net: ipv6: Use struct_size() helper and kcalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes. Also, remove unnecessary
function ipv6_rpl_srh_alloc_size() and replace kzalloc() with kcalloc(),
which has a 2-factor argument form for multiplication.

This code was detected with the help of Coccinelle and, audited and
fixed manually.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/rpl.h

@@ -26,12 +26,6 @@ static inline void rpl_exit(void) {}
 /* Worst decompression memory usage ipv6 address (16) + pad 7 */
 #define IPV6_RPL_SRH_WORST_SWAP_SIZE (sizeof(struct in6_addr) + 7)
 
-static inline size_t ipv6_rpl_srh_alloc_size(unsigned char n)
-{
-	return sizeof(struct ipv6_rpl_sr_hdr) +
-		((n + 1) * sizeof(struct in6_addr));
-}
-
 size_t ipv6_rpl_srh_size(unsigned char n, unsigned char cmpri,
 			 unsigned char cmpre);
 

net/ipv6/exthdrs.c

@@ -580,7 +580,7 @@ looped_back:
 	hdr->segments_left--;
 	i = n - hdr->segments_left;
 
-	buf = kzalloc(ipv6_rpl_srh_alloc_size(n + 1) * 2, GFP_ATOMIC);
+	buf = kcalloc(struct_size(hdr, segments.addr, n + 2), 2, GFP_ATOMIC);
 	if (unlikely(!buf)) {
 		kfree_skb(skb);
 		return -1;

net/ipv6/rpl_iptunnel.c

@@ -136,8 +136,7 @@ static int rpl_do_srh_inline(struct sk_buff *skb, const struct rpl_lwt *rlwt,
 
 	oldhdr = ipv6_hdr(skb);
 
-	buf = kzalloc(ipv6_rpl_srh_alloc_size(srh->segments_left - 1) * 2,
-		      GFP_ATOMIC);
+	buf = kcalloc(struct_size(srh, segments.addr, srh->segments_left), 2, GFP_ATOMIC);
 	if (!buf)
 		return -ENOMEM;