Fwd: Question regarding core dump debugging using gdb on armv4

Pedro Alves palves@redhat.com
Thu Apr 4 09:19:00 GMT 2013

On 04/04/2013 04:30 AM, Abhijit Ray Chaudhury wrote:
> Hi ,
> I am trying to reduce core dump size on target running linux . The
> processor is armv4 family. I tweak elf_core_dump function of linux
> kernel 2.6.23.
> I tweaked the kernel to dump only elf note, registers and stack
> segments of the running process.
> But gdb fails to load required shared object files and fails to give
> the backtraces of the running process.
> Could you please help me ascertain how gdb loads the required
> libraries from the core dump. Which ELF section of the core contains
> the information ?

GDB reads the load map off of structures in the dynamic loader (which runs
in userspace).  The dynamic loader->debugger interface has a 'struct r_debug'
structure in memory that holds the list of loaded libraries.  You need to
preserve that and whatever it references in the core.

In dynamic executables you can find where r_debug is by consulting
the DT_DEBUG dyntag, found in the .dynamic section of the executable,
which in turn can be found in the PT_DYNAMIC program header,
which is found by scanning the OS auxiliary vector, which the kernel
has access to.  All this can be seen in action in gdb's solib-svr4.c.
Dumping all of libc's memory may work too..

Pedro Alves

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