Re: [PATCH 3/8] Deliver signal in hardware single step

[Pedro Alves <palves@redhat.com>]

On 03/04/2016 10:44 AM, Yao Qi wrote:
> GDBserver doesn't deliver signal when stepping over a breakpoint even
> hardware single step is used.  When GDBserver started to step over
> (thread creation) breakpoint for mutlit-threaded debugging in 2002 [1],
> GDBserver behaves this way.
> 
> This behaviour gets trouble on conditional breakpoints on branch to
> self instruction like this,
> 
>     0x00000000004005b6 <+29>:	jmp    0x4005b6 <main+29>
> 
> and I set breakpoint
> 
> $(gdb) break branch-to-self.c:43 if counter > 3
> 
> and the variable counter will be set to 5 in SIGALRM signal handler.
> Since GDBserver keeps stepping over breakpoint, the SIGALRM can never
> be dequeued and delivered to the inferior, so the program can't stop.
> The test can be found in gdb.base/branch-to-self.exp.
> 
> I can understand why does GDBserver queue signal for software single
> step, but I can't figure out a reason we should queue signal for
> hardware single step.  With this patch applied, GDBserver forward the
> signal to inferior and the program can stop correctly.
> 
> [1] PATCH: Multithreaded debugging for gdbserver
>      https://sourceware.org/ml/gdb-patches/2002-06/msg00157.html
> 

Because the signal handler might recurse and call the same code
that had the breakpoint (or some other removed breakpoint), and thus
we'd miss a breakpoint hit in the signal handler.

GDB / infrun.c handles it here:

      if (ecs->event_thread->prev_pc == stop_pc
	  && ecs->event_thread->control.trap_expected
	  && ecs->event_thread->control.step_resume_breakpoint == NULL)
	{
	  int was_in_line;

	  /* We were just starting a new sequence, attempting to
	     single-step off of a breakpoint and expecting a SIGTRAP.
	     Instead this signal arrives.  This signal will take us out
	     of the stepping range so GDB needs to remember to, when
	     the signal handler returns, resume stepping off that
	     breakpoint.  */
	  /* To simplify things, "continue" is forced to use the same
	     code paths as single-step - set a breakpoint at the
	     signal return address and then, once hit, step off that
	     breakpoint.  */

IIRC, some of sigstep.exp, signull.exp, signest.exp exercise this.

Note that this also lets all threads run while the signal
handler runs.

Thanks,
Pedro Alves