[PATCH] gdb: Don't ignore all SIGSTOP when the signal handler is set to pass

[Pedro Alves]

On 9/10/19 6:37 PM, Andrew Burgess wrote:
> It was observed that in a multi-threaded application on GNU/Linux,
> that if the user has set the SIGSTOP to be pass (using GDB's handle
> command) then the inferior would hang upon hitting a breakpoint.
> 
> What happens is that when a thread hits the breakpoint GDB tries to
> stop all of the other threads by sending them a SIGSTOP and setting
> the stop_requested flag in the target_ops structure - this can be seen
> in infrun.c:stop_all_threads.
> 
> GDB then waits for all of the other threads to stop.
> 
> When the SIGSTOP event arrives we eventually end up in
> linux-nat.c:linux_nat_filter_event, which has the job of deciding if
> the event we're looking at (the SIGSTOP arriving in this case) is
> something that should be reported back to the core of GDB.
> 
> One of the final actions of this function is to check if we stopped
> due to a signal, and if we did, and the signal has been set to 'pass'
> by the user then we ignore the event and resume the thread.
> 
> This code already has some conditions in place that mean the event is
> reported to GDB even if the signal is in the set of signals to be
> passed to the inferior.
> 
> In this commit I extend this condition to such that:
> 
>   If the signal is a SIGSTOP, and the thread's stop_requested flag is
>   set (indicating we're waiting for the thread to stop with a SIGSTOP)
>   then we should report this SIGSTOP to GDB and not pass it to the
>   inferior.
> 
> With this change in place the test now passes.  Regression tested on
> x86-64 GNU/Linux with no regressions.

Indeed, makes sense.  

I checked that gdbserver passes the new test too.  (It knows to always
report an event when gdb tries to stop a thread with 'vCont;t'.)

> +/* The worker thread just spins forever.  */
> +void*
> +thread_worker (void *payload)
> +{
> +  while (1)
> +    {
> +      sleep (1);
> +    }
> +
> +  return NULL;
> +}
> +

> +/* A place for GDB to place a breakpoint.   */
> +void
> +breakpt ()
> +{
> +  asm ("" ::: "memory");

I wonder whether this barrier is a good idea, given
that it limits portability to gcc and clang.

I checked that both clang and gcc still manage to
eliminate the function with "-flto -O2", the barrier
didn't help then.  '__attribute__((used))' instead
of the barrier works though.

> +}

> +
> +/* Create a worker thread that just spins forever, then enter a loop
> +   periodically calling the BREAKPT function.  */
> +int main() {

GNU formatting.

> +  spawn_thread ();
> +
> +  while (1)
> +    {
> +      sleep (1);
> +
> +      breakpt ();
> +    }
> +
> +  return 0;
> +}

It's better to avoid that the testcase really runs forever, in case GDB
crashes and the system doesn't kill the process.  Easily fixed with
an "alarm" call at the top of main.

LGTM otherwise.

BTW, FYI, I saw this:

$ git am /tmp/sigstop.mbox
Applying: gdb: Don't ignore all SIGSTOP when the signal handler is set to pass
.git/rebase-apply/patch:104: new blank line at EOF.
+
.git/rebase-apply/patch:162: new blank line at EOF.
+
warning: 2 lines add whitespace errors.

Thanks,
Pedro Alves