Remounts bind mounts if read-only flag is provided

Bind mounts do not honor filesystem mount options. This change will
remount filesystems that are bind mounted if there are changes to
filesystem mount options, specifically if the mount is readonly.

Signed-off-by: Abin Shahab <ashahab@altiscale.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

src/lxc/conf.c

@@ -2054,10 +2054,12 @@ static int mount_entry(const char *fsname, const char *target,
 	if ((mountflags & MS_REMOUNT) || (mountflags & MS_BIND)) {
 		DEBUG("remounting %s on %s to respect bind or remount options",
 		      fsname ? fsname : "(none)", target ? target : "(none)");
-
+		unsigned long rqd_flags = 0;
+		if (mountflags & MS_RDONLY)
+			rqd_flags |= MS_RDONLY;
 #ifdef HAVE_STATVFS
 		if (statvfs(fsname, &sb) == 0) {
-			unsigned long required_flags = 0;
+			unsigned long required_flags = rqd_flags;
 			if (sb.f_flag & MS_NOSUID)
 				required_flags |= MS_NOSUID;
 			if (sb.f_flag & MS_NODEV)
@@ -2073,7 +2075,7 @@ static int mount_entry(const char *fsname, const char *target,
 			 * mountflags, then skip the remount
 			 */
 			if (!(mountflags & MS_REMOUNT)) {
-				if (!(required_flags & ~mountflags)) {
+				if (!(required_flags & ~mountflags) && rqd_flags == 0) {
 					DEBUG("mountflags already was %lu, skipping remount",
 						mountflags);
 					goto skipremount;