FreeBSD/Linux Kernel Cross Reference
sys/Documentation/oops-tracing.txt

     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.

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