[PATCH] ovl_obtain_alias(): don't call d_instantiate_anon() for old dentry

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

[Miklos, Amir - is there anything subtle I'm missing here?]

The situation is the same as for __d_obtain_alias() (which is what
that thing is parallel to) - if we find a preexisting alias, we
want to grab it, drop the inode and return the alias we'd found.

The only thing d_instantiate_anon() does compared to that is
spurious security_d_instiate() that has already been done to
that dentry with exact same arguments.

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
--
diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
index 73c9775215b3..a64195b194c2 100644
--- a/fs/overlayfs/export.c
+++ b/fs/overlayfs/export.c
@@ -317,29 +317,32 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
 		ovl_set_flag(OVL_UPPERDATA, inode);
 
 	dentry = d_find_any_alias(inode);
-	if (!dentry) {
-		dentry = d_alloc_anon(inode->i_sb);
-		if (!dentry)
-			goto nomem;
-		oe = ovl_alloc_entry(lower ? 1 : 0);
-		if (!oe)
-			goto nomem;
-
-		if (lower) {
-			oe->lowerstack->dentry = dget(lower);
-			oe->lowerstack->layer = lowerpath->layer;
-		}
-		dentry->d_fsdata = oe;
-		if (upper_alias)
-			ovl_dentry_set_upper_alias(dentry);
+	if (dentry)
+		goto out_iput;
+
+	dentry = d_alloc_anon(inode->i_sb);
+	if (unlikely(!dentry))
+		goto nomem;
+	oe = ovl_alloc_entry(lower ? 1 : 0);
+	if (!oe)
+		goto nomem;
+
+	if (lower) {
+		oe->lowerstack->dentry = dget(lower);
+		oe->lowerstack->layer = lowerpath->layer;
 	}
+	dentry->d_fsdata = oe;
+	if (upper_alias)
+		ovl_dentry_set_upper_alias(dentry);
 
 	return d_instantiate_anon(dentry, inode);
 
 nomem:
-	iput(inode);
 	dput(dentry);
-	return ERR_PTR(-ENOMEM);
+	dentry = ERR_PTR(-ENOMEM);
+out_iput:
+	iput(inode);
+	return dentry;
 }
 
 /* Get the upper or lower dentry in stach whose on layer @idx */