[PATCH 3/3] ovl: strict upper fs requirements for remote upper fs

[Amir Goldstein <amir73il@xxxxxxxxx>]

Overlayfs works sub-optimally with upper fs that has no
xattr/d_type/O_TMPFILE/RENAME_WHITEOUT support. We should basically
deprecate support for those filesystems, but so far, we only issue a
warning and don't fail the mount for the sake of backward compat.
Some features are already being disabled with no xattr support.

For newly supported remote upper fs, we do not need to worry about
backward compatibility, so we can fail the mount if upper fs is a
sub-optimal filesystem.

This reduces the in-tree remote filesystems supported as upper to
just FUSE, for which the remote upper fs support was added.

Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---
 fs/overlayfs/super.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 7322cf8faea4..6dc45bc7d664 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1136,6 +1136,8 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 {
 	struct vfsmount *mnt = ofs->upper_mnt;
 	struct dentry *temp;
+	bool rename_whiteout;
+	bool d_type;
 	int fh_type;
 	int err;
 
@@ -1161,11 +1163,8 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 	if (err < 0)
 		goto out;
 
-	/*
-	 * We allowed this configuration and don't want to break users over
-	 * kernel upgrade. So warn instead of erroring out.
-	 */
-	if (!err)
+	d_type = err;
+	if (!d_type)
 		pr_warn("upper fs needs to support d_type.\n");
 
 	/* Check if upper/work fs supports O_TMPFILE */
@@ -1182,7 +1181,8 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 	if (err < 0)
 		goto out;
 
-	if (!err)
+	rename_whiteout = err;
+	if (!rename_whiteout)
 		pr_warn("upper fs does not support RENAME_WHITEOUT.\n");
 
 	/*
@@ -1199,6 +1199,18 @@ static int ovl_make_workdir(struct super_block *sb, struct ovl_fs *ofs,
 		vfs_removexattr(ofs->workdir, OVL_XATTR_OPAQUE);
 	}
 
+	/*
+	 * We allowed sub-optimal upper fs configuration and don't want to break
+	 * users over kernel upgrade, but we never allowed remote upper fs, so
+	 * we can enforce strict requirements for remote upper fs.
+	 */
+	if (ovl_dentry_remote(ofs->workdir) &&
+	    (!d_type || !ofs->tmpfile || !rename_whiteout || ofs->noxattr)) {
+		pr_err("upper fs missing required features.\n");
+		err = -EINVAL;
+		goto out;
+	}
+
 	/* Check if upper/work fs supports file handles */
 	fh_type = ovl_can_decode_fh(ofs->workdir->d_sb);
 	if (ofs->config.index && !fh_type) {
-- 
2.17.1