From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Tue, 11 Jan 2022 18:59:15 +0000 (-0500)
Subject: Fix: lttng-ctl: lttng_list_sessions: initialize out_sessions to NULL when returning 0
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=985aea182b618c85c51651f224abedfe367c75ee;p=lttng-tools.git

Fix: lttng-ctl: lttng_list_sessions: initialize out_sessions to NULL when returning 0

Observed issue
==============

Users of lttng-ctl API's lttng_list_sessions observe application crash
when freeing the *out_sessions output value when lttng_list_sessions
returns 0.

Cause
=====

The implementation does not set *out_sessions to NULL when
lttng_ctl_ask_sessiond() sets the sessions variable to NULL.
This causes the user application to attempt to free(3) an uninitialized
pointer.

Solution
========

Initialize out_sessions to NULL before invoking
lttng_ctl_ask_sessiond(), so it is initialized when lttng_list_sessions
returns 0, thus allowing *out_sessions to be subsequently freed.
A free(3) on a NULL pointer is a no-op.

Known drawbacks
===============

None.

History
=======

This was introduced by those two commits:

b178f53e90 ("Generate session name and default output on sessiond's end")
27ea4ba825 ("Fix: error when listing sessions with no session")

This is a regression present in the stable-2.11, stable-2.12,
stable-2.13, and master branches.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I34125d708a32674d79b831e5004c48321ebd711e
---

diff --git a/src/lib/lttng-ctl/lttng-ctl.cpp b/src/lib/lttng-ctl/lttng-ctl.cpp
index 63841df40..b7eaee260 100644
--- a/src/lib/lttng-ctl/lttng-ctl.cpp
+++ b/src/lib/lttng-ctl/lttng-ctl.cpp
@@ -2102,6 +2102,12 @@ int lttng_list_sessions(struct lttng_session **out_sessions)
 
 	memset(&lsm, 0, sizeof(lsm));
 	lsm.cmd_type = LTTNG_LIST_SESSIONS;
+	/*
+	 * Initialize out_sessions to NULL so it is initialized when
+	 * lttng_list_sessions returns 0, thus allowing *out_sessions to
+	 * be subsequently freed.
+	 */
+	*out_sessions = NULL;
 	ret = lttng_ctl_ask_sessiond(&lsm, (void**) &sessions);
 	if (ret <= 0) {
 		goto end;
@@ -2114,7 +2120,6 @@ int lttng_list_sessions(struct lttng_session **out_sessions)
 	if (ret % session_size) {
 		ret = -LTTNG_ERR_UNK;
 		free(sessions);
-		*out_sessions = NULL;
 		goto end;
 	}
 	session_count = (size_t) ret / session_size;