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

User · Answer

Let s go to the source -- 2 6 32  for example   The message is printed by show signal msg   function in arch x86 mm fault c if the show unhandled signals sysctl is set    error  is not an errno nor a signal number  it s a  page fault error code  -- see definition of enum x86 pf error code     7fa44d2f8000 f6f000   is starting address and size of virtual memory area where offending object was mapped at the time of crash   Value of  ip  should fit in this region   With this info in hand  it should be easy to find offending code in gdb

User · Answer

Error 4 means  The cause was a user-mode read resulting in no page being found    There s a tool that decodes it here   Here s the definition from the kernel  Keep in mind that 4 means that bit 2 is set and no other bits are set  If you convert it to binary that becomes clear         Page fault error code bits         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         bit 3    1 means use of reserved bit detected         bit 4    1 means fault was an instruction fetch      define PF PROT          1 lt  lt 0   define PF WRITE         1 lt  lt 1   define PF USER          1 lt  lt 2   define PF RSVD          1 lt  lt 3   define PF INSTR         1 lt  lt 4    Now then   ip 00007f9bebcca90d  means the instruction pointer was at 0x00007f9bebcca90d when the segfault happened    libQtWebKit so 4 5 2 7f9beb83a000 f6f000   tells you    The object the crash was in    libQtWebKit so 4 5 2  The base address of that object  7f9beb83a000  How big that object is    f6f000    If you take the base address and subtract it from the ip  you get the offset into that object   0x00007f9bebcca90d - 0x7f9beb83a000   0x49090D   Then you can run addr2line on it   addr2line -e  usr lib64 qt45 lib libQtWebKit so 4 5 2 -fCi 0x49090D       0   In my case it wasn t successful  either the copy I installed isn t identical to yours  or it s stripped

User · Answer

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

[linux] Interpreting segfault messages

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