netfilter: xtables: use memcmp in unconditional check

Instead of inspecting each u32/char open-coded, clean up and make use
of memcmp. On some arches, memcmp is implemented as assembly or GCC's
__builtin_memcmp which can possibly take advantages of known
alignment.
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

net/ipv4/netfilter/arp_tables.c

@@ -341,15 +341,11 @@ unsigned int arpt_do_table(struct sk_buff *skb,
 }
 
 /* All zeroes == unconditional rule. */
-static inline int unconditional(const struct arpt_arp *arp)
+static inline bool unconditional(const struct arpt_arp *arp)
 {
-	unsigned int i;
-
-	for (i = 0; i < sizeof(*arp)/sizeof(__u32); i++)
-		if (((__u32 *)arp)[i])
-			return 0;
+	static const struct arpt_arp uncond;
 
-	return 1;
+	return memcmp(arp, &uncond, sizeof(uncond)) == 0;
 }
 
 /* Figures out from what hook each rule can be called: returns 0 if

net/ipv4/netfilter/ip_tables.c

@@ -190,16 +190,11 @@ get_entry(void *base, unsigned int offset)
 
 /* All zeroes == unconditional rule. */
 /* Mildly perf critical (only if packet tracing is on) */
-static inline int
-unconditional(const struct ipt_ip *ip)
+static inline bool unconditional(const struct ipt_ip *ip)
 {
-	unsigned int i;
+	static const struct ipt_ip uncond;
 
-	for (i = 0; i < sizeof(*ip)/sizeof(__u32); i++)
-		if (((__u32 *)ip)[i])
-			return 0;
-
-	return 1;
+	return memcmp(ip, &uncond, sizeof(uncond)) == 0;
 #undef FWINV
 }
 

net/ipv6/netfilter/ip6_tables.c

@@ -222,16 +222,11 @@ get_entry(void *base, unsigned int offset)
 
 /* All zeroes == unconditional rule. */
 /* Mildly perf critical (only if packet tracing is on) */
-static inline int
-unconditional(const struct ip6t_ip6 *ipv6)
+static inline bool unconditional(const struct ip6t_ip6 *ipv6)
 {
-	unsigned int i;
-
-	for (i = 0; i < sizeof(*ipv6); i++)
-		if (((char *)ipv6)[i])
-			break;
+	static const struct ip6t_ip6 uncond;
 
-	return (i == sizeof(*ipv6));
+	return memcmp(ipv6, &uncond, sizeof(uncond)) == 0;
 }
 
 #if defined(CONFIG_NETFILTER_XT_TARGET_TRACE) || \