Re: Host GDB disconnect while waiting for tracee status change

Dmitry Antipov
2017-08-23T09:37:52Z

On 08/23/2017 11:18 AM, Yao Qi wrote:

> I didn't try GDBserver under the case you described, but I find
> something related int GDB manual,
>
> https://sourceware.org/gdb/onlinedocs/gdb/Server.html
> "Therefore, when the connection drops unexpectedly, and GDB cannot ask
> gdbserver to kill its debugged processes, gdbserver stays running even
> in the target remote mode."

It's definitely stays running, but seems doesn't listen for remote connection
attempts anymore. For example:

$ cat /tmp/t-simple.c
int main (int argc, char *argv[]) { while (1) (void) argc; return 0; }

$ gdbserver --debug --attach tcp:9999 $(pidof t-simple)

$ gdb -q
(gdb) target extended-remote :9999
Remote debugging using :9999
[...skipped...]
0x00000000004004f2 in main (argc=1, argv=0x7ffec6b6b208) at /tmp/t-simple.c:1
1 int main (int argc, char *argv[]) { while (1) (void) argc; return 0; }
(gdb) c
Continuing.
Segmentation fault (core dumped) ;; Due to killall -11 gdb from another TTY

Immediately after killing gdb, gdbserver reports:
...
client connection closed
my_waitpid (-1, 0x40000001)
my_waitpid (-1, 0x40000001): status(ffffffff), 0
LWFE: waitpid(-1, ...) returned 0, ERRNO-OK
leader_pid=21352, leader_lp!=NULL=1, num_lwps=1, zombie=0
sigsuspend'ing
...

Next, a new instance of gdb is unable to reconnect:

$ gdb -q
(gdb) target extended-remote :9999
Remote debugging using :9999
Ignoring packet error, continuing...
warning: unrecognized item "timeout" in "qSupported" response
Ignoring packet error, continuing...
Remote replied unexpectedly to 'vMustReplyEmpty': timeout

And it seems that there is no similar issue in a non-stop mode. For example:

$ gdb -q
(gdb) set non-stop 1
(gdb) target extended-remote :9999
Remote debugging using :9999
[...skipped...]
Program stopped.
0x00000000004004f2 in main (argc=1, argv=0x7ffc72333088) at /tmp/t-simple.c:1
1 int main (int argc, char *argv[]) { while (1) (void) argc; return 0; }
(gdb) c
Continuing.
Segmentation fault (core dumped) ;; Due to killall -11 gdb from another TTY

Immediately after killing gdb, gdbserver reports:

...
handling possible serial event
Remote side has terminated connection. GDBserver will reopen the connection.
Listening on port 9999
...

This way, a new instance of gdb can reconnect:

$ gdb -q
(gdb) set non-stop 1
(gdb) target extended-remote :9999
Remote debugging using :9999
[...skipped...]
(gdb) interrupt
Program stopped.
0x00000000004004f2 in main (argc=1, argv=0x7ffc72333088) at /tmp/t-simple.c:1
1 int main (int argc, char *argv[]) { while (1) (void) argc; return 0; }
(gdb)

Dmitry