Fwd: Question regarding core dump debugging using gdb on armv4

Abhijit Ray Chaudhury abhijit.ray.chaudhury@gmail.com
Mon Apr 8 03:54:00 GMT 2013


Pedro,

In solib-svr4.c function

static CORE_ADDR
elf_locate_base (void) {
...
/* Find DT_DEBUG.  */
  if (scan_dyntag (DT_DEBUG, exec_bfd, &dyn_ptr)
      || scan_dyntag_auxv (DT_DEBUG, &dyn_ptr))
    return dyn_ptr;

  /* This may be a static executable.  Look for the symbol
     conventionally named _r_debug, as a last resort.  */
  msymbol = lookup_minimal_symbol ("_r_debug", NULL, symfile_objfile);
  if (msymbol != NULL)
    return SYMBOL_VALUE_ADDRESS (msymbol);

Are you asking me going through this section of the code ?

Thanks,
-Abhijit

On Thu, Apr 4, 2013 at 2:49 PM, Pedro Alves <palves@redhat.com> wrote:
> 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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