ipv6: lockless IPV6_MTU implementation

np->frag_size can be read/written without holding socket lock.

Add missing annotations and make IPV6_MTU setsockopt() lockless.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -881,9 +881,11 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 			mtu = IPV6_MIN_MTU;
 	}
 
-	if (np && np->frag_size < mtu) {
-		if (np->frag_size)
-			mtu = np->frag_size;
+	if (np) {
+		u32 frag_size = READ_ONCE(np->frag_size);
+
+		if (frag_size && frag_size < mtu)
+			mtu = frag_size;
 	}
 	if (mtu < hlen + sizeof(struct frag_hdr) + 8)
 		goto fail_toobig;
@@ -1392,7 +1394,7 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
 			  struct rt6_info *rt)
 {
 	struct ipv6_pinfo *np = inet6_sk(sk);
-	unsigned int mtu;
+	unsigned int mtu, frag_size;
 	struct ipv6_txoptions *nopt, *opt = ipc6->opt;
 
 	/* callers pass dst together with a reference, set it first so
@@ -1441,10 +1443,11 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
 	else
 		mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
 			READ_ONCE(rt->dst.dev->mtu) : dst_mtu(xfrm_dst_path(&rt->dst));
-	if (np->frag_size < mtu) {
-		if (np->frag_size)
-			mtu = np->frag_size;
-	}
+
+	frag_size = READ_ONCE(np->frag_size);
+	if (frag_size && frag_size < mtu)
+		mtu = frag_size;
+
 	cork->base.fragsize = mtu;
 	cork->base.gso_size = ipc6->gso_size;
 	cork->base.tx_flags = 0;

net/ipv6/ipv6_sockglue.c

@@ -441,6 +441,13 @@ int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		WRITE_ONCE(np->mcast_hops,
 			   val == -1 ? IPV6_DEFAULT_MCASTHOPS : val);
 		return 0;
+	case IPV6_MTU:
+		if (optlen < sizeof(int))
+			return -EINVAL;
+		if (val && val < IPV6_MIN_MTU)
+			return -EINVAL;
+		WRITE_ONCE(np->frag_size, val);
+		return 0;
 	}
 	if (needs_rtnl)
 		rtnl_lock();
@@ -910,14 +917,6 @@ int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		np->pmtudisc = val;
 		retv = 0;
 		break;
-	case IPV6_MTU:
-		if (optlen < sizeof(int))
-			goto e_inval;
-		if (val && val < IPV6_MIN_MTU)
-			goto e_inval;
-		np->frag_size = val;
-		retv = 0;
-		break;
 	case IPV6_RECVERR:
 		if (optlen < sizeof(int))
 			goto e_inval;