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 <[email protected]>
Signed-off-by: NeilBrown <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

1 files changed, 3 insertions, 4 deletions

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index c9e34842139f..8ec756b5dec4 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -160,18 +160,17 @@ static int dev_mkdir(const char *name, umode_t mode)
 {
 	struct dentry *dentry;
 	struct path path;
-	int err;
 
 	dentry = kern_path_create(AT_FDCWD, name, &path, LOOKUP_DIRECTORY);
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 
-	err = vfs_mkdir(&nop_mnt_idmap, d_inode(path.dentry), dentry, mode);
-	if (!err)
+	dentry = vfs_mkdir(&nop_mnt_idmap, d_inode(path.dentry), dentry, mode);
+	if (!IS_ERR(dentry))
 		/* mark as kernel-created inode */
 		d_inode(dentry)->i_private = &thread;
 	done_path_create(&path, dentry);
-	return err;
+	return PTR_ERR_OR_ZERO(dentry);
 }
 
 static int create_path(const char *nodepath)