From: Linus Torvalds How to track down an Oops.. [originally a mail to linux-kernel] The main trick is having 5 years of experience with those pesky oops messages ;-) Actually, there are things you can do that make this easier. I have two separate approaches: gdb /usr/src/linux/vmlinux gdb> disassemble That's the easy way to find the problem, at least if the bug-report is well made (like this one was - run through ksymoops to get the information of which function and the offset in the function that it happened in). Oh, it helps if the report happens on a kernel that is compiled with the same compiler and similar setups. The other thing to do is disassemble the "Code:" part of the bug report: ksymoops will do this too with the correct tools (and new version of ksymoops), but if you don't have the tools you can just do a silly program: char str[] = "\xXX\xXX\xXX..."; main(){} and compile it with gcc -g and then do "disassemble str" (where the "XX" stuff are the values reported by the Oops - you can just cut-and-paste and do a replace of spaces to "\x" - that's what I do, as I'm too lazy to write a program to automate this all). Finally, if you want to see where the code comes from, you can do cd /usr/src/linux make fs/buffer.s # or whatever file the bug happened in and then you get a better idea of what happens than with the gdb disassembly. Now, the trick is just then to combine all the data you have: the C sources (and general knowledge of what it _should_ do, the assembly listing and the code disassembly (and additionally the register dump you also get from the "oops" message - that can be useful to see _what_ the corrupted pointers were, and when you have the assembler listing you can also match the other registers to whatever C expressions they were used for). Essentially, you just look at what doesn't match (in this case it was the "Code" disassembly that didn't match with what the compiler generated). Then you need to find out _why_ they don't match. Often it's simple - you see that the code uses a NULL pointer and then you look at the code and wonder how the NULL pointer got there, and if it's a valid thing to do you just check against it.. Now, if somebody gets the idea that this is time-consuming and requires some small amount of concentration, you're right. Which is why I will mostly just ignore any panic reports that don't have the symbol table info etc looked up: it simply gets too hard to look it up (I have some programs to search for specific patterns in the kernel code segment, and sometimes I have been able to look up those kinds of panics too, but that really requires pretty good knowledge of the kernel just to be able to pick out the right sequences etc..) _Sometimes_ it happens that I just see the disassembled code sequence from the panic, and I know immediately where it's coming from. That's when I get worried that I've been doing this for too long ;-) Linus --------------------------------------------------------------------------- Notes on Oops tracing with klogd: In order to help Linus and the other kernel developers there has been substantial support incorporated into klogd for processing protection faults. In order to have full support for address resolution at least version 1.3-pl3 of the sysklogd package should be used. When a protection fault occurs the klogd daemon automatically translates important addresses in the kernel log messages to their symbolic equivalents. This translated kernel message is then forwarded through whatever reporting mechanism klogd is using. The protection fault message can be simply cut out of the message files and forwarded to the kernel developers. Two types of address resolution are performed by klogd. The first is static translation and the second is dynamic translation. Static translation uses the file in much the same manner that ksymoops does. In order to do static translation the klogd daemon must be able to find a system map file at daemon initialization time. See the klogd man page for information on how klogd searches for map files. Dynamic address translation is important when kernel loadable modules are being used. Since memory for kernel modules is allocated from the kernel's dynamic memory pools there are no fixed locations for either the start of the module or for functions and symbols in the module. The kernel supports system calls which allow a program to determine which modules are loaded and their location in memory. Using these system calls the klogd daemon builds a symbol table which can be used to debug a protection fault which occurs in a loadable kernel module. At the very minimum klogd will provide the name of the module which generated the protection fault. There may be additional symbolic information available if the developer of the loadable module chose to export symbol information from the module. Since the kernel module environment can be dynamic there must be a mechanism for notifying the klogd daemon when a change in module environment occurs. There are command line options available which allow klogd to signal the currently executing daemon that symbol information should be refreshed. See the klogd manual page for more information. A patch is included with the sysklogd distribution which modifies the modules-2.0.0 package to automatically signal klogd whenever a module is loaded or unloaded. Applying this patch provides essentially seamless support for debugging protection faults which occur with kernel loadable modules. The following is an example of a protection fault in a loadable module processed by klogd: --------------------------------------------------------------------------- Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000 Aug 29 09:51:01 blizard kernel: *pde = 00000000 Aug 29 09:51:01 blizard kernel: Oops: 0002 Aug 29 09:51:01 blizard kernel: CPU: 0 Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868] Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212 Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018 Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000) Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001 Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00 Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036 Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128] 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 --------------------------------------------------------------------------- Dr. G.W. 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