Re: Fwd: Question regarding core dump debugging using gdb on armv4

[Abhijit Ray Chaudhury <abhijit dot ray dot chaudhury at gmail dot com>]

Pedro,

That is exactly what I wanted to do. I want get back traces of all the
running treads and the stack variables.

I would dump the NOTE PHDRS, which has NT_PRSTATTUS, NT_PRPSINFO,
NT_AUXV and a bunch of NT_PRSTATUS structures.
Then I need to decide which of the PHDRS ( which are of load type) ,
to be dumped.

>From your answer it looked like I need lot of ELF parsing , so I think
I need to invoke user level program when core dump happens.

Do you know of any documents describing the shared object loading
process by gdb from core files ?

Thanks in advance,
-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
>