Debugging ld.so in gdb

[Jacob Kroon]

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 ?

Any other ideas I should try ?

Jacob