Re: [PATCH 2/2] Avoid step-over infinite loop in GDBServer

[Yao Qi <qiyaoltc at gmail dot com>]

On Tue, Nov 29, 2016 at 12:07 PM, Antoine Tremblay
<antoine.tremblay@ericsson.com> wrote:
> Before this patch, GDBServer always executed a step-over if it found a
> thread that needed one.
>
> This could be a problem in a situation exposed by non-stop-fair-events.exp
> where the code and the breakpoint placement is like so:
>
> instruction A : has a single-step breakpoint installed for thread 1 and 2
> instruction B : has a single-step breakpoint installed for thread 3
> and is a branch to A.
>

Is instruction B following instruction A?  Is it like

.L1:
 nop
 b .L1

> In this particular case:
>
>  - GDBServer stops on instruction A in thread 1.
>  - Deletes thread 1 single-step breakpoint.
>  - Starts a step-over of thread 1 to step-over the thread 2 breakpoint.
>  - GDBServer finishes a step-over and is at instruction B.
>  - GDBserver starts a step-over of thread 1 to step-over the thread 3
>    breakpoint at instruction B.

Why does GDBserver starts a step-over again?  is it because
need_step_over_p doing checks like this,

  if (breakpoint_here (pc) || fast_tracepoint_jump_here (pc))
    {
      /* Don't step over a breakpoint that GDB expects to hit
         though.  If the condition is being evaluated on the target's side
         and it evaluate to false, step over this breakpoint as well.  */
      if (gdb_breakpoint_here (pc)
          && gdb_condition_true_at_breakpoint (pc)
          && gdb_no_commands_at_breakpoint (pc))
        {
          if (debug_threads)
            debug_printf ("Need step over [LWP %ld]? yes, but found"
                          " GDB breakpoint at 0x%s; skipping step over\n",
                          lwpid_of (thread), paddress (pc));

          current_thread = saved_thread;
          return 0;
        }
      else
        {
          if (debug_threads)
            debug_printf ("Need step over [LWP %ld]? yes, "
                          "found breakpoint at 0x%s\n",
                          lwpid_of (thread), paddress (pc));

          /* We've found an lwp that needs stepping over --- return 1 so
             that find_inferior stops looking.  */
          current_thread = saved_thread;

          return 1;
        }
    }

there is a single step breakpoint on pc, and it is obviously not a
gdb breakpoint, so 1 is returned.

>  - GDBServer stops on instuction A in thread 1.
>  - GDBServer is now in an infinite loop.
>

I am wondering can we take the information that we've already step
over a breakpoint for thread A into need_step_over_p, if we see pc
is on another single step breakpoint for thread B, don't do step over.

-- 
Yao (齐尧)