mpls_iptunnel: use struct_size() helper

One of the more common cases of allocation size calculations is finding the size of a structure that has a zero-sized array at the end, along with memory for some number of elements for that array. For example:

struct foo {
    int stuff;
    struct boo entry[];
};

instance = alloc(sizeof(struct foo) + count * sizeof(struct boo));

Instead of leaving these open-coded and prone to type mistakes, we can now use the new struct_size() helper:

instance = alloc(struct_size(instance, entry, count));

This code was detected with the help of Coccinelle.

---
 net/mpls/mpls_iptunnel.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 94f53a9b7d1a..dda8930f20e7 100644
@@ -183,8 +183,8 @@ static int mpls_build_state(struct nlattr *nla,
 			   &n_labels, NULL, extack))
 		return -EINVAL;
 
-	newts = lwtunnel_state_alloc(sizeof(*tun_encap_info) +
-				     n_labels * sizeof(u32));
+	newts = lwtunnel_state_alloc(struct_size(tun_encap_info, label,
+						 n_labels));
 	if (!newts)
 		return -ENOMEM;
 

2.20.1

From: "Gustavo A. R. Silva" <[email protected]>
Date: Thu, 7 Feb 2019 19:10:52 -0600

Applied.