267 lines
12 KiB
Plaintext
267 lines
12 KiB
Plaintext
NOTE: ksymoops is useless on 2.6. Please use the Oops in its original format
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(from dmesg, etc). Ignore any references in this or other docs to "decoding
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the Oops" or "running it through ksymoops". If you post an Oops from 2.6 that
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has been run through ksymoops, people will just tell you to repost it.
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Quick Summary
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-------------
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Find the Oops and send it to the maintainer of the kernel area that seems to be
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involved with the problem. Don't worry too much about getting the wrong person.
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If you are unsure send it to the person responsible for the code relevant to
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what you were doing. If it occurs repeatably try and describe how to recreate
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it. That's worth even more than the oops.
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If you are totally stumped as to whom to send the report, send it to
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linux-kernel@vger.kernel.org. Thanks for your help in making Linux as
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stable as humanly possible.
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Where is the Oops?
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----------------------
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Normally the Oops text is read from the kernel buffers by klogd and
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handed to syslogd which writes it to a syslog file, typically
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/var/log/messages (depends on /etc/syslog.conf). Sometimes klogd dies,
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in which case you can run dmesg > file to read the data from the kernel
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buffers and save it. Or you can cat /proc/kmsg > file, however you
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have to break in to stop the transfer, kmsg is a "never ending file".
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If the machine has crashed so badly that you cannot enter commands or
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the disk is not available then you have three options :-
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(1) Hand copy the text from the screen and type it in after the machine
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has restarted. Messy but it is the only option if you have not
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planned for a crash. Alternatively, you can take a picture of
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the screen with a digital camera - not nice, but better than
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nothing. If the messages scroll off the top of the console, you
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may find that booting with a higher resolution (eg, vga=791)
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will allow you to read more of the text. (Caveat: This needs vesafb,
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so won't help for 'early' oopses)
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(2) Boot with a serial console (see Documentation/serial-console.txt),
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run a null modem to a second machine and capture the output there
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using your favourite communication program. Minicom works well.
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(3) Use Kdump (see Documentation/kdump/kdump.txt),
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extract the kernel ring buffer from old memory with using dmesg
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gdbmacro in Documentation/kdump/gdbmacros.txt.
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Full Information
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----------------
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NOTE: the message from Linus below applies to 2.4 kernel. I have preserved it
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for historical reasons, and because some of the information in it still
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applies. Especially, please ignore any references to ksymoops.
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From: Linus Torvalds <torvalds@osdl.org>
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How to track down an Oops.. [originally a mail to linux-kernel]
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The main trick is having 5 years of experience with those pesky oops
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messages ;-)
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Actually, there are things you can do that make this easier. I have two
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separate approaches:
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gdb /usr/src/linux/vmlinux
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gdb> disassemble <offending_function>
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That's the easy way to find the problem, at least if the bug-report is
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well made (like this one was - run through ksymoops to get the
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information of which function and the offset in the function that it
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happened in).
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Oh, it helps if the report happens on a kernel that is compiled with the
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same compiler and similar setups.
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The other thing to do is disassemble the "Code:" part of the bug report:
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ksymoops will do this too with the correct tools, but if you don't have
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the tools you can just do a silly program:
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char str[] = "\xXX\xXX\xXX...";
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main(){}
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and compile it with gcc -g and then do "disassemble str" (where the "XX"
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stuff are the values reported by the Oops - you can just cut-and-paste
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and do a replace of spaces to "\x" - that's what I do, as I'm too lazy
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to write a program to automate this all).
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Alternatively, you can use the shell script in scripts/decodecode.
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Its usage is: decodecode < oops.txt
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The hex bytes that follow "Code:" may (in some architectures) have a series
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of bytes that precede the current instruction pointer as well as bytes at and
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following the current instruction pointer. In some cases, one instruction
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byte or word is surrounded by <> or (), as in "<86>" or "(f00d)". These
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<> or () markings indicate the current instruction pointer. Example from
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i386, split into multiple lines for readability:
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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
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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
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7e 04 40 89 42 54 8b 43 04 3b 05 00 f6 52 c0
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Finally, if you want to see where the code comes from, you can do
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cd /usr/src/linux
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make fs/buffer.s # or whatever file the bug happened in
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and then you get a better idea of what happens than with the gdb
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disassembly.
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Now, the trick is just then to combine all the data you have: the C
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sources (and general knowledge of what it _should_ do), the assembly
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listing and the code disassembly (and additionally the register dump you
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also get from the "oops" message - that can be useful to see _what_ the
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corrupted pointers were, and when you have the assembler listing you can
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also match the other registers to whatever C expressions they were used
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for).
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Essentially, you just look at what doesn't match (in this case it was the
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"Code" disassembly that didn't match with what the compiler generated).
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Then you need to find out _why_ they don't match. Often it's simple - you
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see that the code uses a NULL pointer and then you look at the code and
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wonder how the NULL pointer got there, and if it's a valid thing to do
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you just check against it..
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Now, if somebody gets the idea that this is time-consuming and requires
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some small amount of concentration, you're right. Which is why I will
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mostly just ignore any panic reports that don't have the symbol table
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info etc looked up: it simply gets too hard to look it up (I have some
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programs to search for specific patterns in the kernel code segment, and
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sometimes I have been able to look up those kinds of panics too, but
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that really requires pretty good knowledge of the kernel just to be able
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to pick out the right sequences etc..)
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_Sometimes_ it happens that I just see the disassembled code sequence
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from the panic, and I know immediately where it's coming from. That's when
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I get worried that I've been doing this for too long ;-)
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Linus
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---------------------------------------------------------------------------
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Notes on Oops tracing with klogd:
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In order to help Linus and the other kernel developers there has been
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substantial support incorporated into klogd for processing protection
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faults. In order to have full support for address resolution at least
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version 1.3-pl3 of the sysklogd package should be used.
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When a protection fault occurs the klogd daemon automatically
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translates important addresses in the kernel log messages to their
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symbolic equivalents. This translated kernel message is then
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forwarded through whatever reporting mechanism klogd is using. The
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protection fault message can be simply cut out of the message files
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and forwarded to the kernel developers.
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Two types of address resolution are performed by klogd. The first is
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static translation and the second is dynamic translation. Static
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translation uses the System.map file in much the same manner that
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ksymoops does. In order to do static translation the klogd daemon
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must be able to find a system map file at daemon initialization time.
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See the klogd man page for information on how klogd searches for map
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files.
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Dynamic address translation is important when kernel loadable modules
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are being used. Since memory for kernel modules is allocated from the
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kernel's dynamic memory pools there are no fixed locations for either
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the start of the module or for functions and symbols in the module.
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The kernel supports system calls which allow a program to determine
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which modules are loaded and their location in memory. Using these
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system calls the klogd daemon builds a symbol table which can be used
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to debug a protection fault which occurs in a loadable kernel module.
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At the very minimum klogd will provide the name of the module which
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generated the protection fault. There may be additional symbolic
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information available if the developer of the loadable module chose to
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export symbol information from the module.
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Since the kernel module environment can be dynamic there must be a
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mechanism for notifying the klogd daemon when a change in module
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environment occurs. There are command line options available which
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allow klogd to signal the currently executing daemon that symbol
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information should be refreshed. See the klogd manual page for more
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information.
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A patch is included with the sysklogd distribution which modifies the
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modules-2.0.0 package to automatically signal klogd whenever a module
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is loaded or unloaded. Applying this patch provides essentially
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seamless support for debugging protection faults which occur with
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kernel loadable modules.
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The following is an example of a protection fault in a loadable module
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processed by klogd:
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---------------------------------------------------------------------------
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Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
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Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
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Aug 29 09:51:01 blizard kernel: *pde = 00000000
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Aug 29 09:51:01 blizard kernel: Oops: 0002
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Aug 29 09:51:01 blizard kernel: CPU: 0
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Aug 29 09:51:01 blizard kernel: EIP: 0010:[oops:_oops+16/3868]
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Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
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Aug 29 09:51:01 blizard kernel: eax: 315e97cc ebx: 003a6f80 ecx: 001be77b edx: 00237c0c
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Aug 29 09:51:01 blizard kernel: esi: 00000000 edi: bffffdb3 ebp: 00589f90 esp: 00589f8c
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Aug 29 09:51:01 blizard kernel: ds: 0018 es: 0018 fs: 002b gs: 002b ss: 0018
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Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
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Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
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Aug 29 09:51:01 blizard kernel: 00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
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Aug 29 09:51:01 blizard kernel: bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
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Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
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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
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---------------------------------------------------------------------------
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Dr. G.W. Wettstein Oncology Research Div. Computing Facility
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Roger Maris Cancer Center INTERNET: greg@wind.rmcc.com
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820 4th St. N.
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Fargo, ND 58122
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Phone: 701-234-7556
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---------------------------------------------------------------------------
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Tainted kernels:
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Some oops reports contain the string 'Tainted: ' after the program
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counter. This indicates that the kernel has been tainted by some
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mechanism. The string is followed by a series of position-sensitive
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characters, each representing a particular tainted value.
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1: 'G' if all modules loaded have a GPL or compatible license, 'P' if
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any proprietary module has been loaded. Modules without a
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MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
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insmod as GPL compatible are assumed to be proprietary.
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2: 'F' if any module was force loaded by "insmod -f", ' ' if all
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modules were loaded normally.
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3: 'S' if the oops occurred on an SMP kernel running on hardware that
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hasn't been certified as safe to run multiprocessor.
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Currently this occurs only on various Athlons that are not
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SMP capable.
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4: 'R' if a module was force unloaded by "rmmod -f", ' ' if all
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modules were unloaded normally.
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5: 'M' if any processor has reported a Machine Check Exception,
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' ' if no Machine Check Exceptions have occurred.
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6: 'B' if a page-release function has found a bad page reference or
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some unexpected page flags.
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7: 'U' if a user or user application specifically requested that the
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Tainted flag be set, ' ' otherwise.
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8: 'D' if the kernel has died recently, i.e. there was an OOPS or BUG.
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9: 'A' if the ACPI table has been overridden.
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10: 'W' if a warning has previously been issued by the kernel.
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11: 'C' if a staging driver has been loaded.
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The primary reason for the 'Tainted: ' string is to tell kernel
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debuggers if this is a clean kernel or if anything unusual has
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occurred. Tainting is permanent: even if an offending module is
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unloaded, the tainted value remains to indicate that the kernel is not
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trustworthy.
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