Re: [PATCH] gdbserver/linux-low: carry over stop_expected flag after exec (avoid spurious SIGSTOPs)

[Pedro Alves <pedro@palves.net>]

On 2026-06-26 15:32, simon.marchi@polymtl.ca wrote:

> What about GDB
> ==============
> 
> I tried to check if the same bug could happen with GDB's linux-nat
> target, and if the same fix was needed.  linux-nat takes a different
> approach when handling PTRACE_EVENT_EXEC.  It wipes all lwp_infos except
> the leader:
> 
>       for (lwp_info &other_lp : all_lwps_safe ())
> 	if (&other_lp != lp && other_lp.ptid.pid () == lp->ptid.pid ())
> 	  exit_lwp (&other_lp);
> 
> Here, LP is an lwp_info obtained using the event ptid of the
> PTRACE_EVENT_EXEC, therefore the leader's lwp_info (even if the exec was
> done by a non-leader).
> 
> If the exec is done by the leader, as is the case with
> gdb.base/vfork-follow-parent.exp, we are ok.  Because GDB doesn't delete
> and re-create the lwp_info, the equivalent of GDBserver's
> `lwp_info::stop_expected`, `lwp_info::signalled`, survives the exec.
> 
> If the exec is done by a non-leader, then we could be in trouble.  If
> the leader's signalled flag is not set, but the exec'ing non-leader's
> flag is set, then we'll lose it.  I suppose we could fix GDB to use
> PTRACE_GETEVENTMSG to get the exec'ing thread former id, look up the
> lwp_info for that id, and preserve that lwp_info.

Yeah, it'd be good to fix GDB too.  This sort of bug is hard to track
down, and we already know we have it.

> +      unsigned long execing_tid = event_ptid.lwp ();
> +      if (ptrace (PTRACE_GETEVENTMSG, event_ptid.lwp (), (PTRACE_TYPE_ARG3) 0,
> +		  &execing_tid) != 0)
> +	execing_tid = event_ptid.lwp ();

This gave me pause -- this is setting execing_tid to the event lwp if ptrace fails.
But execing_tid is already initialized to the event lwp.  I think it'd be clearer not
to initialize it, like:

      unsigned long execing_tid;
      if (ptrace (PTRACE_GETEVENTMSG, event_ptid.lwp (), (PTRACE_TYPE_ARG3) 0,
		  &execing_tid) != 0)
	execing_tid = event_ptid.lwp ();

Otherwise:

Approved-By: Pedro Alves <pedro@palves.net>

Thanks,
Pedro Alves