mq_notify: rework to fix use-after-close/double-close bugs

in the error path where the mq_notify syscall fails, the initiating
thread may have closed the socket before the worker thread calls recv
on it. even in the absence of such a race, if the recv call failed,
e.g. due to seccomp policy blocking it, the worker thread could
proceed to close, producing a double-close condition.

this can all be simplified by moving the mq_notify syscall into the
new thread, so that the error case does not require pthread_cancel.
now, the initiating thread only needs to read back the error status
after waiting for the worker thread to consume its arguments.

src/mq/mq_notify.c

@@ -10,6 +10,8 @@
 struct args {
 	sem_t sem;
 	int sock;
+	mqd_t mqd;
+	int err;
 	const struct sigevent *sev;
 };
 
@@ -21,8 +23,18 @@ static void *start(void *p)
 	int s = args->sock;
 	void (*func)(union sigval) = args->sev->sigev_notify_function;
 	union sigval val = args->sev->sigev_value;
+	struct sigevent sev2;
+	static const char zeros[32];
+	int err;
 
+	sev2.sigev_notify = SIGEV_THREAD;
+	sev2.sigev_signo = s;
+	sev2.sigev_value.sival_ptr = (void *)&zeros;
+
+	args->err = err = -__syscall(SYS_mq_notify, args->mqd, &sev2);
 	sem_post(&args->sem);
+	if (err) return 0;
+
 	n = recv(s, buf, sizeof(buf), MSG_NOSIGNAL|MSG_WAITALL);
 	close(s);
 	if (n==sizeof buf && buf[sizeof buf - 1] == 1)
@@ -36,8 +48,6 @@ int mq_notify(mqd_t mqd, const struct sigevent *sev)
 	pthread_attr_t attr;
 	pthread_t td;
 	int s;
-	struct sigevent sev2;
-	static const char zeros[32];
 	int cs;
 
 	if (!sev || sev->sigev_notify != SIGEV_THREAD)
@@ -46,6 +56,7 @@ int mq_notify(mqd_t mqd, const struct sigevent *sev)
 	s = socket(AF_NETLINK, SOCK_RAW|SOCK_CLOEXEC, 0);
 	if (s < 0) return -1;
 	args.sock = s;
+	args.mqd = mqd;
 
 	if (sev->sigev_notify_attributes) attr = *sev->sigev_notify_attributes;
 	else pthread_attr_init(&attr);
@@ -63,13 +74,9 @@ int mq_notify(mqd_t mqd, const struct sigevent *sev)
 	pthread_setcancelstate(cs, 0);
 	sem_destroy(&args.sem);
 
-	sev2.sigev_notify = SIGEV_THREAD;
+	if (args.err) {
-	sev2.sigev_signo = s;
-	sev2.sigev_value.sival_ptr = (void *)&zeros;
-
-	if (syscall(SYS_mq_notify, mqd, &sev2) < 0) {
-		pthread_cancel(td);
 		__syscall(SYS_close, s);
+		errno = args.err;
 		return -1;
 	}