inet: implement lockless IP_MTU_DISCOVER

inet->pmtudisc can be read locklessly.

Implement proper lockless reads and writes to inet->pmtudisc

ip_sock_set_mtu_discover() can now be called from arbitrary
contexts.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -434,19 +434,22 @@ int ip_dont_fragment(const struct sock *sk, const struct dst_entry *dst)
 
 static inline bool ip_sk_accept_pmtu(const struct sock *sk)
 {
-	return inet_sk(sk)->pmtudisc != IP_PMTUDISC_INTERFACE &&
-	       inet_sk(sk)->pmtudisc != IP_PMTUDISC_OMIT;
+	u8 pmtudisc = READ_ONCE(inet_sk(sk)->pmtudisc);
+
+	return pmtudisc != IP_PMTUDISC_INTERFACE &&
+	       pmtudisc != IP_PMTUDISC_OMIT;
 }
 
 static inline bool ip_sk_use_pmtu(const struct sock *sk)
 {
-	return inet_sk(sk)->pmtudisc < IP_PMTUDISC_PROBE;
+	return READ_ONCE(inet_sk(sk)->pmtudisc) < IP_PMTUDISC_PROBE;
 }
 
 static inline bool ip_sk_ignore_df(const struct sock *sk)
 {
-	return inet_sk(sk)->pmtudisc < IP_PMTUDISC_DO ||
-	       inet_sk(sk)->pmtudisc == IP_PMTUDISC_OMIT;
+	u8 pmtudisc = READ_ONCE(inet_sk(sk)->pmtudisc);
+
+	return pmtudisc < IP_PMTUDISC_DO || pmtudisc == IP_PMTUDISC_OMIT;
 }
 
 static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,

net/ipv4/ip_output.c

@@ -1387,8 +1387,8 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	struct ip_options *opt = NULL;
 	struct rtable *rt = (struct rtable *)cork->dst;
 	struct iphdr *iph;
+	u8 pmtudisc, ttl;
 	__be16 df = 0;
-	__u8 ttl;
 
 	skb = __skb_dequeue(queue);
 	if (!skb)
@@ -1418,8 +1418,9 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	/* DF bit is set when we want to see DF on outgoing frames.
 	 * If ignore_df is set too, we still allow to fragment this frame
 	 * locally. */
-	if (inet->pmtudisc == IP_PMTUDISC_DO ||
-	    inet->pmtudisc == IP_PMTUDISC_PROBE ||
+	pmtudisc = READ_ONCE(inet->pmtudisc);
+	if (pmtudisc == IP_PMTUDISC_DO ||
+	    pmtudisc == IP_PMTUDISC_PROBE ||
 	    (skb->len <= dst_mtu(&rt->dst) &&
 	     ip_dont_fragment(sk, &rt->dst)))
 		df = htons(IP_DF);

net/ipv4/ip_sockglue.c

@@ -622,9 +622,7 @@ int ip_sock_set_mtu_discover(struct sock *sk, int val)
 {
 	if (val < IP_PMTUDISC_DONT || val > IP_PMTUDISC_OMIT)
 		return -EINVAL;
-	lock_sock(sk);
-	inet_sk(sk)->pmtudisc = val;
-	release_sock(sk);
+	WRITE_ONCE(inet_sk(sk)->pmtudisc, val);
 	return 0;
 }
 EXPORT_SYMBOL(ip_sock_set_mtu_discover);
@@ -1050,6 +1048,8 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 			return -EINVAL;
 		WRITE_ONCE(inet->mc_ttl, val);
 		return 0;
+	case IP_MTU_DISCOVER:
+		return ip_sock_set_mtu_discover(sk, val);
 	}
 
 	err = 0;
@@ -1107,11 +1107,6 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
 	case IP_TOS:	/* This sets both TOS and Precedence */
 		__ip_sock_set_tos(sk, val);
 		break;
-	case IP_MTU_DISCOVER:
-		if (val < IP_PMTUDISC_DONT || val > IP_PMTUDISC_OMIT)
-			goto e_inval;
-		inet->pmtudisc = val;
-		break;
 	case IP_UNICAST_IF:
 	{
 		struct net_device *dev = NULL;
@@ -1595,6 +1590,9 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 	case IP_MULTICAST_TTL:
 		val = READ_ONCE(inet->mc_ttl);
 		goto copyval;
+	case IP_MTU_DISCOVER:
+		val = READ_ONCE(inet->pmtudisc);
+		goto copyval;
 	}
 
 	if (needs_rtnl)
@@ -1634,9 +1632,6 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
 	case IP_TOS:
 		val = inet->tos;
 		break;
-	case IP_MTU_DISCOVER:
-		val = inet->pmtudisc;
-		break;
 	case IP_MTU:
 	{
 		struct dst_entry *dst;

net/ipv4/ping.c

@@ -551,7 +551,7 @@ void ping_err(struct sk_buff *skb, int offset, u32 info)
 		case ICMP_DEST_UNREACH:
 			if (code == ICMP_FRAG_NEEDED) { /* Path MTU discovery */
 				ipv4_sk_update_pmtu(skb, sk, info);
-				if (inet_sock->pmtudisc != IP_PMTUDISC_DONT) {
+				if (READ_ONCE(inet_sock->pmtudisc) != IP_PMTUDISC_DONT) {
 					err = EMSGSIZE;
 					harderr = 1;
 					break;

net/ipv4/raw.c

@@ -239,7 +239,7 @@ static void raw_err(struct sock *sk, struct sk_buff *skb, u32 info)
 		if (code > NR_ICMP_UNREACH)
 			break;
 		if (code == ICMP_FRAG_NEEDED) {
-			harderr = inet->pmtudisc != IP_PMTUDISC_DONT;
+			harderr = READ_ONCE(inet->pmtudisc) != IP_PMTUDISC_DONT;
 			err = EMSGSIZE;
 		} else {
 			err = icmp_err_convert[code].errno;

net/ipv4/udp.c

@@ -750,7 +750,7 @@ int __udp4_lib_err(struct sk_buff *skb, u32 info, struct udp_table *udptable)
 	case ICMP_DEST_UNREACH:
 		if (code == ICMP_FRAG_NEEDED) { /* Path MTU discovery */
 			ipv4_sk_update_pmtu(skb, sk, info);
-			if (inet->pmtudisc != IP_PMTUDISC_DONT) {
+			if (READ_ONCE(inet->pmtudisc) != IP_PMTUDISC_DONT) {
 				err = EMSGSIZE;
 				harderr = 1;
 				break;

net/netfilter/ipvs/ip_vs_sync.c

@@ -1335,7 +1335,7 @@ static void set_mcast_pmtudisc(struct sock *sk, int val)
 
 	/* setsockopt(sock, SOL_IP, IP_MTU_DISCOVER, &val, sizeof(val)); */
 	lock_sock(sk);
-	inet->pmtudisc = val;
+	WRITE_ONCE(inet->pmtudisc, val);
 #ifdef CONFIG_IP_VS_IPV6
 	if (sk->sk_family == AF_INET6) {
 		struct ipv6_pinfo *np = inet6_sk(sk);