4 Install ksymoops from ftp://ftp.ocs.com.au/pub/ksymoops
5 Read the ksymoops man page.
6 ksymoops < the_oops.txt
8 and send the output the maintainer of the kernel area that seems to be
9 involved with the problem, not to the ksymoops maintainer. Don't worry
10 too much about getting the wrong person. If you are unsure send it to
11 the person responsible for the code relevant to what you were doing.
12 If it occurs repeatably try and describe how to recreate it. Thats
13 worth even more than the oops
15 If you are totally stumped as to whom to send the report, send it to
16 linux-kernel@vger.rutgers.edu. Thanks for your help in making Linux as
17 stable as humanly possible.
19 Where is the_oops.txt?
20 ----------------------
22 Normally the Oops text is read from the kernel buffers by klogd and
23 handed to syslogd which writes it to a syslog file, typically
24 /var/log/messages (depends on /etc/syslog.conf). Sometimes klogd dies,
25 in which case you can run dmesg > file to read the data from the kernel
26 buffers and save it. Or you can cat /proc/kmsg > file, however you
27 have to break in to stop the transfer, kmsg is a "never ending file".
28 If the machine has crashed so badly that you cannot enter commands or
29 the 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
35 (2) Boot with a serial console (see Documentation/serial-console.txt),
36 run a null modem to a second machine and capture the output there
37 using your favourite communication program. Minicom works well.
39 (3) Patch the kernel with one of the crash dump patches. These save
40 data to a floppy disk or video rom or a swap partition. None of
41 these are standard kernel patches so you have to find and apply
42 them yourself. Search kernel archives for kmsgdump, lkcd and
45 No matter how you capture the log output, feed the resulting file to
46 ksymoops along with /proc/ksyms and /proc/modules that applied at the
47 time of the crash. /var/log/ksymoops can be useful to capture the
48 latter, man ksymoops for details.
54 From: Linus Torvalds <torvalds@transmeta.com>
56 How to track down an Oops.. [originally a mail to linux-kernel]
58 The main trick is having 5 years of experience with those pesky oops
61 Actually, there are things you can do that make this easier. I have two
64 gdb /usr/src/linux/vmlinux
65 gdb> disassemble <offending_function>
67 That's the easy way to find the problem, at least if the bug-report is
68 well made (like this one was - run through ksymoops to get the
69 information of which function and the offset in the function that it
72 Oh, it helps if the report happens on a kernel that is compiled with the
73 same compiler and similar setups.
75 The other thing to do is disassemble the "Code:" part of the bug report:
76 ksymoops will do this too with the correct tools, but if you don't have
77 the tools you can just do a silly program:
79 char str[] = "\xXX\xXX\xXX...";
82 and compile it with gcc -g and then do "disassemble str" (where the "XX"
83 stuff are the values reported by the Oops - you can just cut-and-paste
84 and do a replace of spaces to "\x" - that's what I do, as I'm too lazy
85 to write a program to automate this all).
87 Finally, if you want to see where the code comes from, you can do
90 make fs/buffer.s # or whatever file the bug happened in
92 and then you get a better idea of what happens than with the gdb
95 Now, the trick is just then to combine all the data you have: the C
96 sources (and general knowledge of what it _should_ do), the assembly
97 listing and the code disassembly (and additionally the register dump you
98 also get from the "oops" message - that can be useful to see _what_ the
99 corrupted pointers were, and when you have the assembler listing you can
100 also match the other registers to whatever C expressions they were used
103 Essentially, you just look at what doesn't match (in this case it was the
104 "Code" disassembly that didn't match with what the compiler generated).
105 Then you need to find out _why_ they don't match. Often it's simple - you
106 see that the code uses a NULL pointer and then you look at the code and
107 wonder how the NULL pointer got there, and if it's a valid thing to do
108 you just check against it..
110 Now, if somebody gets the idea that this is time-consuming and requires
111 some small amount of concentration, you're right. Which is why I will
112 mostly just ignore any panic reports that don't have the symbol table
113 info etc looked up: it simply gets too hard to look it up (I have some
114 programs to search for specific patterns in the kernel code segment, and
115 sometimes I have been able to look up those kinds of panics too, but
116 that really requires pretty good knowledge of the kernel just to be able
117 to pick out the right sequences etc..)
119 _Sometimes_ it happens that I just see the disassembled code sequence
120 from the panic, and I know immediately where it's coming from. That's when
121 I get worried that I've been doing this for too long ;-)
126 ---------------------------------------------------------------------------
127 Notes on Oops tracing with klogd:
129 In order to help Linus and the other kernel developers there has been
130 substantial support incorporated into klogd for processing protection
131 faults. In order to have full support for address resolution at least
132 version 1.3-pl3 of the sysklogd package should be used.
134 When a protection fault occurs the klogd daemon automatically
135 translates important addresses in the kernel log messages to their
136 symbolic equivalents. This translated kernel message is then
137 forwarded through whatever reporting mechanism klogd is using. The
138 protection fault message can be simply cut out of the message files
139 and forwarded to the kernel developers.
141 Two types of address resolution are performed by klogd. The first is
142 static translation and the second is dynamic translation. Static
143 translation uses the System.map file in much the same manner that
144 ksymoops does. In order to do static translation the klogd daemon
145 must be able to find a system map file at daemon initialization time.
146 See the klogd man page for information on how klogd searches for map
149 Dynamic address translation is important when kernel loadable modules
150 are being used. Since memory for kernel modules is allocated from the
151 kernel's dynamic memory pools there are no fixed locations for either
152 the start of the module or for functions and symbols in the module.
154 The kernel supports system calls which allow a program to determine
155 which modules are loaded and their location in memory. Using these
156 system calls the klogd daemon builds a symbol table which can be used
157 to debug a protection fault which occurs in a loadable kernel module.
159 At the very minimum klogd will provide the name of the module which
160 generated the protection fault. There may be additional symbolic
161 information available if the developer of the loadable module chose to
162 export symbol information from the module.
164 Since the kernel module environment can be dynamic there must be a
165 mechanism for notifying the klogd daemon when a change in module
166 environment occurs. There are command line options available which
167 allow klogd to signal the currently executing daemon that symbol
168 information should be refreshed. See the klogd manual page for more
171 A patch is included with the sysklogd distribution which modifies the
172 modules-2.0.0 package to automatically signal klogd whenever a module
173 is loaded or unloaded. Applying this patch provides essentially
174 seamless support for debugging protection faults which occur with
175 kernel loadable modules.
177 The following is an example of a protection fault in a loadable module
179 ---------------------------------------------------------------------------
180 Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
181 Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
182 Aug 29 09:51:01 blizard kernel: *pde = 00000000
183 Aug 29 09:51:01 blizard kernel: Oops: 0002
184 Aug 29 09:51:01 blizard kernel: CPU: 0
185 Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
186 Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
187 Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
188 Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
189 Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
190 Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
191 Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
192 Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
193 Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
194 Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
195 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
196 ---------------------------------------------------------------------------
198 Dr. G.W. Wettstein Oncology Research Div. Computing Facility
199 Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com