vnc_server_cut_text_caps() is not guaranteed to be called only once.
If it called twice, we finally call notifier_list_add() twice with same
element. Which leads to loopback QLIST. So, on next
notifier_list_notify() we'll loop forever and QEMU stuck.
So, let's only register new notifier if it's not yet registered.
Note, that similar check is used in vdagent_chr_recv_caps() (before
call qemu_clipboard_peer_register()), and also before
qemu_clipboard_peer_unregister() call in vdagent_disconnect() and in
vnc_disconnect_finish().
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
Hi all!
After backporting clipboard patches to our Rhel7-based downstream, we
faced Qemu stuck in notifier_list_notify():
(gdb) bt
#0 vnc_clipboard_notify (notifier=0x564427f283f8, data=0x564426c59a70) at
ui/vnc-clipboard.c:193
#1 0x0000564423455887 in notifier_list_notify (list=list@entry=0x564423d2b258
<clipboard_notifiers>, data=data@entry=0x564426c59a70) at util/notify.c:40
#2 0x00005644233273bf in qemu_clipboard_update
(info=info@entry=0x564426c59a70) at ui/clipboard.c:19
#3 0x000056442334efd2 in vnc_client_cut_text_ext (vs=vs@entry=0x564427f18000,
len=len@entry=4, flags=<optimized out>,
data=data@entry=0x5644263cc00c
"\002\f\001\251\020\377\377\377!\377\377\377\314\376\377\377\315\376\377\377
\377\377\377\316\345\241\300\307\376\377\377\310\376\377\377\376\376\377\377\a")
at ui/vnc-clipboard.c:256
#4 0x000056442333b172 in protocol_client_msg (vs=0x564427f18000, data=0x5644263cc000
"\006", len=<optimized out>) at ui/vnc.c:2396
#5 0x0000564423338af6 in vnc_client_read (vs=0x564427f18000) at
ui/vnc.c:1537
#6 vnc_client_io (ioc=<optimized out>, condition=G_IO_IN,
opaque=0x564427f18000) at ui/vnc.c:1559
#7 0x00007f07b02cf147 in g_main_dispatch (context=0x564425546bb0) at
gmain.c:3192
#8 g_main_context_dispatch (context=context@entry=0x564425546bb0) at
gmain.c:3845
#9 0x00005644234468f7 in glib_pollfds_poll () at util/main-loop.c:215
#10 os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:238
#11 main_loop_wait (nonblocking=nonblocking@entry=0) at util/main-loop.c:497
investigations shows, that notifier list has only one element which points to
itself as next. So, we are in the endless loop.
Seems that it's possible, if vnc_server_cut_text_caps() called twice. Then it
registers notifier twice and it added to QLIST twice, which leads to the
situation.
I don't have any reproducer and not sure that bug may be reproduced on
master.
I'm not familiar with ui code - may be vnc_server_cut_text_caps() should
never be called twice? Or notifier should be removed somehow before the
second call? Maybe this patch just shadows another bug?
But what I do know, is that we should not put same element into QLIST
twice. And if the check I propose is not needed we should add an
assertion instead:
assert(!vs->cbpeer.update.notify);
ui/vnc-clipboard.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/ui/vnc-clipboard.c b/ui/vnc-clipboard.c
index 9f077965d0..67284b556c 100644
--- a/ui/vnc-clipboard.c
+++ b/ui/vnc-clipboard.c
@@ -316,8 +316,10 @@ void vnc_server_cut_text_caps(VncState *vs)
caps[1] = 0;
vnc_clipboard_send(vs, 2, caps);
- vs->cbpeer.name = "vnc";
- vs->cbpeer.update.notify = vnc_clipboard_notify;
- vs->cbpeer.request = vnc_clipboard_request;
- qemu_clipboard_peer_register(&vs->cbpeer);
+ if (!vs->cbpeer.update.notify) {
+ vs->cbpeer.name = "vnc";
+ vs->cbpeer.update.notify = vnc_clipboard_notify;
+ vs->cbpeer.request = vnc_clipboard_request;
+ qemu_clipboard_peer_register(&vs->cbpeer);
+ }
}