VFS: Change vfs_mkdir() to return the dentry.

vfs_mkdir() does not guarantee to leave the child dentry hashed or make
it positive on success, and in many such cases the filesystem had to use
a different dentry which it can now return.

This patch changes vfs_mkdir() to return the dentry provided by the
filesystems which is hashed and positive when provided.  This reduces
the number of cases where the resulting dentry is not positive to a
handful which don't deserve extra efforts.

The only callers of vfs_mkdir() which are interested in the resulting
inode are in-kernel filesystem clients: cachefiles, nfsd, smb/server.
The only filesystems that don't reliably provide the inode are:
- kernfs, tracefs which these clients are unlikely to be interested in
- cifs in some configurations would need to do a lookup to find the
  created inode, but doesn't.  cifs cannot be exported via NFS, is
  unlikely to be used by cachefiles, and smb/server only has a soft
  requirement for the inode, so this is unlikely to be a problem in
  practice.
- hostfs, nfs, cifs may need to do a lookup (rarely for NFS) and it is
  possible for a race to make that lookup fail.  Actual failure
  is unlikely and providing callers handle negative dentries graceful
  they will fail-safe.

So this patch removes the lookup code in nfsd and smb/server and adjusts
them to fail safe if a negative dentry is provided:
- cache-files already fails safe by restarting the task from the
  top - it still does with this change, though it no longer calls
  cachefiles_put_directory() as that will crash if the dentry is
  negative.
- nfsd reports "Server-fault" which it what it used to do if the lookup
  failed. This will never happen on any file-systems that it can actually
  export, so this is of no consequence.  I removed the fh_update()
  call as that is not needed and out-of-place.  A subsequent
  nfsd_create_setattr() call will call fh_update() when needed.
- smb/server only wants the inode to call ksmbd_smb_inherit_owner()
  which updates ->i_uid (without calling notify_change() or similar)
  which can be safely skipping on cifs (I hope).

If a different dentry is returned, the first one is put.  If necessary
the fact that it is new can be determined by comparing pointers.  A new
dentry will certainly have a new pointer (as the old is put after the
new is obtained).
Similarly if an error is returned (via ERR_PTR()) the original dentry is
put.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20250227013949.536172-7-neilb@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 9 insertions, 7 deletions

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index 7cf59713f0f7..83a60126de0f 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -128,18 +128,19 @@ retry:
 		ret = security_path_mkdir(&path, subdir, 0700);
 		if (ret < 0)
 			goto mkdir_error;
-		ret = cachefiles_inject_write_error();
-		if (ret == 0)
-			ret = vfs_mkdir(&nop_mnt_idmap, d_inode(dir), subdir, 0700);
-		if (ret < 0) {
+		subdir = ERR_PTR(cachefiles_inject_write_error());
+		if (!IS_ERR(subdir))
+			subdir = vfs_mkdir(&nop_mnt_idmap, d_inode(dir), subdir, 0700);
+		ret = PTR_ERR(subdir);
+		if (IS_ERR(subdir)) {
 			trace_cachefiles_vfs_error(NULL, d_inode(dir), ret,
 						   cachefiles_trace_mkdir_error);
 			goto mkdir_error;
 		}
 		trace_cachefiles_mkdir(dir, subdir);
 
-		if (unlikely(d_unhashed(subdir))) {
-			cachefiles_put_directory(subdir);
+		if (unlikely(d_unhashed(subdir) || d_is_negative(subdir))) {
+			dput(subdir);
 			goto retry;
 		}
 		ASSERT(d_backing_inode(subdir));
@@ -195,7 +196,8 @@ mark_error:
 
 mkdir_error:
 	inode_unlock(d_inode(dir));
-	dput(subdir);
+	if (!IS_ERR(subdir))
+		dput(subdir);
 	pr_err("mkdir %s failed with error %d\n", dirname, ret);
 	return ERR_PTR(ret);