GDB 15/16 crashing in add_thread_silent()

[Paul Smith]

On Fri, 2025-11-14 at 14:25 -0500, Simon Marchi wrote:
> If your core is threaded, you shouldn't get to that fallback "if" at
> all.  This is where GDB should add all your threads:
> 
>   /* Build up thread list from BFD sections, and possibly set the
>      current thread to the .reg/NN section matching the .reg
>      section.  */
>   asection *reg_sect
>     = bfd_get_section_by_name (current_program_space->core_bfd (), ".reg");
>   for (asection *sect : gdb_bfd_sections (current_program_space->core_bfd ()))
>     add_to_thread_list (sect, reg_sect, inf);
> 
> If this doesn't add any threads, then you need to dig to understand
> why BFD doesn't create the .reg pseudo sections.

I applied this patch, which I think is correct:

--- a/gdb/corelow.c     2025-04-20 13:22:05.000000000 -0400
+++ b/gdb/corelow.c     2025-11-14 14:17:57.220145722 -0500
@@ -1120,7 +1120,7 @@
       thread_info *thread = first_thread_of_inferior (inf);

       if (thread == NULL)
-       thread = add_thread_silent (target, ptid_t (CORELOW_PID));
+       thread = add_thread_silent (target, ptid_t (pid));

       switch_to_thread (thread);
     }

Earlier if the PID couldn't be found then pid is set to CORELOW_PID
anyway, so this works and prevents the crash (although, I think GDB
should check for the nullptr return and do _something_ non-crashy...
maybe?)

After preventing the crash I get these errors which seem to align with
your diagnosis:

  warning: Couldn't find general-purpose registers in core file.

  warning: Unexpected size of section `.reg2' in core file.
  Cannot access memory at address 0x84a21264
  Cannot access memory at address 0x84a21260
  Cannot access memory at address 0x84a21260
  Core was generated by `myprogram'.

  warning: Couldn't find general-purpose registers in core file.

  warning: Unexpected size of section `.reg2' in core file.

and the core is unusable:

  (gdb) bt
  #0  <unavailable> in ?? ()
  Backtrace stopped: not enough registers or memory available to unwind further

  (gdb) thr a a bt

  Thread 1 (process 9168):
  #0  <unavailable> in ?? ()
  Backtrace stopped: not enough registers or memory available to unwind further

However, if I use the native GDB 8.2 that comes with Rocky Linux, then
it will open the core file without these errors, and even show me the
backtrace for all threads.