gdbstub: Fix client Ctrl-C handling

The gdb remote protocol has a special interrupt character (0x03) that is
transmitted outside the regular packet processing, and represents a
Ctrl-C pressed in the client. Despite not being a regular packet, it
does expect a regular stop response if the stub successfully stops the
running program.

See: https://sourceware.org/gdb/onlinedocs/gdb/Interrupts.html

Inhibiting the stop reply packet can lead to gdb client hang. So permit
a stop response when receiving a character from gdb that stops the vm.
Additionally, add a warning if that was not a 0x03 character, because
the gdb session is likely to end up getting confused if this happens.

Cc: qemu-stable@nongnu.org
Fixes: 7583700 ("gdbstub: only send stop-reply packets when allowed to")
Reported-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Joel Stanley <joel@jms.id.au>
Message-id: 20230711085903.304496-1-npiggin@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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