On Fri, Feb 22, 2019 at 01:59:12PM +0200, Alberto Garcia wrote:
> There's a race condition in which the tcp_chr_read() ioc handler can
> close a connection that is being written to from another thread.
>
> Running iotest 136 in a loop triggers this problem and crashes QEMU.
>
> (gdb) bt
> #0 0x00005558b842902d in object_get_class (obj=0x0) at qom/object.c:860
> #1 0x00005558b84f92db in qio_channel_writev_full (ioc=0x0,
> iov=0x7ffc355decf0, niov=1, fds=0x0, nfds=0, errp=0x0) at io/channel.c:76
> #2 0x00005558b84e0e9e in io_channel_send_full (ioc=0x0, buf=0x5558baf5beb0,
> len=138, fds=0x0, nfds=0) at chardev/char-io.c:123
> #3 0x00005558b84e4a69 in tcp_chr_write (chr=0x5558ba460380,
> buf=0x5558baf5beb0 "...", len=138) at chardev/char-socket.c:135
> #4 0x00005558b84dca55 in qemu_chr_write_buffer (s=0x5558ba460380,
> buf=0x5558baf5beb0 "...", len=138, offset=0x7ffc355dedd0, write_all=false) at
> chardev/char.c:112
> #5 0x00005558b84dcbc2 in qemu_chr_write (s=0x5558ba460380,
> buf=0x5558baf5beb0 "...", len=138, write_all=false) at chardev/char.c:147
> #6 0x00005558b84dfb26 in qemu_chr_fe_write (be=0x5558ba476610,
> buf=0x5558baf5beb0 "...", len=138) at chardev/char-fe.c:42
> #7 0x00005558b8088c86 in monitor_flush_locked (mon=0x5558ba476610) at
> monitor.c:406
> #8 0x00005558b8088e8c in monitor_puts (mon=0x5558ba476610,
> str=0x5558ba921e49 "") at monitor.c:449
> #9 0x00005558b8089178 in qmp_send_response (mon=0x5558ba476610,
> rsp=0x5558bb161600) at monitor.c:498
> #10 0x00005558b808920c in monitor_qapi_event_emit
> (event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:526
> #11 0x00005558b8089307 in monitor_qapi_event_queue_no_reenter
> (event=QAPI_EVENT_SHUTDOWN, qdict=0x5558bb161600) at monitor.c:551
> #12 0x00005558b80896c0 in qapi_event_emit (event=QAPI_EVENT_SHUTDOWN,
> qdict=0x5558bb161600) at monitor.c:626
> #13 0x00005558b855f23b in qapi_event_send_shutdown (guest=false,
> reason=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at qapi/qapi-events-run-state.c:43
> #14 0x00005558b81911ef in qemu_system_shutdown
> (cause=SHUTDOWN_CAUSE_HOST_QMP_QUIT) at vl.c:1837
> #15 0x00005558b8191308 in main_loop_should_exit () at vl.c:1885
> #16 0x00005558b819140d in main_loop () at vl.c:1924
> #17 0x00005558b8198c84 in main (argc=18, argv=0x7ffc355df3f8,
> envp=0x7ffc355df490) at vl.c:4665
>
> This patch adds a lock to protect tcp_chr_disconnect() and
> socket_reconnect_timeout()
Can you think of any way to test this in the unit tests ? I can understand
if its too difficult but just curious if there's any viable option ?
> diff --git a/chardev/char-socket.c b/chardev/char-socket.c
> index 4fcdd8aedd..67c2eeac6d 100644
> --- a/chardev/char-socket.c
> +++ b/chardev/char-socket.c
> @@ -172,7 +172,9 @@ static int tcp_chr_write(Chardev *chr, const uint8_t
> *buf, int len)
>
> if (ret < 0 && errno != EAGAIN) {
> if (tcp_chr_read_poll(chr) <= 0) {
> + qemu_mutex_unlock(&chr->chr_write_lock);
> tcp_chr_disconnect(chr);
> + qemu_mutex_lock(&chr->chr_write_lock);
The idea of unlock & relocking here, just so tcp_chr_disconnect can
also acquire locks, doesn't make me too happy. This is really an
anti-pattern to me.
I think we should rename the existing method to be
tcp_chr_disconnect_locked() and document that it must only
be called with the write lock held, and call that from
here.
Then introduce a new wrapper for all the other callers to
carry on using which does the locking
static void tcp_chr_disconnect(Chardev *chr)
{
qemu_mutex_lock(&chr->chr_write_lock);
tcp_chr_disconnect_locked(chr);
qemu_mutex_unlock(&chr->chr_write_lock);
}
Regards,
Daniel
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