mount: save ext_real_root for external mounts

need it to check if we can bindmount from external mount

note: when migrating from criu with patch to criu without,
external mount mapping won't work, we do not support it.

v2: s/real_root/ext_real_root/
v4: add comment
v5: use ext_key field for mapping, put NO_ROOT_MOUNT in root
for old externals for which we do not have it.
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/mount.c

@@ -35,6 +35,7 @@
  * debugging.
  */
 #define AUTODETECTED_MOUNT "CRIU:AUTOGENERATED"
+#define NO_ROOT_MOUNT "CRIU:NO_ROOT"
 #define MS_PROPAGATE (MS_SHARED | MS_PRIVATE | MS_UNBINDABLE | MS_SLAVE)
 
 #undef	LOG_PREFIX
@@ -1329,17 +1330,14 @@ static int dump_one_mountpoint(struct mount_info *pm, struct cr_img *img)
 		me.internal_sharing = true;
 	}
 
-	if (pm->external) {
+	if (pm->external)
 		/*
 		 * For external mount points dump the mapping's
-		 * value instead of root. See collect_mnt_from_image
+		 * value, see collect_mnt_from_image -> get_mp_root
 		 * for reverse mapping details.
 		 */
-		me.root	= pm->external;
-		me.has_ext_mount = true;
-		me.ext_mount = true;
-	} else
-		me.root = pm->root;
+		me.ext_key = pm->external;
+	me.root = pm->root;
 
 	if (pb_write_one(img, &me, PB_MNT))
 		return -1;
@@ -2446,11 +2444,27 @@ static int get_mp_root(MntEntry *me, struct mount_info *mi)
 {
 	char *ext = NULL;
 
+	BUG_ON(me->ext_mount && me->ext_key);
+
+	/* Forward compatibility fixup */
+	if (me->ext_mount) {
+		me->ext_key = me->root;
+		/*
+		 * Puting the id of external mount which is provided by user,
+		 * to ->root can confuse mnt_is_external and other functions
+		 * which expect to see the path in the file system to the root
+		 * of these mount (mounts_equal, mnt_build_ids_tree,
+		 * find_wider_shared, find_fsroot_mount_for,
+		 * find_best_external_match, etc.)
+		 */
+		me->root = NO_ROOT_MOUNT;
+	}
+
 	mi->root = xstrdup(me->root);
 	if (!mi->root)
 		return -1;
 
-	if (!me->ext_mount)
+	if (!me->ext_key)
 		goto out;
 
 	/*
@@ -2458,7 +2472,7 @@ static int get_mp_root(MntEntry *me, struct mount_info *mi)
 	 * from the command line and put into root's place
 	 */
 
-	ext = ext_mount_lookup(me->root);
+	ext = ext_mount_lookup(me->ext_key);
 	if (!ext) {
 		if (!opts.autodetect_ext_mounts) {
 			pr_err("No mapping for %s mountpoint\n", me->mountpoint);

images/mnt.proto

@@ -53,4 +53,6 @@ message mnt_entry {
 
 	optional bool		deleted			= 16;
 	optional uint32		sb_flags		= 17 [(criu).hex = true];
+	/* user defined mapping for external mount */
+	optional string		ext_key			= 18;
 }