1Bug hunting
2===========
3
4Kernel bug reports often come with a stack dump like the one below::
5
6	------------[ cut here ]------------
7	WARNING: CPU: 1 PID: 28102 at kernel/module.c:1108 module_put+0x57/0x70
8	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]
9	CPU: 1 PID: 28102 Comm: rmmod Tainted: P        WC O 4.8.4-build.1 #1
10	Hardware name: MSI MS-7309/MS-7309, BIOS V1.12 02/23/2009
11	 00000000 c12ba080 00000000 00000000 c103ed6a c1616014 00000001 00006dc6
12	 c1615862 00000454 c109e8a7 c109e8a7 00000009 ffffffff 00000000 f13f6a10
13	 f5f5a600 c103ee33 00000009 00000000 00000000 c109e8a7 f80ca4d0 c109f617
14	Call Trace:
15	 [<c12ba080>] ? dump_stack+0x44/0x64
16	 [<c103ed6a>] ? __warn+0xfa/0x120
17	 [<c109e8a7>] ? module_put+0x57/0x70
18	 [<c109e8a7>] ? module_put+0x57/0x70
19	 [<c103ee33>] ? warn_slowpath_null+0x23/0x30
20	 [<c109e8a7>] ? module_put+0x57/0x70
21	 [<f80ca4d0>] ? gp8psk_fe_set_frontend+0x460/0x460 [dvb_usb_gp8psk]
22	 [<c109f617>] ? symbol_put_addr+0x27/0x50
23	 [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
24	 [<f80bb3bf>] ? dvb_usb_exit+0x2f/0xd0 [dvb_usb]
25	 [<c13d03bc>] ? usb_disable_endpoint+0x7c/0xb0
26	 [<f80bb48a>] ? dvb_usb_device_exit+0x2a/0x50 [dvb_usb]
27	 [<c13d2882>] ? usb_unbind_interface+0x62/0x250
28	 [<c136b514>] ? __pm_runtime_idle+0x44/0x70
29	 [<c13620d8>] ? __device_release_driver+0x78/0x120
30	 [<c1362907>] ? driver_detach+0x87/0x90
31	 [<c1361c48>] ? bus_remove_driver+0x38/0x90
32	 [<c13d1c18>] ? usb_deregister+0x58/0xb0
33	 [<c109fbb0>] ? SyS_delete_module+0x130/0x1f0
34	 [<c1055654>] ? task_work_run+0x64/0x80
35	 [<c1000fa5>] ? exit_to_usermode_loop+0x85/0x90
36	 [<c10013f0>] ? do_fast_syscall_32+0x80/0x130
37	 [<c1549f43>] ? sysenter_past_esp+0x40/0x6a
38	---[ end trace 6ebc60ef3981792f ]---
39
40Such stack traces provide enough information to identify the line inside the
41Kernel's source code where the bug happened. Depending on the severity of
42the issue, it may also contain the word **Oops**, as on this one::
43
44	BUG: unable to handle kernel NULL pointer dereference at   (null)
45	IP: [<c06969d4>] iret_exc+0x7d0/0xa59
46	*pdpt = 000000002258a001 *pde = 0000000000000000
47	Oops: 0002 [#1] PREEMPT SMP
48	...
49
50Despite being an **Oops** or some other sort of stack trace, the offended
51line is usually required to identify and handle the bug. Along this chapter,
52we'll refer to "Oops" for all kinds of stack traces that need to be analized.
53
54.. note::
55
56  ``ksymoops`` is useless on 2.6 or upper.  Please use the Oops in its original
57  format (from ``dmesg``, etc).  Ignore any references in this or other docs to
58  "decoding the Oops" or "running it through ksymoops".
59  If you post an Oops from 2.6+ that has been run through ``ksymoops``,
60  people will just tell you to repost it.
61
62Where is the Oops message is located?
63-------------------------------------
64
65Normally the Oops text is read from the kernel buffers by klogd and
66handed to ``syslogd`` which writes it to a syslog file, typically
67``/var/log/messages`` (depends on ``/etc/syslog.conf``). On systems with
68systemd, it may also be stored by the ``journald`` daemon, and accessed
69by running ``journalctl`` command.
70
71Sometimes ``klogd`` dies, in which case you can run ``dmesg > file`` to
72read the data from the kernel buffers and save it.  Or you can
73``cat /proc/kmsg > file``, however you have to break in to stop the transfer,
74``kmsg`` is a "never ending file".
75
76If the machine has crashed so badly that you cannot enter commands or
77the disk is not available then you have three options:
78
79(1) Hand copy the text from the screen and type it in after the machine
80    has restarted.  Messy but it is the only option if you have not
81    planned for a crash. Alternatively, you can take a picture of
82    the screen with a digital camera - not nice, but better than
83    nothing.  If the messages scroll off the top of the console, you
84    may find that booting with a higher resolution (eg, ``vga=791``)
85    will allow you to read more of the text. (Caveat: This needs ``vesafb``,
86    so won't help for 'early' oopses)
87
88(2) Boot with a serial console (see
89    :ref:`Documentation/admin-guide/serial-console.rst <serial_console>`),
90    run a null modem to a second machine and capture the output there
91    using your favourite communication program.  Minicom works well.
92
93(3) Use Kdump (see Documentation/kdump/kdump.txt),
94    extract the kernel ring buffer from old memory with using dmesg
95    gdbmacro in Documentation/kdump/gdbmacros.txt.
96
97Finding the bug's location
98--------------------------
99
100Reporting a bug works best if you point the location of the bug at the
101Kernel source file. There are two methods for doing that. Usually, using
102``gdb`` is easier, but the Kernel should be pre-compiled with debug info.
103
104gdb
105^^^
106
107The GNU debug (``gdb``) is the best way to figure out the exact file and line
108number of the OOPS from the ``vmlinux`` file.
109
110The usage of gdb works best on a kernel compiled with ``CONFIG_DEBUG_INFO``.
111This can be set by running::
112
113  $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
114
115On a kernel compiled with ``CONFIG_DEBUG_INFO``, you can simply copy the
116EIP value from the OOPS::
117
118 EIP:    0060:[<c021e50e>]    Not tainted VLI
119
120And use GDB to translate that to human-readable form::
121
122  $ gdb vmlinux
123  (gdb) l *0xc021e50e
124
125If you don't have ``CONFIG_DEBUG_INFO`` enabled, you use the function
126offset from the OOPS::
127
128 EIP is at vt_ioctl+0xda8/0x1482
129
130And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled::
131
132  $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO
133  $ make vmlinux
134  $ gdb vmlinux
135  (gdb) l *vt_ioctl+0xda8
136  0x1888 is in vt_ioctl (drivers/tty/vt/vt_ioctl.c:293).
137  288	{
138  289		struct vc_data *vc = NULL;
139  290		int ret = 0;
140  291
141  292		console_lock();
142  293		if (VT_BUSY(vc_num))
143  294			ret = -EBUSY;
144  295		else if (vc_num)
145  296			vc = vc_deallocate(vc_num);
146  297		console_unlock();
147
148or, if you want to be more verbose::
149
150  (gdb) p vt_ioctl
151  $1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl>
152  (gdb) l *0xae0+0xda8
153
154You could, instead, use the object file::
155
156  $ make drivers/tty/
157  $ gdb drivers/tty/vt/vt_ioctl.o
158  (gdb) l *vt_ioctl+0xda8
159
160If you have a call trace, such as::
161
162     Call Trace:
163      [<ffffffff8802c8e9>] :jbd:log_wait_commit+0xa3/0xf5
164      [<ffffffff810482d9>] autoremove_wake_function+0x0/0x2e
165      [<ffffffff8802770b>] :jbd:journal_stop+0x1be/0x1ee
166      ...
167
168this shows the problem likely in the :jbd: module. You can load that module
169in gdb and list the relevant code::
170
171  $ gdb fs/jbd/jbd.ko
172  (gdb) l *log_wait_commit+0xa3
173
174.. note::
175
176     You can also do the same for any function call at the stack trace,
177     like this one::
178
179	 [<f80bc9ca>] ? dvb_usb_adapter_frontend_exit+0x3a/0x70 [dvb_usb]
180
181     The position where the above call happened can be seen with::
182
183	$ gdb drivers/media/usb/dvb-usb/dvb-usb.o
184	(gdb) l *dvb_usb_adapter_frontend_exit+0x3a
185
186objdump
187^^^^^^^
188
189To debug a kernel, use objdump and look for the hex offset from the crash
190output to find the valid line of code/assembler. Without debug symbols, you
191will see the assembler code for the routine shown, but if your kernel has
192debug symbols the C code will also be available. (Debug symbols can be enabled
193in the kernel hacking menu of the menu configuration.) For example::
194
195    $ objdump -r -S -l --disassemble net/dccp/ipv4.o
196
197.. note::
198
199   You need to be at the top level of the kernel tree for this to pick up
200   your C files.
201
202If you don't have access to the code you can also debug on some crash dumps
203e.g. crash dump output as shown by Dave Miller::
204
205     EIP is at 	+0x14/0x4c0
206      ...
207     Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00
208     00 00 55 57  56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08
209     <8b> 83 3c 01 00 00 89 44  24 14 8b 45 28 85 c0 89 44 24 18 0f 85
210
211     Put the bytes into a "foo.s" file like this:
212
213            .text
214            .globl foo
215     foo:
216            .byte  .... /* bytes from Code: part of OOPS dump */
217
218     Compile it with "gcc -c -o foo.o foo.s" then look at the output of
219     "objdump --disassemble foo.o".
220
221     Output:
222
223     ip_queue_xmit:
224         push       %ebp
225         push       %edi
226         push       %esi
227         push       %ebx
228         sub        $0xbc, %esp
229         mov        0xd0(%esp), %ebp        ! %ebp = arg0 (skb)
230         mov        0x8(%ebp), %ebx         ! %ebx = skb->sk
231         mov        0x13c(%ebx), %eax       ! %eax = inet_sk(sk)->opt
232
233Reporting the bug
234-----------------
235
236Once you find where the bug happened, by inspecting its location,
237you could either try to fix it yourself or report it upstream.
238
239In order to report it upstream, you should identify the mailing list
240used for the development of the affected code. This can be done by using
241the ``get_maintainer.pl`` script.
242
243For example, if you find a bug at the gspca's sonixj.c file, you can get
244their maintainers with::
245
246	$ ./scripts/get_maintainer.pl -f drivers/media/usb/gspca/sonixj.c
247	Hans Verkuil <hverkuil@xs4all.nl> (odd fixer:GSPCA USB WEBCAM DRIVER,commit_signer:1/1=100%)
248	Mauro Carvalho Chehab <mchehab@kernel.org> (maintainer:MEDIA INPUT INFRASTRUCTURE (V4L/DVB),commit_signer:1/1=100%)
249	Tejun Heo <tj@kernel.org> (commit_signer:1/1=100%)
250	Bhaktipriya Shridhar <bhaktipriya96@gmail.com> (commit_signer:1/1=100%,authored:1/1=100%,added_lines:4/4=100%,removed_lines:9/9=100%)
251	linux-media@vger.kernel.org (open list:GSPCA USB WEBCAM DRIVER)
252	linux-kernel@vger.kernel.org (open list)
253
254Please notice that it will point to:
255
256- The last developers that touched on the source code. On the above example,
257  Tejun and Bhaktipriya (in this specific case, none really envolved on the
258  development of this file);
259- The driver maintainer (Hans Verkuil);
260- The subsystem maintainer (Mauro Carvalho Chehab);
261- The driver and/or subsystem mailing list (linux-media@vger.kernel.org);
262- the Linux Kernel mailing list (linux-kernel@vger.kernel.org).
263
264Usually, the fastest way to have your bug fixed is to report it to mailing
265list used for the development of the code (linux-media ML) copying the driver maintainer (Hans).
266
267If you are totally stumped as to whom to send the report, and
268``get_maintainer.pl`` didn't provide you anything useful, send it to
269linux-kernel@vger.kernel.org.
270
271Thanks for your help in making Linux as stable as humanly possible.
272
273Fixing the bug
274--------------
275
276If you know programming, you could help us by not only reporting the bug,
277but also providing us with a solution. After all, open source is about
278sharing what you do and don't you want to be recognised for your genius?
279
280If you decide to take this way, once you have worked out a fix please submit
281it upstream.
282
283Please do read
284:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though
285to help your code get accepted.
286
287
288---------------------------------------------------------------------------
289
290Notes on Oops tracing with ``klogd``
291------------------------------------
292
293In order to help Linus and the other kernel developers there has been
294substantial support incorporated into ``klogd`` for processing protection
295faults.  In order to have full support for address resolution at least
296version 1.3-pl3 of the ``sysklogd`` package should be used.
297
298When a protection fault occurs the ``klogd`` daemon automatically
299translates important addresses in the kernel log messages to their
300symbolic equivalents.  This translated kernel message is then
301forwarded through whatever reporting mechanism ``klogd`` is using.  The
302protection fault message can be simply cut out of the message files
303and forwarded to the kernel developers.
304
305Two types of address resolution are performed by ``klogd``.  The first is
306static translation and the second is dynamic translation.  Static
307translation uses the System.map file in much the same manner that
308ksymoops does.  In order to do static translation the ``klogd`` daemon
309must be able to find a system map file at daemon initialization time.
310See the klogd man page for information on how ``klogd`` searches for map
311files.
312
313Dynamic address translation is important when kernel loadable modules
314are being used.  Since memory for kernel modules is allocated from the
315kernel's dynamic memory pools there are no fixed locations for either
316the start of the module or for functions and symbols in the module.
317
318The kernel supports system calls which allow a program to determine
319which modules are loaded and their location in memory.  Using these
320system calls the klogd daemon builds a symbol table which can be used
321to debug a protection fault which occurs in a loadable kernel module.
322
323At the very minimum klogd will provide the name of the module which
324generated the protection fault.  There may be additional symbolic
325information available if the developer of the loadable module chose to
326export symbol information from the module.
327
328Since the kernel module environment can be dynamic there must be a
329mechanism for notifying the ``klogd`` daemon when a change in module
330environment occurs.  There are command line options available which
331allow klogd to signal the currently executing daemon that symbol
332information should be refreshed.  See the ``klogd`` manual page for more
333information.
334
335A patch is included with the sysklogd distribution which modifies the
336``modules-2.0.0`` package to automatically signal klogd whenever a module
337is loaded or unloaded.  Applying this patch provides essentially
338seamless support for debugging protection faults which occur with
339kernel loadable modules.
340
341The following is an example of a protection fault in a loadable module
342processed by ``klogd``::
343
344	Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
345	Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
346	Aug 29 09:51:01 blizard kernel: *pde = 00000000
347	Aug 29 09:51:01 blizard kernel: Oops: 0002
348	Aug 29 09:51:01 blizard kernel: CPU:    0
349	Aug 29 09:51:01 blizard kernel: EIP:    0010:[oops:_oops+16/3868]
350	Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
351	Aug 29 09:51:01 blizard kernel: eax: 315e97cc   ebx: 003a6f80   ecx: 001be77b   edx: 00237c0c
352	Aug 29 09:51:01 blizard kernel: esi: 00000000   edi: bffffdb3   ebp: 00589f90   esp: 00589f8c
353	Aug 29 09:51:01 blizard kernel: ds: 0018   es: 0018   fs: 002b   gs: 002b   ss: 0018
354	Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
355	Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
356	Aug 29 09:51:01 blizard kernel:        00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
357	Aug 29 09:51:01 blizard kernel:        bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
358	Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
359	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
360
361---------------------------------------------------------------------------
362
363::
364
365  Dr. G.W. Wettstein           Oncology Research Div. Computing Facility
366  Roger Maris Cancer Center    INTERNET: greg@wind.rmcc.com
367  820 4th St. N.
368  Fargo, ND  58122
369  Phone: 701-234-7556
370