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Linus Torvalds1da177e2005-04-16 15:20:36 -07001NOTE: ksymoops is useless on 2.6. Please use the Oops in its original format
2(from dmesg, etc). Ignore any references in this or other docs to "decoding
Jesper Juhl62a07e62005-11-07 01:01:03 -08003the Oops" or "running it through ksymoops". If you post an Oops from 2.6 that
Linus Torvalds1da177e2005-04-16 15:20:36 -07004has been run through ksymoops, people will just tell you to repost it.
5
6Quick Summary
7-------------
8
9Find the Oops and send it to the maintainer of the kernel area that seems to be
10involved with the problem. Don't worry too much about getting the wrong person.
11If you are unsure send it to the person responsible for the code relevant to
12what you were doing. If it occurs repeatably try and describe how to recreate
13it. That's worth even more than the oops.
14
15If you are totally stumped as to whom to send the report, send it to
16linux-kernel@vger.kernel.org. Thanks for your help in making Linux as
17stable as humanly possible.
18
19Where is the Oops?
20----------------------
21
22Normally the Oops text is read from the kernel buffers by klogd and
23handed to syslogd which writes it to a syslog file, typically
24/var/log/messages (depends on /etc/syslog.conf). Sometimes klogd dies,
25in which case you can run dmesg > file to read the data from the kernel
26buffers and save it. Or you can cat /proc/kmsg > file, however you
27have to break in to stop the transfer, kmsg is a "never ending file".
28If the machine has crashed so badly that you cannot enter commands or
29the disk is not available then you have three options :-
30
31(1) Hand copy the text from the screen and type it in after the machine
32 has restarted. Messy but it is the only option if you have not
Diego Calleja36174492005-11-13 16:07:40 -080033 planned for a crash. Alternatively, you can take a picture of
34 the screen with a digital camera - not nice, but better than
Dave Jonese1f1def2005-11-15 00:09:24 -080035 nothing. If the messages scroll off the top of the console, you
36 may find that booting with a higher resolution (eg, vga=791)
37 will allow you to read more of the text. (Caveat: This needs vesafb,
38 so won't help for 'early' oopses)
Linus Torvalds1da177e2005-04-16 15:20:36 -070039
40(2) Boot with a serial console (see Documentation/serial-console.txt),
41 run a null modem to a second machine and capture the output there
42 using your favourite communication program. Minicom works well.
43
Akinobu Mita75ba0862006-01-11 12:17:31 -080044(3) Use Kdump (see Documentation/kdump/kdump.txt),
45 extract the kernel ring buffer from old memory with using dmesg
46 gdbmacro in Documentation/kdump/gdbmacros.txt.
Linus Torvalds1da177e2005-04-16 15:20:36 -070047
48
49Full Information
50----------------
51
52NOTE: the message from Linus below applies to 2.4 kernel. I have preserved it
53for historical reasons, and because some of the information in it still
54applies. Especially, please ignore any references to ksymoops.
55
56From: Linus Torvalds <torvalds@osdl.org>
57
58How to track down an Oops.. [originally a mail to linux-kernel]
59
60The main trick is having 5 years of experience with those pesky oops
61messages ;-)
62
63Actually, there are things you can do that make this easier. I have two
64separate approaches:
65
66 gdb /usr/src/linux/vmlinux
67 gdb> disassemble <offending_function>
68
69That's the easy way to find the problem, at least if the bug-report is
70well made (like this one was - run through ksymoops to get the
71information of which function and the offset in the function that it
72happened in).
73
74Oh, it helps if the report happens on a kernel that is compiled with the
75same compiler and similar setups.
76
77The other thing to do is disassemble the "Code:" part of the bug report:
78ksymoops will do this too with the correct tools, but if you don't have
79the tools you can just do a silly program:
80
81 char str[] = "\xXX\xXX\xXX...";
82 main(){}
83
84and compile it with gcc -g and then do "disassemble str" (where the "XX"
85stuff are the values reported by the Oops - you can just cut-and-paste
86and do a replace of spaces to "\x" - that's what I do, as I'm too lazy
87to write a program to automate this all).
88
89Finally, if you want to see where the code comes from, you can do
90
91 cd /usr/src/linux
92 make fs/buffer.s # or whatever file the bug happened in
93
94and then you get a better idea of what happens than with the gdb
95disassembly.
96
97Now, the trick is just then to combine all the data you have: the C
98sources (and general knowledge of what it _should_ do), the assembly
99listing and the code disassembly (and additionally the register dump you
100also get from the "oops" message - that can be useful to see _what_ the
101corrupted pointers were, and when you have the assembler listing you can
102also match the other registers to whatever C expressions they were used
103for).
104
105Essentially, you just look at what doesn't match (in this case it was the
106"Code" disassembly that didn't match with what the compiler generated).
107Then you need to find out _why_ they don't match. Often it's simple - you
108see that the code uses a NULL pointer and then you look at the code and
109wonder how the NULL pointer got there, and if it's a valid thing to do
110you just check against it..
111
112Now, if somebody gets the idea that this is time-consuming and requires
113some small amount of concentration, you're right. Which is why I will
114mostly just ignore any panic reports that don't have the symbol table
115info etc looked up: it simply gets too hard to look it up (I have some
116programs to search for specific patterns in the kernel code segment, and
117sometimes I have been able to look up those kinds of panics too, but
118that really requires pretty good knowledge of the kernel just to be able
119to pick out the right sequences etc..)
120
121_Sometimes_ it happens that I just see the disassembled code sequence
122from the panic, and I know immediately where it's coming from. That's when
123I get worried that I've been doing this for too long ;-)
124
125 Linus
126
127
128---------------------------------------------------------------------------
129Notes on Oops tracing with klogd:
130
131In order to help Linus and the other kernel developers there has been
132substantial support incorporated into klogd for processing protection
133faults. In order to have full support for address resolution at least
134version 1.3-pl3 of the sysklogd package should be used.
135
136When a protection fault occurs the klogd daemon automatically
137translates important addresses in the kernel log messages to their
138symbolic equivalents. This translated kernel message is then
139forwarded through whatever reporting mechanism klogd is using. The
140protection fault message can be simply cut out of the message files
141and forwarded to the kernel developers.
142
143Two types of address resolution are performed by klogd. The first is
144static translation and the second is dynamic translation. Static
145translation uses the System.map file in much the same manner that
146ksymoops does. In order to do static translation the klogd daemon
147must be able to find a system map file at daemon initialization time.
148See the klogd man page for information on how klogd searches for map
149files.
150
151Dynamic address translation is important when kernel loadable modules
152are being used. Since memory for kernel modules is allocated from the
153kernel's dynamic memory pools there are no fixed locations for either
154the start of the module or for functions and symbols in the module.
155
156The kernel supports system calls which allow a program to determine
157which modules are loaded and their location in memory. Using these
158system calls the klogd daemon builds a symbol table which can be used
159to debug a protection fault which occurs in a loadable kernel module.
160
161At the very minimum klogd will provide the name of the module which
162generated the protection fault. There may be additional symbolic
163information available if the developer of the loadable module chose to
164export symbol information from the module.
165
166Since the kernel module environment can be dynamic there must be a
167mechanism for notifying the klogd daemon when a change in module
168environment occurs. There are command line options available which
169allow klogd to signal the currently executing daemon that symbol
170information should be refreshed. See the klogd manual page for more
171information.
172
173A patch is included with the sysklogd distribution which modifies the
174modules-2.0.0 package to automatically signal klogd whenever a module
175is loaded or unloaded. Applying this patch provides essentially
176seamless support for debugging protection faults which occur with
177kernel loadable modules.
178
179The following is an example of a protection fault in a loadable module
180processed by klogd:
181---------------------------------------------------------------------------
182Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
183Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
184Aug 29 09:51:01 blizard kernel: *pde = 00000000
185Aug 29 09:51:01 blizard kernel: Oops: 0002
186Aug 29 09:51:01 blizard kernel: CPU: 0
187Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
188Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
189Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
190Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
191Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
192Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
193Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
194Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
195Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
196Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
197Aug 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
198---------------------------------------------------------------------------
199
200Dr. G.W. Wettstein Oncology Research Div. Computing Facility
201Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com
202820 4th St. N.
203Fargo, ND 58122
204Phone: 701-234-7556
205
206
207---------------------------------------------------------------------------
208Tainted kernels:
209
210Some oops reports contain the string 'Tainted: ' after the program
Randy Dunlap1cc57532005-09-13 01:25:46 -0700211counter. This indicates that the kernel has been tainted by some
212mechanism. The string is followed by a series of position-sensitive
Linus Torvalds1da177e2005-04-16 15:20:36 -0700213characters, each representing a particular tainted value.
214
215 1: 'G' if all modules loaded have a GPL or compatible license, 'P' if
216 any proprietary module has been loaded. Modules without a
217 MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
218 insmod as GPL compatible are assumed to be proprietary.
219
Randy Dunlap1cc57532005-09-13 01:25:46 -0700220 2: 'F' if any module was force loaded by "insmod -f", ' ' if all
Linus Torvalds1da177e2005-04-16 15:20:36 -0700221 modules were loaded normally.
222
223 3: 'S' if the oops occurred on an SMP kernel running on hardware that
Randy Dunlap1cc57532005-09-13 01:25:46 -0700224 hasn't been certified as safe to run multiprocessor.
225 Currently this occurs only on various Athlons that are not
226 SMP capable.
227
228 4: 'R' if a module was force unloaded by "rmmod -f", ' ' if all
229 modules were unloaded normally.
230
231 5: 'M' if any processor has reported a Machine Check Exception,
232 ' ' if no Machine Check Exceptions have occurred.
233
234 6: 'B' if a page-release function has found a bad page reference or
235 some unexpected page flags.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700236
237The primary reason for the 'Tainted: ' string is to tell kernel
238debuggers if this is a clean kernel or if anything unusual has
Randy Dunlap1cc57532005-09-13 01:25:46 -0700239occurred. Tainting is permanent: even if an offending module is
240unloaded, the tainted value remains to indicate that the kernel is not
Linus Torvalds1da177e2005-04-16 15:20:36 -0700241trustworthy.