Merge pull request #850 from hallyn/2016-02-25/cgfs.lxcfs

src/lxc/cgfs.c

@@ -636,6 +636,11 @@ static struct cgroup_hierarchy *lxc_cgroup_find_hierarchy(struct cgroup_meta_dat
 	return NULL;
 }
 
+static bool mountpoint_is_accessible(struct cgroup_mount_point *mp)
+{
+	return mp && access(mp->mount_point, F_OK) == 0;
+}
+
 static struct cgroup_mount_point *lxc_cgroup_find_mount_point(struct cgroup_hierarchy *hierarchy, const char *group, bool should_be_writable)
 {
 	struct cgroup_mount_point **mps;
@@ -643,9 +648,9 @@ static struct cgroup_mount_point *lxc_cgroup_find_mount_point(struct cgroup_hier
 	ssize_t quality = -1;
 
 	/* trivial case */
-	if (hierarchy->rw_absolute_mount_point)
+	if (mountpoint_is_accessible(hierarchy->rw_absolute_mount_point))
 		return hierarchy->rw_absolute_mount_point;
-	if (!should_be_writable && hierarchy->ro_absolute_mount_point)
+	if (!should_be_writable && mountpoint_is_accessible(hierarchy->ro_absolute_mount_point))
 		return hierarchy->ro_absolute_mount_point;
 
 	for (mps = hierarchy->all_mount_points; mps && *mps; mps++) {
@@ -655,6 +660,9 @@ static struct cgroup_mount_point *lxc_cgroup_find_mount_point(struct cgroup_hier
 		if (prefix_len == 1 && mp->mount_prefix[0] == '/')
 			prefix_len = 0;
 
+		if (!mountpoint_is_accessible(mp))
+			continue;
+
 		if (should_be_writable && mp->read_only)
 			continue;
 
@@ -1396,8 +1404,9 @@ static bool cgroupfs_mount_cgroup(void *hdata, const char *root, int type)
 	for (info = base_info; info; info = info->next) {
 		size_t subsystem_count, i;
 		struct cgroup_mount_point *mp = info->designated_mount_point;
-		if (!mp)
+		if (!mountpoint_is_accessible(mp))
 			mp = lxc_cgroup_find_mount_point(info->hierarchy, info->cgroup_path, true);
+
 		if (!mp) {
 			SYSERROR("could not find original mount point for cgroup hierarchy while trying to mount cgroup filesystem");
 			goto out_error;