mnt: Fix validation of dumpable mountpoints

This patch consists of 3 unsplittable (from my POV) fixes.

1. Remove messy check from dump_one_mountpoint() -- we have
   validate_mounts to check whether we can dump the tree
   or not.

2. Other than being in the wron place the mentioned check
   is wrong. Comparing of the length of the mp->source-s
   makes no sense -- it should be mp->root, but even this
   would be wrong...

3. ... instead, we should check for bind mount root path
   being accessible from the target mount root path, i.e.
   the bind->root should start with src->root.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>

mount.c

@@ -313,7 +313,7 @@ static int validate_mounts(struct mount_info *info, bool call_plugins)
 	struct mount_info *m, *t;
 
 	for (m = info; m; m = m->next) {
-		if (m->parent == NULL)
+		if (m->parent == NULL || m->is_ns_root)
 			/* root mount can be any */
 			continue;
 
@@ -337,11 +337,31 @@ static int validate_mounts(struct mount_info *info, bool call_plugins)
 			}
 		}
 
-		if (!fsroot_mounted(m)) {
+		/*
+		 * Mountpoint can point to / of an FS. In that case this FS
+		 * should be of some known type so that we can just mount one.
+		 *
+		 * Otherwise it's a bindmount mountpoint and we try to find
+		 * what fsroot mountpoint it's bound to. If this point is the
+		 * root mount, the path to bindmount root should be accessible
+		 * form the rootmount path (the strstartswith check in the
+		 * else branch below).
+		 */
+
+		if (fsroot_mounted(m)) {
+			if (m->fstype->code == FSTYPE__UNSUPPORTED) {
+				pr_err("FS mnt %s dev %#x root %s unsupported id %x\n",
+						m->mountpoint, m->s_dev, m->root, m->mnt_id);
+				return -1;
+			}
+		} else {
 			list_for_each_entry(t, &m->mnt_bind, mnt_bind) {
-				if (fsroot_mounted(t) || t->parent == NULL)
+				if (fsroot_mounted(t) ||
+						(t->parent == NULL &&
+						 strstartswith(m->root, t->root)))
 					break;
 			}
+
 			if (&t->mnt_bind == &m->mnt_bind) {
 				int ret;
 
@@ -783,22 +803,6 @@ static int dump_one_mountpoint(struct mount_info *pm, int fd)
 			pm->root, pm->mountpoint);
 
 	me.fstype		= pm->fstype->code;
-	if ((me.fstype == FSTYPE__UNSUPPORTED) && !is_root_mount(pm)) {
-		struct mount_info *t;
-
-		/* Is it a bind-mount of the root mount */
-		list_for_each_entry(t, &pm->mnt_bind, mnt_bind)
-			if (t->parent == NULL)
-				break;
-
-		if (&t->mnt_bind == &pm->mnt_bind ||
-		    strlen(t->source) > strlen(pm->source)) {
-			pr_err("FS mnt %s dev %#x root %s unsupported\n",
-					pm->mountpoint, pm->s_dev, pm->root);
-			return -1;
-		}
-	}
-
 	if (!pm->need_plugin && pm->fstype->dump && pm->fstype->dump(pm))
 		return -1;