ovl: fix corner case of non-constant st_dev;st_ino

On non-samefs overlay without xino, non pure upper inodes should use a
pseudo_dev assigned to each unique lower fs, but if lower layer is on the
same fs and upper layer, it has no pseudo_dev assigned.

In this overlay layers setup:
 - two filesystems, A and B
 - upper layer is on A
 - lower layer 1 is also on A
 - lower layer 2 is on B

Non pure upper overlay inode, whose origin is in layer 1 will have the
st_dev;st_ino values of the real lower inode before copy up and the
st_dev;st_ino values of the real upper inode after copy up.

Fix this inconsitency by assigning a unique pseudo_dev also for upper fs,
that will be used as st_dev value along with the lower inode st_dev for
overlay inodes in the case above.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -121,13 +121,12 @@ static int ovl_map_dev_ino(struct dentry *dentry, struct kstat *stat, int fsid)
 		 */
 		stat->dev = dentry->d_sb->s_dev;
 		stat->ino = dentry->d_inode->i_ino;
-	} else if (fsid) {
+	} else {
 		/*
 		 * For non-samefs setup, if we cannot map all layers st_ino
 		 * to a unified address space, we need to make sure that st_dev
-		 * is unique per lower fs. Upper layer uses real st_dev and
-		 * lower layers use the unique anonymous bdev assigned to the
-		 * lower fs.
+		 * is unique per underlying fs, so we use the unique anonymous
+		 * bdev assigned to the underlying fs.
 		 */
 		stat->dev = OVL_FS(dentry->d_sb)->fs[fsid].pseudo_dev;
 	}
@@ -143,7 +142,6 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 	struct path realpath;
 	const struct cred *old_cred;
 	bool is_dir = S_ISDIR(dentry->d_inode->i_mode);
-	bool samefs = ovl_same_fs(dentry->d_sb);
 	int fsid = 0;
 	int err;
 	bool metacopy_blocks = false;
@@ -198,13 +196,7 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 			    (!ovl_verify_lower(dentry->d_sb) &&
 			     (is_dir || lowerstat.nlink == 1))) {
 				fsid = ovl_layer_lower(dentry)->fsid;
-				/*
-				 * Cannot use origin st_dev;st_ino because
-				 * origin inode content may differ from overlay
-				 * inode content.
-				 */
-				if (samefs || fsid)
-					stat->ino = lowerstat.ino;
+				stat->ino = lowerstat.ino;
 			}
 
 			/*

fs/overlayfs/super.c

@@ -229,8 +229,7 @@ static void ovl_free_fs(struct ovl_fs *ofs)
 		mntput(ofs->layers[i].mnt);
 	}
 	kfree(ofs->layers);
-	/* fs[0].pseudo_dev is either null or real upper st_dev */
-	for (i = 1; i < ofs->numfs; i++)
+	for (i = 0; i < ofs->numfs; i++)
 		free_anon_bdev(ofs->fs[i].pseudo_dev);
 	kfree(ofs->fs);
 
@@ -1341,13 +1340,19 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
 	ofs->numlayer = 1;
 
 	/*
-	 * All lower layers that share the same fs as upper layer, use the real
-	 * upper st_dev.
+	 * All lower layers that share the same fs as upper layer, use the same
+	 * pseudo_dev as upper layer.  Allocate fs[0].pseudo_dev even for lower
+	 * only overlay to simplify ovl_fs_free().
 	 * is_lower will be set if upper fs is shared with a lower layer.
 	 */
+	err = get_anon_bdev(&ofs->fs[0].pseudo_dev);
+	if (err) {
+		pr_err("failed to get anonymous bdev for upper fs\n");
+		goto out;
+	}
+
 	if (ofs->upper_mnt) {
 		ofs->fs[0].sb = ofs->upper_mnt->mnt_sb;
-		ofs->fs[0].pseudo_dev = ofs->upper_mnt->mnt_sb->s_dev;
 		ofs->fs[0].is_lower = false;
 	}