Re: [PATCH 7/8] Resume the inferior with signal rather than stepping over

[Pedro Alves <palves at redhat dot com>]

On 03/04/2016 10:44 AM, Yao Qi wrote:
> When GDBserver steps over a breakpoint using software single step, it
> enqueues the signal, single step and deliver the signal in the next
> resume if step over is not needed.  In this way, the program won't
> receive the signal if the conditional breakpoint is set a branch to
> self instruction, because the step over is always needed.
> 
> This patch removes the restriction that don't deliver the signal to
> the inferior if we are trying to reinsert a breakpoint for software
> single step and change the decision on resume vs. step-over when the
> LWP has pending signals to deliver.

Once the handler returns, it'll retrap the same breakpoint we already stopped
for, and thus I think we'll count a spurious tracepoint/breakpoint hit.

Sounds like we'll need to do like GDB does and remember that we are advancing
past a signal handler, and need to get back to stepping over the breakpoint
if/when the handler returns successfully?

> 
> gdb/gdbserver:
> 
> 2016-03-04  Yao Qi  <yao.qi@linaro.org>
> 
> 	* linux-low.c (LWP_SIGNAL_CAN_BE_DELIVERED): Adjust.
> 	(need_step_over_p): Return zero if the LWP has pending signals
> 	can be delivered on software single step target.
> ---
>   gdb/gdbserver/linux-low.c | 18 +++++++++++++++---
>   1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
> index 2330e67..9bae787 100644
> --- a/gdb/gdbserver/linux-low.c
> +++ b/gdb/gdbserver/linux-low.c
> @@ -4119,12 +4119,10 @@ single_step (struct lwp_info* lwp)
>   }
>   
>   /* The signal can not be delivered to the inferior if we are trying to
> -   reinsert a breakpoint for software single step or we're trying to
>      finish a fast tracepoint collect.  */
>   
>   #define LWP_SIGNAL_CAN_BE_DELIVERED(LWP)			\
> -  !(((LWP)->bp_reinsert != 0 && can_software_single_step ())	\
> -    || (LWP)->collecting_fast_tracepoint)
> +  !((LWP)->collecting_fast_tracepoint)
>   
>   /* Resume execution of LWP.  If STEP is nonzero, single-step it.  If
>      SIGNAL is nonzero, give it that signal.  */
> @@ -4572,6 +4570,20 @@ need_step_over_p (struct inferior_list_entry *entry, void *dummy)
>         return 0;
>       }
>   
> +  /* On software single step target, resume the inferior with signal
> +     rather than stepping over.  */
> +  if (can_software_single_step ()
> +      && lwp->pending_signals != NULL
> +      && LWP_SIGNAL_CAN_BE_DELIVERED (lwp))
> +    {
> +      if (debug_threads)
> +	debug_printf ("Need step over [LWP %ld]? Ignoring, has pending"
> +		      " signals.\n",
> +		      lwpid_of (thread));
> +
> +      return 0;

I notice this missed restoring the current thread (below).

> +    }
> +
>     saved_thread = current_thread;
>     current_thread = thread;
>   
> 


-- 
Thanks,
Pedro Alves