How do you read a segfault kernel log message

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This can be a very simple question, I'm am attempting to debug an application which generates the following segfault error in the kern.log

kernel: myapp[15514]: segfault at 794ef0 ip 080513b sp 794ef0 error 6 in myapp[8048000+24000]

Here are my questions:

  1. Is there any documentation as to what are the diff error numbers on segfault, in this instance it is error 6, but i've seen error 4, 5

  2. What is the meaning of the information at bf794ef0 ip 0805130b sp bf794ef0 and myapp[8048000+24000]?

So far i was able to compile with symbols, and when i do a x 0x8048000+24000 it returns a symbol, is that the correct way of doing it? My assumptions thus far are the following:

  • sp = stack pointer?
  • ip = instruction pointer
  • at = ????
  • myapp[8048000+24000] = address of symbol?
syslog segmentation-fault unix c c++
3 Answers

Based on my limited knowledge, your assumptions are correct.

  • sp = stack pointer
  • ip = instruction pointer
  • myapp[8048000+24000] = address

If I were debugging the problem I would modify the code to produce a core dump or log a stack backtrace on the crash. You might also run the program under (or attach) GDB.

The error code is just the architectural error code for page faults and seems to be architecture specific. They are often documented in arch/*/mm/fault.c in the kernel source. My copy of Linux/arch/i386/mm/fault.c has the following definition for error_code:

  • bit 0 == 0 means no page found, 1 means protection fault
  • bit 1 == 0 means read, 1 means write
  • bit 2 == 0 means kernel, 1 means user-mode

My copy of Linux/arch/x86_64/mm/fault.c adds the following:

  • bit 3 == 1 means fault was an instruction fetch
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