Debugging ld.so in gdb

[Jacob Kroon]

On 2/7/22 09:36, Jacob Kroon wrote:
> Hi Florian,
> 
> On 2/4/22 18:15, Florian Weimer wrote:
>> I suspect we are writing beyond the start of the array passed to
>> _dl_sort_maps.
>>
> 
> It looks like it is writing passed the beginning of the rpo[] array in
> _dl_sort_maps_dfs(). The output below is right before the crash happens
> (stepping one instruction garbles the backtrace):
> 
>> (gdb) bt
>> #0  dfs_traversal (rpo=rpo@entry=0x7fffffffd320, map=0x7ffff7fad590, do_reldeps=do_reldeps@entry=0x0) at dl-sort-maps.c:175
>> #1  0x00007ffff7fd85d4 in dfs_traversal (do_reldeps=0x0, map=<optimized out>, rpo=0x7fffffffd320) at dl-sort-maps.c:143
>> #2  dfs_traversal (rpo=rpo@entry=0x7fffffffd320, map=0x7ffff7fadb70, do_reldeps=do_reldeps@entry=0x0) at dl-sort-maps.c:155
>> #3  0x00007ffff7fd89cd in dfs_traversal (do_reldeps=0x0, map=<optimized out>, rpo=0x7fffffffd320) at dl-sort-maps.c:143
>> #4  _dl_sort_maps_dfs (skip=<optimized out>, for_fini=<optimized out>, nmaps=15, maps=0x7ffff7953de0) at dl-sort-maps.c:233
>> #5  _dl_sort_maps (maps=maps@entry=0x7ffff7953de0, nmaps=nmaps@entry=15, skip=<optimized out>, for_fini=for_fini@entry=false) at dl-sort-maps.c:299
>> #6  0x00007ffff7fcaf0f in _dl_map_object_deps (map=<optimized out>, preloads=<optimized out>, npreloads=<optimized out>, trace_mode=<optimized out>, 
>>     open_mode=<optimized out>) at dl-deps.c:616
>> #7  0x00007ffff7fe6970 in dl_main (phdr=<optimized out>, phnum=<optimized out>, user_entry=<optimized out>, auxv=<optimized out>) at rtld.c:1968
>> #8  0x00007ffff7fe2c7c in _dl_sysdep_start (start_argptr=<optimized out>, dl_main=0x7ffff7fe4bb0 <dl_main>) at ../elf/dl-sysdep.c:264
>> #9  0x00007ffff7fe4678 in _dl_start_final (arg=0x7fffffffdec0) at rtld.c:493
>> #10 _dl_start (arg=0x7fffffffdec0) at rtld.c:587
>> #11 0x00007ffff7fe36a8 in _start ()
>> (gdb) f 0
>> #0  dfs_traversal (rpo=rpo@entry=0x7fffffffd320, map=0x7ffff7fad590, do_reldeps=do_reldeps@entry=0x0) at dl-sort-maps.c:175
>> 175       **rpo = map;
>> (gdb) print *rpo
>> $62 = (struct link_map **) 0x7fffffffd238
>> (gdb) f 4
>> #4  _dl_sort_maps_dfs (skip=<optimized out>, for_fini=<optimized out>, nmaps=15, maps=0x7ffff7953de0) at dl-sort-maps.c:233
>> 233           dfs_traversal (&rpo_head, maps[i], do_reldeps_ref);
>> (gdb) print &rpo[-1]
>> $63 = (struct link_map **) 0x7fffffffd238
> 
> I inspected the "maps" vector and it containes *multiple* entries to
> "libjvm.so", is that allowed ? I wonder if "nmaps" is calculated
> correctly, since that determines the array size. Can I verify that somehow ?
> 

Actually that is was not correct. "maps[]->l_name" does not contain any
"libjvm.so" at all, but the resulting rpo[] does contain several
"libjvm.so" entries.

Jacob