From: Serge Hallyn <serge.hallyn@ubuntu.com>
Date: Thu, 25 Feb 2016 19:01:12 +0000 (-0800)
Subject: cgfs: make sure we use valid cgroup mountpoints
X-Git-Tag: lxc-1.0.9~85
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f2e8fceaa21fd5c3c0bfe1d763416c5c3c0f21ec;p=thirdparty%2Flxc.git

cgfs: make sure we use valid cgroup mountpoints

If lxcfs starts before cgroup-lite, then the first cgroup mountpoints in
/proc/self/mountinfo are /run/lxcfs/*.  Unprivileged users cannot access
these.  So privileged containers are ok, and unprivileged containers are ok
since they won't cache those to begin with.  But unprivileged root-owned
containers cache /run/lxcfs/* and then try to use them.

So when doing cgroup automounting check whether the mountpoints we have
stored are accessible, and if not look for a new one to use.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
---

diff --git a/src/lxc/cgfs.c b/src/lxc/cgfs.c
index bb3c88293..e0f1ce3da 100644
--- a/src/lxc/cgfs.c
+++ b/src/lxc/cgfs.c
@@ -635,6 +635,11 @@ 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;
@@ -642,9 +647,9 @@ 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++) {
@@ -654,6 +659,9 @@ 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;
 
@@ -1392,8 +1400,9 @@ 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;