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-sta...@nongnu.org Cc: Matheus Tavares Bernardino <quic_mathb...@quicinc.com> Cc: Alex Bennée <alex.ben...@linaro.org> Cc: Taylor Simpson <tsimp...@quicinc.com> Reported-by: Frederic Barrat <fbar...@linux.ibm.com> Fixes: 758370052fb ("gdbstub: only send stop-reply packets when allowed to") Signed-off-by: Nicholas Piggin <npig...@gmail.com> --- Hey Fred, I'm not sure if this is the gdb hang you were seeing, but it is the one I could reproduce. Could be worth checking there are no more corner case hangs after this. Thanks, Nick gdbstub/gdbstub.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) 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 -- 2.40.1