linux - Interpreting segfault messages

Question

What is the correct interpretation of the following segfault messages?

segfault at 10 ip 00007f9bebcca90d sp 00007fffb62705f0 error 4 in libQtWebKit.so.4.5.2[7f9beb83a000+f6f000]
segfault at 10 ip 00007fa44d78890d sp 00007fff43f6b720 error 4 in libQtWebKit.so.4.5.2[7fa44d2f8000+f6f000]
segfault at 11 ip 00007f2b0022acee sp 00007fff368ea610 error 4 in libQtWebKit.so.4.5.2[7f2aff9f7000+f6f000]
segfault at 11 ip 00007f24b21adcee sp 00007fff7379ded0 error 4 in libQtWebKit.so.4.5.2[7f24b197a000+f6f000]

Answer 1

This is a segfault due to following a null pointer trying to find code to run (that is, during an instruction fetch).

If this were a program, not a shared library

Run addr2line -e yourSegfaultingProgram 00007f9bebcca90d (and repeat for the other instruction pointer values given) to see where the error is happening. Better, get a debug-instrumented build, and reproduce the problem under a debugger such as gdb.

Since it's a shared library

You're hosed, unfortunately; it's not possible to know where the libraries were placed in memory by the dynamic linker after-the-fact. Reproduce the problem under gdb.

What the error means

Here's the breakdown of the fields:

• address (after the at) - the location in memory the code is trying to access (it's likely that 10 and 11 are offsets from a pointer we expect to be set to a valid value but which is instead pointing to 0)
• ip - instruction pointer, ie. where the code which is trying to do this lives
• sp - stack pointer

• error - An error code for page faults; see below for what this means on x86.

  /*
   * Page fault error code bits:
   *
   *   bit 0 ==    0: no page found       1: protection fault
   *   bit 1 ==    0: read access         1: write access
   *   bit 2 ==    0: kernel-mode access  1: user-mode access
   *   bit 3 ==                           1: use of reserved bit detected
   *   bit 4 ==                           1: fault was an instruction fetch
   */