[PATCH v2 4/5] ovl: relax redirect/metacopy requirements for lower -> data redirect

[Miklos Szeredi <mszeredi@redhat.com>]

Allow the special case of a redirect from a lower layer to a data layer
without having to turn on metacopy.  This makes the feature work with
userxattr, which in turn allows data layers to be usable in user
namespaces.

Minimize the risk by only enabling redirect from a single lower layer to a
data layer iff a data layer is specified.  The only way to access a data
layer is to enable this, so there's really no reason no to enable this.

This can be used safely if the lower layer is read-only and the
user.overlay.redirect xattr cannot be modified.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 Documentation/filesystems/overlayfs.rst |  7 ++++++
 fs/overlayfs/namei.c                    | 32 ++++++++++++++-----------
 fs/overlayfs/params.c                   |  5 ----
 3 files changed, 25 insertions(+), 19 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 6245b67ae9e0..5d277d79cf2f 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -429,6 +429,13 @@ Only the data of the files in the "data-only" lower layers may be visible
 when a "metacopy" file in one of the lower layers above it, has a "redirect"
 to the absolute path of the "lower data" file in the "data-only" lower layer.
 
+Instead of explicitly enabling "metacopy=on" it is sufficient to specify at
+least one data-only layer to enable redirection of data to a data-only layer.
+In this case other forms of metacopy are rejected.  Note: this way data-only
+layers may be used toghether with "userxattr", in which case careful attention
+must be given to privileges needed to change the "user.overlay.redirect" xattr
+to prevent misuse.
+
 Since kernel version v6.8, "data-only" lower layers can also be added using
 the "datadir+" mount options and the fsconfig syscall from new mount api.
 For example::
diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index da322e9768d1..f9dc71b70beb 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -1042,6 +1042,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	char *upperredirect = NULL;
 	bool nextredirect = false;
 	bool nextmetacopy = false;
+	bool check_redirect = (ovl_redirect_follow(ofs) || ofs->numdatalayer);
 	struct dentry *this;
 	unsigned int i;
 	int err;
@@ -1053,7 +1054,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		.is_dir = false,
 		.opaque = false,
 		.stop = false,
-		.last = ovl_redirect_follow(ofs) ? false : !ovl_numlower(poe),
+		.last = check_redirect ? false : !ovl_numlower(poe),
 		.redirect = NULL,
 		.metacopy = 0,
 	};
@@ -1141,7 +1142,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			goto out_put;
 		}
 
-		if (!ovl_redirect_follow(ofs))
+		if (!check_redirect)
 			d.last = i == ovl_numlower(poe) - 1;
 		else if (d.is_dir || !ofs->numdatalayer)
 			d.last = lower.layer->idx == ovl_numlower(roe);
@@ -1222,21 +1223,24 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		}
 	}
 
-	/* Defer lookup of lowerdata in data-only layers to first access */
+	/*
+	 * Defer lookup of lowerdata in data-only layers to first access.
+	 * Don't require redirect=follow and metacopy=on in this case.
+	 */
 	if (d.metacopy && ctr && ofs->numdatalayer && d.absolute_redirect) {
 		d.metacopy = 0;
 		ctr++;
-	}
-
-	if (nextmetacopy && !ofs->config.metacopy) {
-		pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry);
-		err = -EPERM;
-		goto out_put;
-	}
-	if (nextredirect && !ovl_redirect_follow(ofs)) {
-		pr_warn_ratelimited("refusing to follow redirect for (%pd2)\n", dentry);
-		err = -EPERM;
-		goto out_put;
+	} else {
+		if (nextmetacopy && !ofs->config.metacopy) {
+			pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry);
+			err = -EPERM;
+			goto out_put;
+		}
+		if (nextredirect && !ovl_redirect_follow(ofs)) {
+			pr_warn_ratelimited("refusing to follow redirect for (%pd2)\n", dentry);
+			err = -EPERM;
+			goto out_put;
+		}
 	}
 
 	/*
diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
index 1115c22deca0..54468b2b0fba 100644
--- a/fs/overlayfs/params.c
+++ b/fs/overlayfs/params.c
@@ -1000,11 +1000,6 @@ int ovl_fs_params_verify(const struct ovl_fs_context *ctx,
 		 */
 	}
 
-	if (ctx->nr_data > 0 && !config->metacopy) {
-		pr_err("lower data-only dirs require metacopy support.\n");
-		return -EINVAL;
-	}
-
 	return 0;
 }
 
-- 
2.49.0