Debugging ld.so in gdb

[Florian Weimer]

* Jacob Kroon via Gdb:

> Since a week or two I have started to see a segfault on my updated
> Fedora 35 system. I suspect the segfault is related to a recent glibc
> update.
>
> The segfault I see happens when I run the following:
>
> $ ldd ./mylib.so
>
> I narrowed it down to running:
>
> $ LD_TRACE_LOADED_OBJECTS=1 /lib64/ld-linux-x86-64.so.2 ./mylib.so
>
> "coredumpctl info" gives me:
>
>> Stack trace of thread 143567:
>> #0  0x00007ff428f73590 n/a (n/a + 0x0)
>> #1  0x00007ff428f8af0f _dl_map_object_deps (/usr/lib64/ld-linux-x86-64.so.2 + 0x3f0f)
>> #2  0x00007ff428fa6970 dl_main (/usr/lib64/ld-linux-x86-64.so.2 + 0x1f970)
>> #3  0x00007ff428fa2c7c _dl_sysdep_start (/usr/lib64/ld-linux-x86-64.so.2 + 0x1bc7c)
>> #4  0x00007ff428fa4678 _dl_start_final (/usr/lib64/ld-linux-x86-64.so.2 + 0x1d678)
>> #5  0x00007ff428fa36a8 _start (/usr/lib64/ld-linux-x86-64.so.2 + 0x1c6a8)
>
> but inspecting in gdb using "coredumpctl debug" doesn't give me any sane
> backtrace.
>
> The .so is part of a Yocto build. If I copy the file out from its build
> directory to $HOME and run ldd on it, then there is no crash. So I
> suspect RUNPATH is involved somehow since it contains $ORIGIN.
>
> Any ideas of what I can do to investigate further ?

I suggest to run ld.so under GDB, with

  set startup-with-shell off
  set environment LD_TRACE_LOADED_OBJECTS 1
  b _start
  run ./mylib.so
  record btrace pt
  continue

And after the crash, look at

  record instruction-history

to see how it reached the crash.  This assumes that you have an
execution environment that supports branch tracing.

Thanks,
Florian