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

[Alex Bennée]

Matheus Tavares Bernardino <quic_mathbern@quicinc.com> writes:

> Hi, Nick.
>
>> Nicholas Piggin <npiggin@gmail.com> wrote:
>>
>> 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.
>
> I just noticed now (as I was integrating the latest upstream patches
> with our downstream qemu-system-hexagon) that this causes the
> gdbstub-untimely-packet tcg test to fail.
>
> My first thought was that, if 0x3 is the only valid case where we will
> read a char when the cpu is running, perhaps not issuing the stop-reply
> isn't that bad as GDB would ignore it anyways. E.g. from a `set debug
> remote 1` output:
>
>   Sending packet: $qSupported:multiprocess+;swbreak+;hwbreak+;qRelocInsn+;
>                    fork-events+;vfork-events+;exec-events+;vContSupported+;
>                  QThreadEvents+;no-resumed+;
>                  xmlRegisters=i386#6a...
>   Packet instead of Ack, ignoring it
>
> So, perhaps, we could do:
>
> diff --git a/gdbstub/gdbstub.c b/gdbstub/gdbstub.c
> index f123b40ce7..8af066301a 100644
> --- a/gdbstub/gdbstub.c
> +++ b/gdbstub/gdbstub.c
> @@ -2055,8 +2055,9 @@ void gdb_read_byte(uint8_t ch)
>           */
>          if (ch != 0x03) {
>              warn_report("gdbstub: client sent packet while target
> running\n");

This warning seems to be triggering either way, investigating now.

> +        } else {
> +            gdbserver_state.allow_stop_reply = true;
>          }
> -        gdbserver_state.allow_stop_reply = true;
>          vm_stop(RUN_STATE_PAUSED);
>      } else
>  #endif
> -- >8 --
>
> Alternatively, since GDB ignores the packet anyways, should we just let
> this be and refactor/remove the test?


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro