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 63841df403f249d5aeb681ce81a0ec671b7d4242..b7eaee2602bc10540fd5d9902f0781888de0c33a 100644 (file)
--- 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;