ovl: support encoding non-decodable file handles

When all layers support file handles, we support encoding non-decodable
file handles (a.k.a. fid) even with nfs_export=off.

When file handles do not need to be decoded, we do not need to copy up
redirected lower directories on encode, and we encode also non-indexed
upper with lower file handle, so fid will not change on copy up.

This enables reporting fanotify events with file handles on overlayfs
with default config/mount options.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

fs/overlayfs/export.c

@@ -174,28 +174,37 @@ static int ovl_connect_layer(struct dentry *dentry)
  * U = upper file handle
  * L = lower file handle
  *
- * (*) Connecting an overlay dir from real lower dentry is not always
+ * (*) Decoding a connected overlay dir from real lower dentry is not always
  * possible when there are redirects in lower layers and non-indexed merge dirs.
  * To mitigate those case, we may copy up the lower dir ancestor before encode
- * a lower dir file handle.
+ * of a decodable file handle for non-upper dir.
  *
  * Return 0 for upper file handle, > 0 for lower file handle or < 0 on error.
  */
 static int ovl_check_encode_origin(struct dentry *dentry)
 {
 	struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
+	bool decodable = ofs->config.nfs_export;
+
+	/* Lower file handle for non-upper non-decodable */
+	if (!ovl_dentry_upper(dentry) && !decodable)
+		return 0;
 
 	/* Upper file handle for pure upper */
 	if (!ovl_dentry_lower(dentry))
 		return 0;
 
 	/*
-	 * Upper file handle for non-indexed upper.
-	 *
 	 * Root is never indexed, so if there's an upper layer, encode upper for
 	 * root.
 	 */
-	if (ovl_dentry_upper(dentry) &&
+	if (dentry == dentry->d_sb->s_root)
+		return 0;
+
+	/*
+	 * Upper decodable file handle for non-indexed upper.
+	 */
+	if (ovl_dentry_upper(dentry) && decodable &&
 	    !ovl_test_flag(OVL_INDEX, d_inode(dentry)))
 		return 0;
 
@@ -205,7 +214,7 @@ static int ovl_check_encode_origin(struct dentry *dentry)
 	 * ovl_connect_layer() will try to make origin's layer "connected" by
 	 * copying up a "connectable" ancestor.
 	 */
-	if (d_is_dir(dentry) && ovl_upper_mnt(ofs))
+	if (d_is_dir(dentry) && ovl_upper_mnt(ofs) && decodable)
 		return ovl_connect_layer(dentry);
 
 	/* Lower file handle for indexed and non-upper dir/non-dir */
@@ -876,3 +885,8 @@ const struct export_operations ovl_export_operations = {
 	.get_name	= ovl_get_name,
 	.get_parent	= ovl_get_parent,
 };
+
+/* encode_fh() encodes non-decodable file handles with nfs_export=off */
+const struct export_operations ovl_export_fid_operations = {
+	.encode_fh	= ovl_encode_fh,
+};

fs/overlayfs/inode.c

@@ -1311,7 +1311,7 @@ static bool ovl_hash_bylower(struct super_block *sb, struct dentry *upper,
 		return false;
 
 	/* No, if non-indexed upper with NFS export */
-	if (sb->s_export_op && upper)
+	if (ofs->config.nfs_export && upper)
 		return false;
 
 	/* Otherwise, hash by lower inode for fsnotify */

fs/overlayfs/overlayfs.h

@@ -799,6 +799,7 @@ int ovl_set_origin(struct ovl_fs *ofs, struct dentry *lower,
 
 /* export.c */
 extern const struct export_operations ovl_export_operations;
+extern const struct export_operations ovl_export_fid_operations;
 
 /* super.c */
 int ovl_fill_super(struct super_block *sb, struct fs_context *fc);

fs/overlayfs/ovl_entry.h

@@ -82,6 +82,7 @@ struct ovl_fs {
 	const struct cred *creator_cred;
 	bool tmpfile;
 	bool noxattr;
+	bool nofh;
 	/* Did we take the inuse lock? */
 	bool upperdir_locked;
 	bool workdir_locked;

fs/overlayfs/super.c

@@ -400,6 +400,7 @@ static int ovl_lower_dir(const char *name, struct path *path,
 		pr_warn("fs on '%s' does not support file handles, falling back to index=off,nfs_export=off.\n",
 			name);
 	}
+	ofs->nofh |= !fh_type;
 	/*
 	 * Decoding origin file handle is required for persistent st_ino.
 	 * Without persistent st_ino, xino=auto falls back to xino=off.
@@ -818,6 +819,7 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 		ofs->config.index = false;
 		pr_warn("upper fs does not support file handles, falling back to index=off.\n");
 	}
+	ofs->nofh |= !fh_type;
 
 	/* Check if upper fs has 32bit inode numbers */
 	if (fh_type != FILEID_INO32_GEN)
@@ -1452,8 +1454,15 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 		ofs->config.nfs_export = false;
 	}
 
+	/*
+	 * Support encoding decodable file handles with nfs_export=on
+	 * and encoding non-decodable file handles with nfs_export=off
+	 * if all layers support file handles.
+	 */
 	if (ofs->config.nfs_export)
 		sb->s_export_op = &ovl_export_operations;
+	else if (!ofs->nofh)
+		sb->s_export_op = &ovl_export_fid_operations;
 
 	/* Never override disk quota limits or use reserved space */
 	cap_lower(cred->cap_effective, CAP_SYS_RESOURCE);