new helper: lookup_positive_unlocked()

Most of the callers of lookup_one_len_unlocked() treat negatives are
ERR_PTR(-ENOENT).  Provide a helper that would do just that.  Note
that a pinned positive dentry remains positive - it's ->d_inode is
stable, etc.; a pinned _negative_ dentry can become positive at any
point as long as you are not holding its parent at least shared.
So using lookup_one_len_unlocked() needs to be careful;
lookup_positive_unlocked() is safer and that's what the callers
end up open-coding anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/overlayfs/namei.c

@@ -200,7 +200,7 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 	int err;
 	bool last_element = !post[0];
 
-	this = lookup_one_len_unlocked(name, base, namelen);
+	this = lookup_positive_unlocked(name, base, namelen);
 	if (IS_ERR(this)) {
 		err = PTR_ERR(this);
 		this = NULL;
@@ -208,8 +208,6 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 			goto out;
 		goto out_err;
 	}
-	if (!this->d_inode)
-		goto put_and_out;
 
 	if (ovl_dentry_weird(this)) {
 		/* Don't support traversing automounts and other weirdness */
@@ -651,7 +649,7 @@ struct dentry *ovl_get_index_fh(struct ovl_fs *ofs, struct ovl_fh *fh)
 	if (err)
 		return ERR_PTR(err);
 
-	index = lookup_one_len_unlocked(name.name, ofs->indexdir, name.len);
+	index = lookup_positive_unlocked(name.name, ofs->indexdir, name.len);
 	kfree(name.name);
 	if (IS_ERR(index)) {
 		if (PTR_ERR(index) == -ENOENT)
@@ -659,9 +657,7 @@ struct dentry *ovl_get_index_fh(struct ovl_fs *ofs, struct ovl_fh *fh)
 		return index;
 	}
 
-	if (d_is_negative(index))
-		err = 0;
-	else if (ovl_is_whiteout(index))
+	if (ovl_is_whiteout(index))
 		err = -ESTALE;
 	else if (ovl_dentry_weird(index))
 		err = -EIO;
@@ -685,7 +681,7 @@ struct dentry *ovl_lookup_index(struct ovl_fs *ofs, struct dentry *upper,
 	if (err)
 		return ERR_PTR(err);
 
-	index = lookup_one_len_unlocked(name.name, ofs->indexdir, name.len);
+	index = lookup_positive_unlocked(name.name, ofs->indexdir, name.len);
 	if (IS_ERR(index)) {
 		err = PTR_ERR(index);
 		if (err == -ENOENT) {
@@ -700,9 +696,7 @@ struct dentry *ovl_lookup_index(struct ovl_fs *ofs, struct dentry *upper,
 	}
 
 	inode = d_inode(index);
-	if (d_is_negative(index)) {
-		goto out_dput;
-	} else if (ovl_is_whiteout(index) && !verify) {
+	if (ovl_is_whiteout(index) && !verify) {
 		/*
 		 * When index lookup is called with !verify for decoding an
 		 * overlay file handle, a whiteout index implies that decode
@@ -1131,7 +1125,7 @@ bool ovl_lower_positive(struct dentry *dentry)
 		struct dentry *this;
 		struct dentry *lowerdir = poe->lowerstack[i].dentry;
 
-		this = lookup_one_len_unlocked(name->name, lowerdir,
+		this = lookup_positive_unlocked(name->name, lowerdir,
 					       name->len);
 		if (IS_ERR(this)) {
 			switch (PTR_ERR(this)) {
@@ -1148,10 +1142,8 @@ bool ovl_lower_positive(struct dentry *dentry)
 				break;
 			}
 		} else {
-			if (this->d_inode) {
-				positive = !ovl_is_whiteout(this);
-				done = true;
-			}
+			positive = !ovl_is_whiteout(this);
+			done = true;
 			dput(this);
 		}
 	}