1NOTE: 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
3the Oops" or "running it through ksymoops". If you post an Oops from 2.6 that
4has been run through ksymoops, people will just tell you to repost it.
6Quick Summary
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.
15If you are totally stumped as to whom to send the report, send it to Thanks for your help in making Linux as
17stable as humanly possible.
19Where is the Oops?
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 :-
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
33    planned for a crash. Alternatively, you can take a picture of
34    the screen with a digital camera - not nice, but better than
35    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)
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.
44(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.
49Full Information
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.
56From: Linus Torvalds <>
58How to track down an Oops.. [originally a mail to linux-kernel]
60The main trick is having 5 years of experience with those pesky oops
61messages ;-)
63Actually, there are things you can do that make this easier. I have two
64separate approaches:
66    gdb /usr/src/linux/vmlinux
67    gdb> disassemble <offending_function>
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).
74Oh, it helps if the report happens on a kernel that is compiled with the
75same compiler and similar setups.
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:
81    char str[] = "\xXX\xXX\xXX...";
82    main(){}
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).
89Alternatively, you can use the shell script in scripts/decodecode.
90Its usage is: decodecode < oops.txt
92The hex bytes that follow "Code:" may (in some architectures) have a series
93of bytes that precede the current instruction pointer as well as bytes at and
94following the current instruction pointer. In some cases, one instruction
95byte or word is surrounded by <> or (), as in "<86>" or "(f00d)". These
96<> or () markings indicate the current instruction pointer. Example from
97i386, split into multiple lines for readability:
99Code: f9 0f 8d f9 00 00 00 8d 42 0c e8 dd 26 11 c7 a1 60 ea 2b f9 8b 50 08 a1
10064 ea 2b f9 8d 34 82 8b 1e 85 db 74 6d 8b 15 60 ea 2b f9 <8b> 43 04 39 42 54
1017e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0
103Finally, if you want to see where the code comes from, you can do
105    cd /usr/src/linux
106    make fs/buffer.s # or whatever file the bug happened in
108and then you get a better idea of what happens than with the gdb
111Now, the trick is just then to combine all the data you have: the C
112sources (and general knowledge of what it _should_ do), the assembly
113listing and the code disassembly (and additionally the register dump you
114also get from the "oops" message - that can be useful to see _what_ the
115corrupted pointers were, and when you have the assembler listing you can
116also match the other registers to whatever C expressions they were used
119Essentially, you just look at what doesn't match (in this case it was the
120"Code" disassembly that didn't match with what the compiler generated).
121Then you need to find out _why_ they don't match. Often it's simple - you
122see that the code uses a NULL pointer and then you look at the code and
123wonder how the NULL pointer got there, and if it's a valid thing to do
124you just check against it..
126Now, if somebody gets the idea that this is time-consuming and requires
127some small amount of concentration, you're right. Which is why I will
128mostly just ignore any panic reports that don't have the symbol table
129info etc looked up: it simply gets too hard to look it up (I have some
130programs to search for specific patterns in the kernel code segment, and
131sometimes I have been able to look up those kinds of panics too, but
132that really requires pretty good knowledge of the kernel just to be able
133to pick out the right sequences etc..)
135_Sometimes_ it happens that I just see the disassembled code sequence
136from the panic, and I know immediately where it's coming from. That's when
137I get worried that I've been doing this for too long ;-)
139        Linus
143Notes on Oops tracing with klogd:
145In order to help Linus and the other kernel developers there has been
146substantial support incorporated into klogd for processing protection
147faults. In order to have full support for address resolution at least
148version 1.3-pl3 of the sysklogd package should be used.
150When a protection fault occurs the klogd daemon automatically
151translates important addresses in the kernel log messages to their
152symbolic equivalents. This translated kernel message is then
153forwarded through whatever reporting mechanism klogd is using. The
154protection fault message can be simply cut out of the message files
155and forwarded to the kernel developers.
157Two types of address resolution are performed by klogd. The first is
158static translation and the second is dynamic translation. Static
159translation uses the file in much the same manner that
160ksymoops does. In order to do static translation the klogd daemon
161must be able to find a system map file at daemon initialization time.
162See the klogd man page for information on how klogd searches for map
165Dynamic address translation is important when kernel loadable modules
166are being used. Since memory for kernel modules is allocated from the
167kernel's dynamic memory pools there are no fixed locations for either
168the start of the module or for functions and symbols in the module.
170The kernel supports system calls which allow a program to determine
171which modules are loaded and their location in memory. Using these
172system calls the klogd daemon builds a symbol table which can be used
173to debug a protection fault which occurs in a loadable kernel module.
175At the very minimum klogd will provide the name of the module which
176generated the protection fault. There may be additional symbolic
177information available if the developer of the loadable module chose to
178export symbol information from the module.
180Since the kernel module environment can be dynamic there must be a
181mechanism for notifying the klogd daemon when a change in module
182environment occurs. There are command line options available which
183allow klogd to signal the currently executing daemon that symbol
184information should be refreshed. See the klogd manual page for more
187A patch is included with the sysklogd distribution which modifies the
188modules-2.0.0 package to automatically signal klogd whenever a module
189is loaded or unloaded. Applying this patch provides essentially
190seamless support for debugging protection faults which occur with
191kernel loadable modules.
193The following is an example of a protection fault in a loadable module
194processed by klogd:
196Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
197Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
198Aug 29 09:51:01 blizard kernel: *pde = 00000000
199Aug 29 09:51:01 blizard kernel: Oops: 0002
200Aug 29 09:51:01 blizard kernel: CPU: 0
201Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
202Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
203Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
204Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
205Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
206Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
207Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
208Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
209Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
210Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
211Aug 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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222Tainted kernels:
224Some oops reports contain the string 'Tainted: ' after the program
225counter. This indicates that the kernel has been tainted by some
226mechanism. The string is followed by a series of position-sensitive
227characters, each representing a particular tainted value.
229  1: 'G' if all modules loaded have a GPL or compatible license, 'P' if
230     any proprietary module has been loaded. Modules without a
231     MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
232     insmod as GPL compatible are assumed to be proprietary.
234  2: 'F' if any module was force loaded by "insmod -f", ' ' if all
235     modules were loaded normally.
237  3: 'S' if the oops occurred on an SMP kernel running on hardware that
238     hasn't been certified as safe to run multiprocessor.
239     Currently this occurs only on various Athlons that are not
240     SMP capable.
242  4: 'R' if a module was force unloaded by "rmmod -f", ' ' if all
243     modules were unloaded normally.
245  5: 'M' if any processor has reported a Machine Check Exception,
246     ' ' if no Machine Check Exceptions have occurred.
248  6: 'B' if a page-release function has found a bad page reference or
249     some unexpected page flags.
251  7: 'U' if a user or user application specifically requested that the
252     Tainted flag be set, ' ' otherwise.
254  8: 'D' if the kernel has died recently, i.e. there was an OOPS or BUG.
256  9: 'A' if the ACPI table has been overridden.
258 10: 'W' if a warning has previously been issued by the kernel.
260 11: 'C' if a staging driver has been loaded.
262The primary reason for the 'Tainted: ' string is to tell kernel
263debuggers if this is a clean kernel or if anything unusual has
264occurred. Tainting is permanent: even if an offending module is
265unloaded, the tainted value remains to indicate that the kernel is not

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