Adding a missing NT_FILES note to a core file so gdb can load solibs for it

Bogdan Harjoc harjoc@gmail.com
Mon Jan 25 15:56:00 GMT 2016


Thanks, a comment in elf_locate_base() confirms NT_FILES wouldn't work:

/* Look for DT_MIPS_RLD_MAP first.  MIPS executables use this instead
of DT_DEBUG [...] */

Solib loading apparently fails here:

#0 read_memory_typed_address (info->debug_base + lmo->r_map_offset)
the map
#1 solib_svr4_r_map

because read_memory...() reads 0x0 from info->debug_base + lmo->r_map_offset.

What I can't figure out is where the rld map should be in the core file.

Thanks.

On Mon, Jan 25, 2016 at 4:22 PM, Jan Kratochvil
<jan.kratochvil@redhat.com> wrote:
> On Mon, 25 Jan 2016 15:06:05 +0100, Bogdan Harjoc wrote:
>> So I'd like to ask whether this has any chance of working.
>
> No.  GDB does not support NT_FILE reading.  GDB follows DT_DEBUG:
>         readelf -d execbinary|grep -w DEBUG
>
> Besides that it would not be completely correct, depending on the point of
> view.  mmap()ped shared library will be present in NT_FILE but GDB will not
> shows it as it was not dlopen()ed (either by a call or via DT_NEEDED).
>
> When GDB cannot read shared libraries it usually means your executable does
> not exactly match the one that was core dumped.  To make them matching one
> should match build-id from the core file with that of the executable:
>         eu-unstrip -n --core=corefile
>         readelf -n execbinary|grep -A1 NT_GNU_BUILD_ID
> IIRC ld-linux.so also needs to match, I cannot remember why now.
>
>
> Jan



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