linux-kernel - [PATCH 10/13] ovl: change ovl_create_real() to get a new lock when re-opening created file.

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Message-ID: <20260204050726.177283-11-neilb@ownmail.net>
Date: Wed,  4 Feb 2026 15:57:54 +1100
From: NeilBrown <neilb@...mail.net>
To: Christian Brauner <brauner@...nel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Jan Kara <jack@...e.cz>,
	Chuck Lever <chuck.lever@...cle.com>,
	Jeff Layton <jlayton@...nel.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Amir Goldstein <amir73il@...il.com>,
	John Johansen <john.johansen@...onical.com>,
	Paul Moore <paul@...l-moore.com>,
	James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Stephen Smalley <stephen.smalley.work@...il.com>
Cc: linux-kernel@...r.kernel.org,
	netfs@...ts.linux.dev,
	linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org,
	linux-unionfs@...r.kernel.org,
	apparmor@...ts.ubuntu.com,
	linux-security-module@...r.kernel.org,
	selinux@...r.kernel.org
Subject: [PATCH 10/13] ovl: change ovl_create_real() to get a new lock when re-opening created file.

From: NeilBrown <neil@...wn.name>

When ovl_create_real() is used to create a file on the upper filesystem
it needs to return the resulting dentry - positive and hashed.
It is usually the case the that dentry passed to the create function
(e.g.  vfs_create()) will be suitable but this is not guaranteed.  The
filesystem may unhash that dentry forcing a repeat lookup next time the
name is wanted.

So ovl_create_real() must be (and is) aware of this and prepared to
perform that lookup to get a hash positive dentry.

This is currently done under that same directory lock that provided
exclusion for the create.  Proposed changes to locking will make this
not possible - as the name, rather than the directory, will be locked.
The new APIs provided for lookup and locking do not and cannot support
this pattern.

The lock isn't needed.  ovl_create_real() can drop the lock and then get
a new lock for the lookup - then check that the lookup returned the
correct inode.  In a well-behaved configuration where the upper
filesystem is not being modified by a third party, this will always work
reliably, and if there are separate modification it will fail cleanly.

So change ovl_create_real() to drop the lock and call
ovl_start_creating_upper() to find the correct dentry.  Note that
start_creating doesn't fail if the name already exists.

This removes the only remaining use of ovl_lookup_upper, so it is
removed.

Signed-off-by: NeilBrown <neil@...wn.name>
---
 fs/overlayfs/dir.c       | 24 ++++++++++++++++++------
 fs/overlayfs/overlayfs.h |  7 -------
 2 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index ff3dbd1ca61f..ec08904d084d 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -219,21 +219,33 @@ struct dentry *ovl_create_real(struct ovl_fs *ofs, struct dentry *parent,
 		err = -EIO;
 	} else if (d_unhashed(newdentry)) {
 		struct dentry *d;
+		struct name_snapshot name;
 		/*
 		 * Some filesystems (i.e. casefolded) may return an unhashed
-		 * negative dentry from the ovl_lookup_upper() call before
+		 * negative dentry from the ovl_start_creating_upper() call before
 		 * ovl_create_real().
 		 * In that case, lookup again after making the newdentry
 		 * positive, so ovl_create_upper() always returns a hashed
 		 * positive dentry.
+		 * As we have to drop the lock before the lookup a race
+		 * could result in a lookup failure.  In that case we return
+		 * an error.
 		 */
-		d = ovl_lookup_upper(ofs, newdentry->d_name.name, parent,
-				     newdentry->d_name.len);
-		dput(newdentry);
-		if (IS_ERR_OR_NULL(d))
+		take_dentry_name_snapshot(&name, newdentry);
+		end_creating_keep(newdentry);
+		d = ovl_start_creating_upper(ofs, parent, &name.name);
+		release_dentry_name_snapshot(&name);
+
+		if (IS_ERR_OR_NULL(d)) {
 			err = d ? PTR_ERR(d) : -ENOENT;
-		else
+		} else if (d->d_inode != newdentry->d_inode) {
+			err = -EIO;
+			dput(newdentry);
+		} else {
+			dput(newdentry);
 			return d;
+		}
+		return ERR_PTR(err);
 	}
 out:
 	if (err) {
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 315882a360ce..4fb4750a83e4 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -406,13 +406,6 @@ static inline struct file *ovl_do_tmpfile(struct ovl_fs *ofs,
 	return file;
 }
 
-static inline struct dentry *ovl_lookup_upper(struct ovl_fs *ofs,
-					      const char *name,
-					      struct dentry *base, int len)
-{
-	return lookup_one(ovl_upper_mnt_idmap(ofs), &QSTR_LEN(name, len), base);
-}
-
 static inline struct dentry *ovl_lookup_upper_unlocked(struct ovl_fs *ofs,
 						       const char *name,
 						       struct dentry *base,
-- 
2.50.0.107.gf914562f5916.dirty