VFS/nfsd/cachefiles/ovl: add start_creating() and end_creating()

start_creating() is similar to simple_start_creating() but is not so
simple.
It takes a qstr for the name, includes permission checking, and does NOT
report an error if the name already exists, returning a positive dentry
instead.

This is currently used by nfsd, cachefiles, and overlayfs.

end_creating() is called after the dentry has been used.
end_creating() drops the reference to the dentry as it is generally no
longer needed.  This is exactly the first section of end_creating_path()
so that function is changed to call the new end_creating()

These calls help encapsulate locking rules so that directory locking can
be changed.

Occasionally this change means that the parent lock is held for a
shorter period of time, for example in cachefiles_commit_tmpfile().
As this function now unlocks after an unlink and before the following
lookup, it is possible that the lookup could again find a positive
dentry, so a while loop is introduced there.

In overlayfs the ovl_lookup_temp() function has ovl_tempname()
split out to be used in ovl_start_creating_temp().  The other use
of ovl_lookup_temp() is preparing for a rename.  When rename handling
is updated, ovl_lookup_temp() will be removed.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-5-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/include/linux/namei.h b/include/linux/namei.h
index fed86221c69c..3f92c1a16878 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -88,6 +88,39 @@ struct dentry *lookup_one_positive_killable(struct mnt_idmap *idmap,
 					    struct qstr *name,
 					    struct dentry *base);
 
+struct dentry *start_creating(struct mnt_idmap *idmap, struct dentry *parent,
+			      struct qstr *name);
+
+/**
+ * end_creating - finish action started with start_creating
+ * @child:  dentry returned by start_creating() or vfs_mkdir()
+ * @parent: dentry given to start_creating(),
+ *
+ * Unlock and release the child.
+ *
+ * Unlike end_dirop() this can only be called if start_creating() succeeded.
+ * It handles @child being and error as vfs_mkdir() might have converted the
+ * dentry to an error - in that case the parent still needs to be unlocked.
+ *
+ * If vfs_mkdir() was called then the value returned from that function
+ * should be given for @child rather than the original dentry, as vfs_mkdir()
+ * may have provided a new dentry.  Even if vfs_mkdir() returns an error
+ * it must be given to end_creating().
+ *
+ * If vfs_mkdir() was not called, then @child will be a valid dentry and
+ * @parent will be ignored.
+ */
+static inline void end_creating(struct dentry *child, struct dentry *parent)
+{
+	if (IS_ERR(child))
+		/* The parent is still locked despite the error from
+		 * vfs_mkdir() - must unlock it.
+		 */
+		inode_unlock(parent->d_inode);
+	else
+		end_dirop(child);
+}
+
 extern int follow_down_one(struct path *);
 extern int follow_down(struct path *path, unsigned int flags);
 extern int follow_up(struct path *);