mchehab@kernel.org> (maintainer:MEDIA INPUT INFRASTRUCTURE (V4L/DVB),commit_signer:1/1=100%)
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Tejun Heo <tj@kernel.org> (commit_signer:1/1=100%)
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Bhaktipriya Shridhar <bhaktipriya96@gmail.com> (commit_signer:1/1=100%,authored:1/1=100%,added_lines:4/4=100%,removed_lines:9/9=100%)
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linux-media@vger.kernel.org (open list:GSPCA USB WEBCAM DRIVER)
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linux-kernel@vger.kernel.org (open list)
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Please notice that it will point to:
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- The last developers that touched on the source code. On the above example,
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Tejun and Bhaktipriya (in this specific case, none really envolved on the
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development of this file);
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- The driver maintainer (Hans Verkuil);
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- The subsystem maintainer (Mauro Carvalho Chehab);
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- The driver and/or subsystem mailing list (linux-media@vger.kernel.org);
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- the Linux Kernel mailing list (linux-kernel@vger.kernel.org).
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Usually, the fastest way to have your bug fixed is to report it to mailing
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list used for the development of the code (linux-media ML) copying the driver maintainer (Hans).
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If you are totally stumped as to whom to send the report, and
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``get_maintainer.pl`` didn't provide you anything useful, send it to
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linux-kernel@vger.kernel.org.
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Thanks for your help in making Linux as stable as humanly possible.
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Fixing the bug
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--------------
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If you know programming, you could help us by not only reporting the bug,
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but also providing us with a solution. After all, open source is about
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sharing what you do and don't you want to be recognised for your genius?
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If you decide to take this way, once you have worked out a fix please submit
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it upstream.
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Please do read
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:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though
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to help your code get accepted.
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---------------------------------------------------------------------------
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Notes on Oops tracing with ``klogd``
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------------------------------------
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In order to help Linus and the other kernel developers there has been
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substantial support incorporated into ``klogd`` for processing protection
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faults. In order to have full support for address resolution at least
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version 1.3-pl3 of the ``sysklogd`` package should be used.
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When a protection fault occurs the ``klogd`` daemon automatically
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translates important addresses in the kernel log messages to their
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symbolic equivalents. This translated kernel message is then
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forwarded through whatever reporting mechanism ``klogd`` is using. The
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protection fault message can be simply cut out of the message files
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and forwarded to the kernel developers.
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Two types of address resolution are performed by ``klogd``. The first is
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static translation and the second is dynamic translation. Static
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translation uses the System.map file in much the same manner that
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ksymoops does. In order to do static translation the ``klogd`` daemon
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must be able to find a system map file at daemon initialization time.
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See the klogd man page for information on how ``klogd`` searches for map
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files.
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Dynamic address translation is important when kernel loadable modules
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are being used. Since memory for kernel modules is allocated from the
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kernel's dynamic memory pools there are no fixed locations for either
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the start of the module or for functions and symbols in the module.
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The kernel supports system calls which allow a program to determine
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which modules are loaded and their location in memory. Using these
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system calls the klogd daemon builds a symbol table which can be used
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to debug a protection fault which occurs in a loadable kernel module.
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At the very minimum klogd will provide the name of the module which
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generated the protection fault. There may be additional symbolic
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information available if the developer of the loadable module chose to
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export symbol information from the module.
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Since the kernel module environment can be dynamic there must be a
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mechanism for notifying the ``klogd`` daemon when a change in module
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environment occurs. There are command line options available which
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allow klogd to signal the currently executing daemon that symbol
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information should be refreshed. See the ``klogd`` manual page for more
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information.
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A patch is included with the sysklogd distribution which modifies the
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``modules-2.0.0`` package to automatically signal klogd whenever a module
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is loaded or unloaded. Applying this patch provides essentially
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seamless support for debugging protection faults which occur with
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kernel loadable modules.
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The following is an example of a protection fault in a loadable module
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processed by ``klogd``::
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Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
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Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
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Aug 29 09:51:01 blizard kernel: *pde = 00000000
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Aug 29 09:51:01 blizard kernel: Oops: 0002
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Aug 29 09:51:01 blizard kernel: CPU: 0
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Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
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Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
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Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
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Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
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Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
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Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
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Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
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Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
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Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
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Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
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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
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---------------------------------------------------------------------------
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::
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Dr. G.W. Wettstein Oncology Research Div. Computing Facility
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Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com
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820 4th St. N.
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Fargo, ND 58122
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Phone: 701-234-7556
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