Documentation/oops-tracing.txt

   1 NOTE: ksymoops is useless on 2.6.  Please use the Oops in its original format
   2 (from dmesg, etc).  Ignore any references in this or other docs to "decoding
   3 the Oops" or "running it through ksymoops".  If you post an Oops from 2.6 that
   4 has been run through ksymoops, people will just tell you to repost it.
   5
   6 Quick Summary
   7 -------------
   8
   9 Find the Oops and send it to the maintainer of the kernel area that seems to be
  10 involved with the problem.  Don't worry too much about getting the wrong person.
  11 If you are unsure send it to the person responsible for the code relevant to
  12 what you were doing.  If it occurs repeatably try and describe how to recreate
  13 it.  That's worth even more than the oops.
  14
  15 If you are totally stumped as to whom to send the report, send it to
  16 linux-kernel@vger.kernel.org. Thanks for your help in making Linux as
  17 stable as humanly possible.
  18
  19 Where is the Oops?
  20 ----------------------
  21
  22 Normally the Oops text is read from the kernel buffers by klogd and
  23 handed to syslogd which writes it to a syslog file, typically
  24 /var/log/messages (depends on /etc/syslog.conf).  Sometimes klogd dies,
  25 in which case you can run dmesg > file to read the data from the kernel
  26 buffers and save it.  Or you can cat /proc/kmsg > file, however you
  27 have to break in to stop the transfer, kmsg is a "never ending file".
  28 If the machine has crashed so badly that you cannot enter commands or
  29 the disk is not available then you have three options :-
  30
  31 (1) Hand copy the text from the screen and type it in after the machine
  32     has restarted.  Messy but it is the only option if you have not
  33     planned for a crash. Alternatively, you can take a picture of
  34     the screen with a digital camera - not nice, but better than
  35     nothing.  If the messages scroll off the top of the console, you
  36     may find that booting with a higher resolution (eg, vga=791)
  37     will allow you to read more of the text. (Caveat: This needs vesafb,
  38     so won't help for 'early' oopses)
  39
  40 (2) Boot with a serial console (see Documentation/serial-console.txt),
  41     run a null modem to a second machine and capture the output there
  42     using your favourite communication program.  Minicom works well.
  43
  44 (3) Patch the kernel with one of the crash dump patches.  These save
  45     data to a floppy disk or video rom or a swap partition.  None of
  46     these are standard kernel patches so you have to find and apply
  47     them yourself.  Search kernel archives for kmsgdump, lkcd and
  48     oops+smram.
  49
  50
  51 Full Information
  52 ----------------
  53
  54 NOTE: the message from Linus below applies to 2.4 kernel.  I have preserved it
  55 for historical reasons, and because some of the information in it still
  56 applies.  Especially, please ignore any references to ksymoops.
  57
  58 From: Linus Torvalds <torvalds@osdl.org>
  59
  60 How to track down an Oops.. [originally a mail to linux-kernel]
  61
  62 The main trick is having 5 years of experience with those pesky oops
  63 messages ;-)
  64
  65 Actually, there are things you can do that make this easier. I have two
  66 separate approaches:
  67
  68         gdb /usr/src/linux/vmlinux
  69         gdb> disassemble <offending_function>
  70
  71 That's the easy way to find the problem, at least if the bug-report is
  72 well made (like this one was - run through ksymoops to get the
  73 information of which function and the offset in the function that it
  74 happened in).
  75
  76 Oh, it helps if the report happens on a kernel that is compiled with the
  77 same compiler and similar setups.
  78
  79 The other thing to do is disassemble the "Code:" part of the bug report:
  80 ksymoops will do this too with the correct tools, but if you don't have
  81 the tools you can just do a silly program:
  82
  83         char str[] = "\xXX\xXX\xXX...";
  84         main(){}
  85
  86 and compile it with gcc -g and then do "disassemble str" (where the "XX"
  87 stuff are the values reported by the Oops - you can just cut-and-paste
  88 and do a replace of spaces to "\x" - that's what I do, as I'm too lazy
  89 to write a program to automate this all).
  90
  91 Finally, if you want to see where the code comes from, you can do
  92
  93         cd /usr/src/linux
  94         make fs/buffer.s        # or whatever file the bug happened in
  95
  96 and then you get a better idea of what happens than with the gdb
  97 disassembly.
  98
  99 Now, the trick is just then to combine all the data you have: the C
 100 sources (and general knowledge of what it _should_ do), the assembly
 101 listing and the code disassembly (and additionally the register dump you
 102 also get from the "oops" message - that can be useful to see _what_ the
 103 corrupted pointers were, and when you have the assembler listing you can
 104 also match the other registers to whatever C expressions they were used
 105 for).
 106
 107 Essentially, you just look at what doesn't match (in this case it was the
 108 "Code" disassembly that didn't match with what the compiler generated).
 109 Then you need to find out _why_ they don't match. Often it's simple - you
 110 see that the code uses a NULL pointer and then you look at the code and
 111 wonder how the NULL pointer got there, and if it's a valid thing to do
 112 you just check against it..
 113
 114 Now, if somebody gets the idea that this is time-consuming and requires
 115 some small amount of concentration, you're right. Which is why I will
 116 mostly just ignore any panic reports that don't have the symbol table
 117 info etc looked up: it simply gets too hard to look it up (I have some
 118 programs to search for specific patterns in the kernel code segment, and
 119 sometimes I have been able to look up those kinds of panics too, but
 120 that really requires pretty good knowledge of the kernel just to be able
 121 to pick out the right sequences etc..)
 122
 123 _Sometimes_ it happens that I just see the disassembled code sequence
 124 from the panic, and I know immediately where it's coming from. That's when
 125 I get worried that I've been doing this for too long ;-)
 126
 127                 Linus
 128
 129
 130 ---------------------------------------------------------------------------
 131 Notes on Oops tracing with klogd:
 132
 133 In order to help Linus and the other kernel developers there has been
 134 substantial support incorporated into klogd for processing protection
 135 faults.  In order to have full support for address resolution at least
 136 version 1.3-pl3 of the sysklogd package should be used.
 137
 138 When a protection fault occurs the klogd daemon automatically
 139 translates important addresses in the kernel log messages to their
 140 symbolic equivalents.  This translated kernel message is then
 141 forwarded through whatever reporting mechanism klogd is using.  The
 142 protection fault message can be simply cut out of the message files
 143 and forwarded to the kernel developers.
 144
 145 Two types of address resolution are performed by klogd.  The first is
 146 static translation and the second is dynamic translation.  Static
 147 translation uses the System.map file in much the same manner that
 148 ksymoops does.  In order to do static translation the klogd daemon
 149 must be able to find a system map file at daemon initialization time.
 150 See the klogd man page for information on how klogd searches for map
 151 files.
 152
 153 Dynamic address translation is important when kernel loadable modules
 154 are being used.  Since memory for kernel modules is allocated from the
 155 kernel's dynamic memory pools there are no fixed locations for either
 156 the start of the module or for functions and symbols in the module.
 157
 158 The kernel supports system calls which allow a program to determine
 159 which modules are loaded and their location in memory.  Using these
 160 system calls the klogd daemon builds a symbol table which can be used
 161 to debug a protection fault which occurs in a loadable kernel module.
 162
 163 At the very minimum klogd will provide the name of the module which
 164 generated the protection fault.  There may be additional symbolic
 165 information available if the developer of the loadable module chose to
 166 export symbol information from the module.
 167
 168 Since the kernel module environment can be dynamic there must be a
 169 mechanism for notifying the klogd daemon when a change in module
 170 environment occurs.  There are command line options available which
 171 allow klogd to signal the currently executing daemon that symbol
 172 information should be refreshed.  See the klogd manual page for more
 173 information.
 174
 175 A patch is included with the sysklogd distribution which modifies the
 176 modules-2.0.0 package to automatically signal klogd whenever a module
 177 is loaded or unloaded.  Applying this patch provides essentially
 178 seamless support for debugging protection faults which occur with
 179 kernel loadable modules.
 180
 181 The following is an example of a protection fault in a loadable module
 182 processed by klogd:
 183 ---------------------------------------------------------------------------
 184 Aug 29 09:51:01 blizard kernel: Unable to handle kernel paging request at virtual address f15e97cc
 185 Aug 29 09:51:01 blizard kernel: current->tss.cr3 = 0062d000, %cr3 = 0062d000
 186 Aug 29 09:51:01 blizard kernel: *pde = 00000000
 187 Aug 29 09:51:01 blizard kernel: Oops: 0002
 188 Aug 29 09:51:01 blizard kernel: CPU:    0
 189 Aug 29 09:51:01 blizard kernel: EIP:    0010:[oops:_oops+16/3868]
 190 Aug 29 09:51:01 blizard kernel: EFLAGS: 00010212
 191 Aug 29 09:51:01 blizard kernel: eax: 315e97cc   ebx: 003a6f80   ecx: 001be77b   edx: 00237c0c
 192 Aug 29 09:51:01 blizard kernel: esi: 00000000   edi: bffffdb3   ebp: 00589f90   esp: 00589f8c
 193 Aug 29 09:51:01 blizard kernel: ds: 0018   es: 0018   fs: 002b   gs: 002b   ss: 0018
 194 Aug 29 09:51:01 blizard kernel: Process oops_test (pid: 3374, process nr: 21, stackpage=00589000)
 195 Aug 29 09:51:01 blizard kernel: Stack: 315e97cc 00589f98 0100b0b4 bffffed4 0012e38e 00240c64 003a6f80 00000001
 196 Aug 29 09:51:01 blizard kernel:        00000000 00237810 bfffff00 0010a7fa 00000003 00000001 00000000 bfffff00
 197 Aug 29 09:51:01 blizard kernel:        bffffdb3 bffffed4 ffffffda 0000002b 0007002b 0000002b 0000002b 00000036
 198 Aug 29 09:51:01 blizard kernel: Call Trace: [oops:_oops_ioctl+48/80] [_sys_ioctl+254/272] [_system_call+82/128]
 199 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
 200 ---------------------------------------------------------------------------
 201
 202 Dr. G.W. Wettstein           Oncology Research Div. Computing Facility
 203 Roger Maris Cancer Center    INTERNET: greg@wind.rmcc.com
 204 820 4th St. N.
 205 Fargo, ND  58122
 206 Phone: 701-234-7556
 207
 208
 209 ---------------------------------------------------------------------------
 210 Tainted kernels:
 211
 212 Some oops reports contain the string 'Tainted: ' after the program
 213 counter. This indicates that the kernel has been tainted by some
 214 mechanism.  The string is followed by a series of position-sensitive
 215 characters, each representing a particular tainted value.
 216
 217   1: 'G' if all modules loaded have a GPL or compatible license, 'P' if
 218      any proprietary module has been loaded.  Modules without a
 219      MODULE_LICENSE or with a MODULE_LICENSE that is not recognised by
 220      insmod as GPL compatible are assumed to be proprietary.
 221
 222   2: 'F' if any module was force loaded by "insmod -f", ' ' if all
 223      modules were loaded normally.
 224
 225   3: 'S' if the oops occurred on an SMP kernel running on hardware that
 226      hasn't been certified as safe to run multiprocessor.
 227      Currently this occurs only on various Athlons that are not
 228      SMP capable.
 229
 230   4: 'R' if a module was force unloaded by "rmmod -f", ' ' if all
 231      modules were unloaded normally.
 232
 233   5: 'M' if any processor has reported a Machine Check Exception,
 234      ' ' if no Machine Check Exceptions have occurred.
 235
 236   6: 'B' if a page-release function has found a bad page reference or
 237      some unexpected page flags.
 238
 239 The primary reason for the 'Tainted: ' string is to tell kernel
 240 debuggers if this is a clean kernel or if anything unusual has
 241 occurred.  Tainting is permanent: even if an offending module is
 242 unloaded, the tainted value remains to indicate that the kernel is not
 243 trustworthy.