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diff --git a/Documentation/oops-tracing.txt b/Documentation/oops-tracing.txt new file mode 100644 index 00000000..13032c01 --- /dev/null +++ b/Documentation/oops-tracing.txt @@ -0,0 +1,272 @@ +NOTE: ksymoops is useless on 2.6. Please use the Oops in its original format +(from dmesg, etc). Ignore any references in this or other docs to "decoding +the Oops" or "running it through ksymoops". If you post an Oops from 2.6 that +has been run through ksymoops, people will just tell you to repost it. + +Quick Summary +------------- + +Find the Oops and send it to the maintainer of the kernel area that seems to be +involved with the problem. Don't worry too much about getting the wrong person. +If you are unsure send it to the person responsible for the code relevant to +what you were doing. If it occurs repeatably try and describe how to recreate +it. That's worth even more than the oops. + +If you are totally stumped as to whom to send the report, send it to +linux-kernel@vger.kernel.org. Thanks for your help in making Linux as +stable as humanly possible. + +Where is the Oops? +---------------------- + +Normally the Oops text is read from the kernel buffers by klogd and +handed to syslogd which writes it to a syslog file, typically +/var/log/messages (depends on /etc/syslog.conf). Sometimes klogd dies, +in which case you can run dmesg > file to read the data from the kernel +buffers and save it. Or you can cat /proc/kmsg > file, however you +have to break in to stop the transfer, kmsg is a "never ending file". +If the machine has crashed so badly that you cannot enter commands or +the disk is not available then you have three options :- + +(1) Hand copy the text from the screen and type it in after the machine + has restarted. Messy but it is the only option if you have not + planned for a crash. Alternatively, you can take a picture of + the screen with a digital camera - not nice, but better than + nothing. If the messages scroll off the top of the console, you + may find that booting with a higher resolution (eg, vga=791) + will allow you to read more of the text. (Caveat: This needs vesafb, + so won't help for 'early' oopses) + +(2) Boot with a serial console (see Documentation/serial-console.txt), + run a null modem to a second machine and capture the output there + using your favourite communication program. Minicom works well. + +(3) Use Kdump (see Documentation/kdump/kdump.txt), + extract the kernel ring buffer from old memory with using dmesg + gdbmacro in Documentation/kdump/gdbmacros.txt. + + +Full Information +---------------- + +NOTE: the message from Linus below applies to 2.4 kernel. I have preserved it +for historical reasons, and because some of the information in it still +applies. Especially, please ignore any references to ksymoops. + +From: Linus Torvalds <torvalds@osdl.org> + +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, 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). + +Alternatively, you can use the shell script in scripts/decodecode. +Its usage is: decodecode < oops.txt + +The hex bytes that follow "Code:" may (in some architectures) have a series +of bytes that precede the current instruction pointer as well as bytes at and +following the current instruction pointer. In some cases, one instruction +byte or word is surrounded by <> or (), as in "<86>" or "(f00d)". These +<> or () markings indicate the current instruction pointer. Example from +i386, split into multiple lines for readability: + +Code: f9 0f 8d f9 00 00 00 8d 42 0c e8 dd 26 11 c7 a1 60 ea 2b f9 8b 50 08 a1 +64 ea 2b f9 8d 34 82 8b 1e 85 db 74 6d 8b 15 60 ea 2b f9 <8b> 43 04 39 42 54 +7e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0 + +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 + + +--------------------------------------------------------------------------- +Tainted kernels: + +Some oops reports contain the string 'Tainted: ' after the program +counter. This indicates that the kernel has been tainted by some +mechanism. The string is followed by a series of position-sensitive +characters, each representing a particular tainted value. + + 1: 'G' if all modules loaded have a GPL or compatible license, 'P' if + any proprietary module has been loaded. Modules without a + MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by + insmod as GPL compatible are assumed to be proprietary. + + 2: 'F' if any module was force loaded by "insmod -f", ' ' if all + modules were loaded normally. + + 3: 'S' if the oops occurred on an SMP kernel running on hardware that + hasn't been certified as safe to run multiprocessor. + Currently this occurs only on various Athlons that are not + SMP capable. + + 4: 'R' if a module was force unloaded by "rmmod -f", ' ' if all + modules were unloaded normally. + + 5: 'M' if any processor has reported a Machine Check Exception, + ' ' if no Machine Check Exceptions have occurred. + + 6: 'B' if a page-release function has found a bad page reference or + some unexpected page flags. + + 7: 'U' if a user or user application specifically requested that the + Tainted flag be set, ' ' otherwise. + + 8: 'D' if the kernel has died recently, i.e. there was an OOPS or BUG. + + 9: 'A' if the ACPI table has been overridden. + + 10: 'W' if a warning has previously been issued by the kernel. + (Though some warnings may set more specific taint flags.) + + 11: 'C' if a staging driver has been loaded. + + 12: 'I' if the kernel is working around a severe bug in the platform + firmware (BIOS or similar). + + 13: 'O' if an externally-built ("out-of-tree") module has been loaded. + +The primary reason for the 'Tainted: ' string is to tell kernel +debuggers if this is a clean kernel or if anything unusual has +occurred. Tainting is permanent: even if an offending module is +unloaded, the tainted value remains to indicate that the kernel is not +trustworthy. |