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diff --git a/Documentation/oops-tracing.txt b/Documentation/oops-tracing.txt new file mode 100644 index 000000000..2fb21a726 --- /dev/null +++ b/Documentation/oops-tracing.txt @@ -0,0 +1,150 @@ +From: Linus Torvalds <torvalds@cs.helsinki.fi> + +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 <offending_function> + +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 System.map 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. Wettstein Oncology Research Div. Computing Facility +Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com +820 4th St. N. +Fargo, ND 58122 +Phone: 701-234-7556 |