Re: [PATCH] gdbstub: Fix client Ctrl-C handling

["Nicholas Piggin" <npiggin@gmail.com>]

On Tue Jul 11, 2023 at 9:03 PM AEST, Matheus Tavares Bernardino wrote:
> > Nicholas Piggin <npiggin@gmail.com> wrote:
> >
> > diff --git a/gdbstub/gdbstub.c b/gdbstub/gdbstub.c
> > index 6911b73c07..ce8b42eb15 100644
> > --- a/gdbstub/gdbstub.c
> > +++ b/gdbstub/gdbstub.c
> > @@ -2051,8 +2051,17 @@ void gdb_read_byte(uint8_t ch)
> >              return;
> >      }
> >      if (runstate_is_running()) {
> > -        /* when the CPU is running, we cannot do anything except stop
> > -           it when receiving a char */
> > +        /*
> > +         * When the CPU is running, we cannot do anything except stop
> > +         * it when receiving a char. This is expected on a Ctrl-C in the
> > +         * gdb client. Because we are in all-stop mode, gdb sends a
> > +         * 0x03 byte which is not a usual packet, so we handle it specially
> > +         * here, but it does expect a stop reply.
> > +         */
> > +        if (ch != 0x03) {
> > +            warn_report("gdbstub: client sent packet while target running\n");
> > +        }
> > +        gdbserver_state.allow_stop_reply = true;
> >          vm_stop(RUN_STATE_PAUSED);
> >      } else
> >  #endif
>
> Makes sense to me, but shouldn't we send the stop-reply packet only for
> Ctrl+C/0x03?

Good question.

I think if we get a character here that's not a 3, we're already in
trouble, and we eat it so even worse. Since we only send a stop packet
back when the vm stops, then if we don't send one now we might never
send it. At least if we send one then the client might have some chance
to get back to a sane state. And this does at least take us back to
behaviour before the stop filtering patch.

Could go further and only stop the machine if it was a 3, or send a
stop packet even if we were stopped, etc. but all that get further from
a minimal fix.

Thanks,
Nick