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[Qemu-devel] Possible bug in monitor code

From: Stratos Psomadakis
Subject: [Qemu-devel] Possible bug in monitor code
Date: Wed, 22 Jan 2014 17:53:52 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130801 Thunderbird/17.0.8


we've encountered a weird issue regarding monitor (qmp and hmp) behavior with qemu-1.7 (and qemu-1.5). The following steps will reproduce the issue:
1) Client A connects to qmp socket with socat
2) Client A gets greeting message {"QMP": {"version": ..}
3) Client A waits (select on the socket's fd)
4) Client B tries to connect to the *same* qmp socket with socat
5) Client B does *NOT* get any greating message
6) Client B waits (select on the socket's fd)
7) Client B closes connection (kill socat)
8) Client A quits too
9) Client C connects to qmp socket
10) Client C gets *two* greeting messages!!!
After some investigation, we traced it down to the monitor_flush() function in monitor.c. Specifically, when a second client connects to the qmp (client B), while another one is already using it (client A), we get the following from stracing the second client (client B):
connect(3, {sa_family=AF_FILE, path="foo.mon"}, 9) = 0
getsockname(3, {sa_family=AF_FILE, NULL}, [2]) = 0
select(4, [0 3], [1 3], [], NULL)       = 2 (out [1 3])
select(4, [0 3], [], [], NULL
So, the connect() syscall from client B succeeds, although client B connection has not yet been accepted by the qmp server (it's still in the backlog of the qmp listening socket).

After killing client B and then client A, we see the following when stracing the qemu proc:
22363 accept4(6, {sa_family=AF_FILE, NULL}, [2], SOCK_CLOEXEC) = 9
22363 fcntl(9, F_GETFL)                 = 0x2 (flags O_RDWR)
22363 fcntl(9, F_SETFL, O_RDWR|O_NONBLOCK) = 0
22363 fstat(9, {st_mode=S_IFSOCK|0777, st_size=0, ...}) = 0
22363 fcntl(9, F_GETFL)                 = 0x802 (flags O_RDWR|O_NONBLOCK)
22363 write(9, "{\"QMP\": {\"version\": {\"qemu\": {\"m"..., 127) = -1 EPIPE (Broken pipe)
22363 --- SIGPIPE (Broken pipe) @ 0 (0) ---
The qmp server / qemu accepts the connection from client B (who has now closed the connection) and tries to write the greeting message to the socket fd. This results in write returning an error (EPIPE).

The monitor_flush() function doesn't seem to handle this case (write error). Instead, it adds a watch / handler to retry the write operation. Thus, mon->outbuf is not cleaned up properly, which results in duplicate greeting messages for the next client to connect.

The following seems to do the trick.

diff --git a/monitor.c b/monitor.c
index 845f608..5622f20 100644
--- a/monitor.c
+++ b/monitor.c
@@ -288,8 +288,8 @@ void monitor_flush(Monitor *mon)
     if (len && !mon->mux_out) {
         rc = qemu_chr_fe_write(mon->chr, (const uint8_t *) buf, len);
-        if (rc == len) {
-            /* all flushed */
+        if ((rc < 0 && errno != EAGAIN) || (rc == len)) {
+            /* all flushed or error */
             mon->outbuf = qstring_new();



Stratos Psomadakis

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