VFS: introduce start_dirop() and end_dirop()

The fact that directory operations (create,remove,rename) are protected
by a lock on the parent is known widely throughout the kernel.
In order to change this - to instead lock the target dentry  - it is
best to centralise this knowledge so it can be changed in one place.

This patch introduces start_dirop() which is local to VFS code.
It performs the required locking for create and remove.  Rename
will be handled separately.

Various functions with names like start_creating() or start_removing_path(),
some of which already exist, will export this functionality beyond the VFS.

end_dirop() is the partner of start_dirop().  It drops the lock and
releases the reference on the dentry.
It *is* exported so that various end_creating etc functions can be inline.

As vfs_mkdir() drops the dentry on error we cannot use end_dirop() as
that won't unlock when the dentry IS_ERR().  For now we need an explicit
unlock when dentry IS_ERR().  I hope to change vfs_mkdir() to unlock
when it drops a dentry so that explicit unlock can go away.

end_dirop() can always be called on the result of start_dirop(), but not
after vfs_mkdir().  After a vfs_mkdir() we still may need the explicit
unlock as seen in end_creating_path().

As well as adding start_dirop() and end_dirop()
this patch uses them in:
 - simple_start_creating (which requires sharing lookup_noperm_common()
        with libfs.c)
 - start_removing_path / start_removing_user_path_at
 - filename_create / end_creating_path()
 - do_rmdir(), do_unlinkat()

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-3-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 17 insertions, 19 deletions

diff --git a/fs/libfs.c b/fs/libfs.c
index ce8c496a6940..02371f45ef7d 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -2289,27 +2289,25 @@ void stashed_dentry_prune(struct dentry *dentry)
 	cmpxchg(stashed, dentry, NULL);
 }
 
-/* parent must be held exclusive */
+/**
+ * simple_start_creating - prepare to create a given name
+ * @parent: directory in which to prepare to create the name
+ * @name:   the name to be created
+ *
+ * Required lock is taken and a lookup in performed prior to creating an
+ * object in a directory.  No permission checking is performed.
+ *
+ * Returns: a negative dentry on which vfs_create() or similar may
+ *  be attempted, or an error.
+ */
 struct dentry *simple_start_creating(struct dentry *parent, const char *name)
 {
-	struct dentry *dentry;
-	struct inode *dir = d_inode(parent);
+	struct qstr qname = QSTR(name);
+	int err;
 
-	inode_lock(dir);
-	if (unlikely(IS_DEADDIR(dir))) {
-		inode_unlock(dir);
-		return ERR_PTR(-ENOENT);
-	}
-	dentry = lookup_noperm(&QSTR(name), parent);
-	if (IS_ERR(dentry)) {
-		inode_unlock(dir);
-		return dentry;
-	}
-	if (dentry->d_inode) {
-		dput(dentry);
-		inode_unlock(dir);
-		return ERR_PTR(-EEXIST);
-	}
-	return dentry;
+	err = lookup_noperm_common(&qname, parent);
+	if (err)
+		return ERR_PTR(err);
+	return start_dirop(parent, &qname, LOOKUP_CREATE | LOOKUP_EXCL);
 }
 EXPORT_SYMBOL(simple_start_creating);