ovl: allow upperdir inside lowerdir

stable inclusion
from linux-4.19.190
commit a130ed5212c3dfff88065db4f62c29bb184748d4

--------------------------------

commit 708fa015 upstream.

Commit 146d62e5 ("ovl: detect overlapping layers") made sure we don't
have overlapping layers, but it also broke the arguably valid use case of

 mount -olowerdir=/,upperdir=/subdir,..

where upperdir overlaps lowerdir on the same filesystem.  This has been
causing regressions.

Revert the check, but only for the specific case where upperdir and/or
workdir are subdirectories of lowerdir.  Any other overlap (e.g. lowerdir
is subdirectory of upperdir, etc) case is crazy, so leave the check in
place for those.

Overlaps are detected at lookup time too, so reverting the mount time check
should be safe.

Fixes: 146d62e5 ("ovl: detect overlapping layers")
Cc: <stable@vger.kernel.org> # v5.2
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Conflicts:
  fs/overlayfs/super.c
[yyl: keep ovl_check_overlapping_layers() same as mainline]
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/overlayfs/super.c

@@ -1538,7 +1538,8 @@ static struct ovl_entry *ovl_get_lowerstack(struct super_block *sb,
  * - upper/work dir of any overlayfs instance
  */
 static int ovl_check_layer(struct super_block *sb, struct ovl_fs *ofs,
-			   struct dentry *dentry, const char *name)
+			   struct dentry *dentry, const char *name,
+			   bool is_lower)
 {
 	struct dentry *next = dentry, *parent;
 	int err = 0;
@@ -1550,7 +1551,7 @@ static int ovl_check_layer(struct super_block *sb, struct ovl_fs *ofs,
 
 	/* Walk back ancestors to root (inclusive) looking for traps */
 	while (!err && parent != next) {
-		if (ovl_lookup_trap_inode(sb, parent)) {
+		if (is_lower && ovl_lookup_trap_inode(sb, parent)) {
 			err = -ELOOP;
 			pr_err("overlayfs: overlapping %s path\n", name);
 		} else if (ovl_is_inuse(parent)) {
@@ -1576,7 +1577,7 @@ static int ovl_check_overlapping_layers(struct super_block *sb,
 
 	if (ofs->upper_mnt) {
 		err = ovl_check_layer(sb, ofs, ofs->upper_mnt->mnt_root,
-				      "upperdir");
+				      "upperdir", false);
 		if (err)
 			return err;
 
@@ -1587,7 +1588,8 @@ static int ovl_check_overlapping_layers(struct super_block *sb,
 		 * workbasedir.  In that case, we already have their traps in
 		 * inode cache and we will catch that case on lookup.
 		 */
-		err = ovl_check_layer(sb, ofs, ofs->workbasedir, "workdir");
+		err = ovl_check_layer(sb, ofs, ofs->workbasedir, "workdir",
+				      false);
 		if (err)
 			return err;
 	}
@@ -1595,7 +1597,7 @@ static int ovl_check_overlapping_layers(struct super_block *sb,
 	for (i = 1; i < ofs->numlayer; i++) {
 		err = ovl_check_layer(sb, ofs,
 				      ofs->layers[i].mnt->mnt_root,
-				      "lowerdir");
+				      "lowerdir", true);
 		if (err)
 			return err;
 	}