[lttng-dev] [RFC PATCH LTTNG-UST] Fix: Decrement sem_count when disabling local apps

[Jérémie Galarneau]

LTTng-UST will deadlock waiting on the "constructor_wait" semaphore
if local apps, handled the session daemon running under the current UID,
are disabled or "not_allowed".

This deadlock can be triggered by setting an infinite registration
timeout and clearing the HOME environment variable, which will cause
setup_local_apps() to fail to determine the local_apps sock_path, thus
leaving local_apps.allowed == 0.

Given that local_apps are not allowed at this point, the local_apps
listener thread will not be launched. Therefore, the constructor_wait
will not be waited on.

The fix proposed here consists in decrementing "sem_count" if an error
precluding the launch the the local_apps listener thread occurs.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau at efficios.com>
---
 liblttng-ust/lttng-ust-comm.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
index b290204..c91627f 100644
--- a/liblttng-ust/lttng-ust-comm.c
+++ b/liblttng-ust/lttng-ust-comm.c
@@ -373,6 +373,7 @@ void print_cmd(int cmd, int handle)
 static
 int setup_local_apps(void)
 {
+	int ret = 0;
 	const char *home_dir;
 	uid_t uid;
 
@@ -382,13 +383,16 @@ int setup_local_apps(void)
 	 */
 	if (uid != geteuid()) {
 		assert(local_apps.allowed == 0);
-		return 0;
+		goto end;
 	}
 	home_dir = get_lttng_home_dir();
 	if (!home_dir) {
 		WARN("HOME environment variable not set. Disabling LTTng-UST per-user tracing.");
 		assert(local_apps.allowed == 0);
-		return -ENOENT;
+		ret = -ENOENT;
+		goto end;
+	} else {
+		DBG("home_dir: %s", home_dir);
 	}
 	local_apps.allowed = 1;
 	snprintf(local_apps.sock_path, PATH_MAX, "%s/%s/%s",
@@ -398,7 +402,15 @@ int setup_local_apps(void)
 	snprintf(local_apps.wait_shm_path, PATH_MAX, "/%s-%u",
 		LTTNG_UST_WAIT_FILENAME,
 		uid);
-	return 0;
+end:
+	if (!local_apps.allowed) {
+		/*
+		 * Don't wait on the "local" listener thread since we won't be
+		 * communicating with the local session daemon.
+		 */
+		uatomic_add(&sem_count, -1);
+	}
+	return ret;
 }
 
 /*
-- 
2.4.4