fs: kernfs: Fix possible null-pointer dereferences in kernfs_path_from_node_locked()

In kernfs_path_from_node_locked(), there is an if statement on line 147
to check whether buf is NULL:
    if (buf)

When buf is NULL, it is used on line 151:
    len += strlcpy(buf + len, parent_str, ...)
and line 158:
    len += strlcpy(buf + len, "/", ...)
and line 160:
    len += strlcpy(buf + len, kn->name, ...)

Thus, possible null-pointer dereferences may occur.

To fix these possible bugs, buf is checked before being used.
If it is NULL, -EINVAL is returned.

These bugs are found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Link: https://lore.kernel.org/r/20190724022242.27505-1-baijiaju1990@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/dir.c

@@ -137,6 +137,9 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 	if (kn_from == kn_to)
 		return strlcpy(buf, "/", buflen);
 
+	if (!buf)
+		return -EINVAL;
+
 	common = kernfs_common_ancestor(kn_from, kn_to);
 	if (WARN_ON(!common))
 		return -EINVAL;
@@ -144,8 +147,7 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 	depth_to = kernfs_depth(common, kn_to);
 	depth_from = kernfs_depth(common, kn_from);
 
-	if (buf)
-		buf[0] = '\0';
+	buf[0] = '\0';
 
 	for (i = 0; i < depth_from; i++)
 		len += strlcpy(buf + len, parent_str,