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953ab835 | 1 | Bug hunting |
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2 | =========== |
3 | ||
4 | Kernel 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 | ||
40 | Such stack traces provide enough information to identify the line inside the | |
41 | Kernel's source code where the bug happened. Depending on the severity of | |
42 | the 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 | ||
50 | Despite being an **Oops** or some other sort of stack trace, the offended | |
51 | line is usually required to identify and handle the bug. Along this chapter, | |
52 | we'll refer to "Oops" for all kinds of stack traces that need to be analized. | |
43019a56 | 53 | |
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54 | .. note:: |
55 | ||
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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 | ||
62 | Where is the Oops message is located? | |
63 | ------------------------------------- | |
64 | ||
65 | Normally the Oops text is read from the kernel buffers by klogd and | |
66 | handed to ``syslogd`` which writes it to a syslog file, typically | |
67 | ``/var/log/messages`` (depends on ``/etc/syslog.conf``). On systems with | |
68 | systemd, it may also be stored by the ``journald`` daemon, and accessed | |
69 | by running ``journalctl`` command. | |
70 | ||
71 | Sometimes ``klogd`` dies, in which case you can run ``dmesg > file`` to | |
72 | read 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 | ||
76 | If the machine has crashed so badly that you cannot enter commands or | |
77 | the 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 | ||
330d4810 | 93 | (3) Use Kdump (see Documentation/admin-guide/kdump/kdump.rst), |
f226e460 | 94 | extract the kernel ring buffer from old memory with using dmesg |
330d4810 | 95 | gdbmacro in Documentation/admin-guide/kdump/gdbmacros.txt. |
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96 | |
97 | Finding the bug's location | |
98 | -------------------------- | |
99 | ||
100 | Reporting a bug works best if you point the location of the bug at the | |
101 | Kernel 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. | |
43019a56 | 103 | |
ab0e44c1 | 104 | gdb |
f226e460 | 105 | ^^^ |
ab0e44c1 | 106 | |
f226e460 | 107 | The GNU debug (``gdb``) is the best way to figure out the exact file and line |
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108 | number of the OOPS from the ``vmlinux`` file. |
109 | ||
f226e460 | 110 | The usage of gdb works best on a kernel compiled with ``CONFIG_DEBUG_INFO``. |
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111 | This can be set by running:: |
112 | ||
113 | $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO | |
114 | ||
115 | On a kernel compiled with ``CONFIG_DEBUG_INFO``, you can simply copy the | |
116 | EIP value from the OOPS:: | |
926b2898 PE |
117 | |
118 | EIP: 0060:[<c021e50e>] Not tainted VLI | |
119 | ||
953ab835 | 120 | And use GDB to translate that to human-readable form:: |
926b2898 | 121 | |
ab0e44c1 | 122 | $ gdb vmlinux |
926b2898 PE |
123 | (gdb) l *0xc021e50e |
124 | ||
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125 | If you don't have ``CONFIG_DEBUG_INFO`` enabled, you use the function |
126 | offset from the OOPS:: | |
926b2898 PE |
127 | |
128 | EIP is at vt_ioctl+0xda8/0x1482 | |
129 | ||
953ab835 | 130 | And recompile the kernel with ``CONFIG_DEBUG_INFO`` enabled:: |
926b2898 | 131 | |
f226e460 | 132 | $ ./scripts/config -d COMPILE_TEST -e DEBUG_KERNEL -e DEBUG_INFO |
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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 | ||
148 | or, if you want to be more verbose:: | |
149 | ||
926b2898 | 150 | (gdb) p vt_ioctl |
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151 | $1 = {int (struct tty_struct *, unsigned int, unsigned long)} 0xae0 <vt_ioctl> |
152 | (gdb) l *0xae0+0xda8 | |
953ab835 | 153 | |
ab0e44c1 | 154 | You could, instead, use the object file:: |
953ab835 | 155 | |
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156 | $ make drivers/tty/ |
157 | $ gdb drivers/tty/vt/vt_ioctl.o | |
158 | (gdb) l *vt_ioctl+0xda8 | |
dcc85cb6 | 159 | |
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160 | If 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 | ||
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168 | this shows the problem likely in the :jbd: module. You can load that module |
169 | in gdb and list the relevant code:: | |
dcc85cb6 | 170 | |
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171 | $ gdb fs/jbd/jbd.ko |
172 | (gdb) l *log_wait_commit+0xa3 | |
926b2898 | 173 | |
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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 | ||
186 | objdump | |
187 | ^^^^^^^ | |
188 | ||
189 | To debug a kernel, use objdump and look for the hex offset from the crash | |
190 | output to find the valid line of code/assembler. Without debug symbols, you | |
191 | will see the assembler code for the routine shown, but if your kernel has | |
192 | debug symbols the C code will also be available. (Debug symbols can be enabled | |
193 | in the kernel hacking menu of the menu configuration.) For example:: | |
194 | ||
195 | $ objdump -r -S -l --disassemble net/dccp/ipv4.o | |
196 | ||
197 | .. note:: | |
43019a56 | 198 | |
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199 | You need to be at the top level of the kernel tree for this to pick up |
200 | your C files. | |
201 | ||
202 | If you don't have access to the code you can also debug on some crash dumps | |
203 | e.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 | ||
233 | Reporting the bug | |
234 | ----------------- | |
235 | ||
236 | Once you find where the bug happened, by inspecting its location, | |
237 | you could either try to fix it yourself or report it upstream. | |
238 | ||
239 | In order to report it upstream, you should identify the mailing list | |
240 | used for the development of the affected code. This can be done by using | |
241 | the ``get_maintainer.pl`` script. | |
242 | ||
ed6e26ba | 243 | For example, if you find a bug at the gspca's sonixj.c file, you can get |
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244 | their 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 | ||
254 | Please 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); | |
ed6e26ba | 260 | - The subsystem maintainer (Mauro Carvalho Chehab); |
f226e460 MCC |
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 | ||
264 | Usually, the fastest way to have your bug fixed is to report it to mailing | |
265 | list used for the development of the code (linux-media ML) copying the driver maintainer (Hans). | |
266 | ||
267 | If 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 | |
269 | linux-kernel@vger.kernel.org. | |
270 | ||
271 | Thanks for your help in making Linux as stable as humanly possible. | |
272 | ||
273 | Fixing the bug | |
274 | -------------- | |
275 | ||
276 | If you know programming, you could help us by not only reporting the bug, | |
ed6e26ba | 277 | but also providing us with a solution. After all, open source is about |
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278 | sharing what you do and don't you want to be recognised for your genius? |
279 | ||
280 | If you decide to take this way, once you have worked out a fix please submit | |
281 | it upstream. | |
43019a56 | 282 | |
8c27ceff | 283 | Please do read |
ed6e26ba | 284 | :ref:`Documentation/process/submitting-patches.rst <submittingpatches>` though |
8c27ceff | 285 | to help your code get accepted. |
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286 | |
287 | ||
288 | --------------------------------------------------------------------------- | |
289 | ||
290 | Notes on Oops tracing with ``klogd`` | |
291 | ------------------------------------ | |
292 | ||
293 | In order to help Linus and the other kernel developers there has been | |
294 | substantial support incorporated into ``klogd`` for processing protection | |
295 | faults. In order to have full support for address resolution at least | |
296 | version 1.3-pl3 of the ``sysklogd`` package should be used. | |
297 | ||
298 | When a protection fault occurs the ``klogd`` daemon automatically | |
299 | translates important addresses in the kernel log messages to their | |
300 | symbolic equivalents. This translated kernel message is then | |
301 | forwarded through whatever reporting mechanism ``klogd`` is using. The | |
302 | protection fault message can be simply cut out of the message files | |
303 | and forwarded to the kernel developers. | |
304 | ||
305 | Two types of address resolution are performed by ``klogd``. The first is | |
306 | static translation and the second is dynamic translation. Static | |
307 | translation uses the System.map file in much the same manner that | |
308 | ksymoops does. In order to do static translation the ``klogd`` daemon | |
309 | must be able to find a system map file at daemon initialization time. | |
310 | See the klogd man page for information on how ``klogd`` searches for map | |
311 | files. | |
312 | ||
313 | Dynamic address translation is important when kernel loadable modules | |
314 | are being used. Since memory for kernel modules is allocated from the | |
315 | kernel's dynamic memory pools there are no fixed locations for either | |
316 | the start of the module or for functions and symbols in the module. | |
317 | ||
318 | The kernel supports system calls which allow a program to determine | |
319 | which modules are loaded and their location in memory. Using these | |
320 | system calls the klogd daemon builds a symbol table which can be used | |
321 | to debug a protection fault which occurs in a loadable kernel module. | |
322 | ||
323 | At the very minimum klogd will provide the name of the module which | |
324 | generated the protection fault. There may be additional symbolic | |
325 | information available if the developer of the loadable module chose to | |
326 | export symbol information from the module. | |
327 | ||
328 | Since the kernel module environment can be dynamic there must be a | |
329 | mechanism for notifying the ``klogd`` daemon when a change in module | |
330 | environment occurs. There are command line options available which | |
331 | allow klogd to signal the currently executing daemon that symbol | |
332 | information should be refreshed. See the ``klogd`` manual page for more | |
333 | information. | |
334 | ||
335 | A patch is included with the sysklogd distribution which modifies the | |
336 | ``modules-2.0.0`` package to automatically signal klogd whenever a module | |
337 | is loaded or unloaded. Applying this patch provides essentially | |
338 | seamless support for debugging protection faults which occur with | |
339 | kernel loadable modules. | |
340 | ||
341 | The following is an example of a protection fault in a loadable module | |
342 | processed 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 |