[PATCH mptcp-next v6 2/4] mptcp: propagate RECVERR sockopts to subflows

[David Carlier <devnexen@gmail.com>]

Propagate IP_RECVERR/IP_RECVERR_RFC4884 and
IPV6_RECVERR/IPV6_RECVERR_RFC4884 from the MPTCP socket to existing
and future subflows.

mptcp_setsockopt_recverr() snapshots optval into a local int, applies
it to the parent socket via ip_setsockopt() / ipv6_setsockopt(), bumps
msk->setsockopt_seq, and forwards to every subflow via
mptcp_setsockopt_all_sf(). Newly-joining subflows pick up the four
RECVERR bits through sync_socket_options() now that
MPTCP_INET_FLAGS_MASK covers them.

mptcp_setsockopt_all_sf() skips IPv4 subflows when called with
SOL_IPV6 to avoid the -ENOPROTOOPT that ip_setsockopt() returns on
level mismatch in AF_INET6 msks carrying IPv4 subflows.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Assisted-by: Codex:gpt-5
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 net/mptcp/sockopt.c | 123 ++++++++++++++++++++++++++++++++++++--------
 1 file changed, 101 insertions(+), 22 deletions(-)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 20c737b2e704..3046e9eb2c33 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -8,6 +8,8 @@
 
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <net/ip.h>
+#include <net/ipv6.h>
 #include <net/sock.h>
 #include <net/protocol.h>
 #include <net/tcp.h>
@@ -19,7 +21,11 @@
 #define MPTCP_INET_FLAGS_MASK \
 	(BIT(INET_FLAGS_TRANSPARENT) | \
 	 BIT(INET_FLAGS_FREEBIND) | \
-	 BIT(INET_FLAGS_BIND_ADDRESS_NO_PORT))
+	 BIT(INET_FLAGS_BIND_ADDRESS_NO_PORT) | \
+	 BIT(INET_FLAGS_RECVERR) | \
+	 BIT(INET_FLAGS_RECVERR_RFC4884) | \
+	 BIT(INET_FLAGS_RECVERR6) | \
+	 BIT(INET_FLAGS_RECVERR6_RFC4884))
 
 static struct sock *__mptcp_tcp_fallback(struct mptcp_sock *msk)
 {
@@ -388,6 +394,81 @@ static int mptcp_setsockopt_sol_socket(struct mptcp_sock *msk, int optname,
 	return -EOPNOTSUPP;
 }
 
+static int mptcp_setsockopt_all_sf(struct mptcp_sock *msk, int level,
+				   int optname, sockptr_t optval,
+				   unsigned int optlen)
+{
+	struct mptcp_subflow_context *subflow;
+	int ret = 0;
+
+	mptcp_for_each_subflow(msk, subflow) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+		if (level == SOL_IPV6 && ssk->sk_family != AF_INET6)
+			continue;
+
+		ret = tcp_setsockopt(ssk, level, optname, optval, optlen);
+		if (ret)
+			break;
+	}
+
+	if (!ret)
+		sockopt_seq_inc(msk);
+
+	return ret;
+}
+
+static int mptcp_setsockopt_recverr(struct mptcp_sock *msk, int level,
+				    int optname, sockptr_t optval,
+				    unsigned int optlen)
+{
+	struct sock *sk = (struct sock *)msk;
+	int val, ret;
+
+	/* Let ip_setsockopt() / ipv6_setsockopt() validate optval and optlen
+	 * (so 1-byte boolean writes keep the same ABI as plain TCP) and update
+	 * the parent's RECVERR bit. Re-read that bit under lock_sock() and
+	 * push it to the subflows: concurrent setsockopt callers cannot leave
+	 * parent and subflows desynchronized this way.
+	 */
+	if (level == SOL_IP)
+		ret = ip_setsockopt(sk, level, optname, optval, optlen);
+#if IS_ENABLED(CONFIG_IPV6)
+	else if (level == SOL_IPV6)
+		ret = ipv6_setsockopt(sk, level, optname, optval, optlen);
+#endif
+	else
+		return -EOPNOTSUPP;
+	if (ret)
+		return ret;
+
+	lock_sock(sk);
+	switch (optname) {
+	case IP_RECVERR:
+		val = inet_test_bit(RECVERR, sk);
+		break;
+	case IP_RECVERR_RFC4884:
+		val = inet_test_bit(RECVERR_RFC4884, sk);
+		break;
+#if IS_ENABLED(CONFIG_IPV6)
+	case IPV6_RECVERR:
+		val = inet6_test_bit(RECVERR6, sk);
+		break;
+	case IPV6_RECVERR_RFC4884:
+		val = inet6_test_bit(RECVERR6_RFC4884, sk);
+		break;
+#endif
+	default:
+		release_sock(sk);
+		return -EOPNOTSUPP;
+	}
+
+	ret = mptcp_setsockopt_all_sf(msk, level, optname,
+				      KERNEL_SOCKPTR(&val), sizeof(val));
+	release_sock(sk);
+	return ret;
+}
+
 static int mptcp_setsockopt_v6(struct mptcp_sock *msk, int optname,
 			       sockptr_t optval, unsigned int optlen)
 {
@@ -430,6 +511,10 @@ static int mptcp_setsockopt_v6(struct mptcp_sock *msk, int optname,
 
 		release_sock(sk);
 		break;
+	case IPV6_RECVERR:
+	case IPV6_RECVERR_RFC4884:
+		ret = mptcp_setsockopt_recverr(msk, SOL_IPV6, optname, optval, optlen);
+		break;
 	}
 
 	return ret;
@@ -775,6 +860,9 @@ static int mptcp_setsockopt_v4(struct mptcp_sock *msk, int optname,
 		return mptcp_setsockopt_sol_ip_set(msk, optname, optval, optlen);
 	case IP_TOS:
 		return mptcp_setsockopt_v4_set_tos(msk, optname, optval, optlen);
+	case IP_RECVERR:
+	case IP_RECVERR_RFC4884:
+		return mptcp_setsockopt_recverr(msk, SOL_IP, optname, optval, optlen);
 	}
 
 	return -EOPNOTSUPP;
@@ -802,27 +890,6 @@ static int mptcp_setsockopt_first_sf_only(struct mptcp_sock *msk, int level, int
 	return ret;
 }
 
-static int mptcp_setsockopt_all_sf(struct mptcp_sock *msk, int level,
-				   int optname, sockptr_t optval,
-				   unsigned int optlen)
-{
-	struct mptcp_subflow_context *subflow;
-	int ret = 0;
-
-	mptcp_for_each_subflow(msk, subflow) {
-		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
-
-		ret = tcp_setsockopt(ssk, level, optname, optval, optlen);
-		if (ret)
-			break;
-	}
-
-	if (!ret)
-		sockopt_seq_inc(msk);
-
-	return ret;
-}
-
 static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 				    sockptr_t optval, unsigned int optlen)
 {
@@ -1467,6 +1534,12 @@ static int mptcp_getsockopt_v4(struct mptcp_sock *msk, int optname,
 	case IP_LOCAL_PORT_RANGE:
 		return mptcp_put_int_option(msk, optval, optlen,
 				READ_ONCE(inet_sk(sk)->local_port_range));
+	case IP_RECVERR:
+		return mptcp_put_int_option(msk, optval, optlen,
+				inet_test_bit(RECVERR, sk));
+	case IP_RECVERR_RFC4884:
+		return mptcp_put_int_option(msk, optval, optlen,
+				inet_test_bit(RECVERR_RFC4884, sk));
 	}
 
 	return -EOPNOTSUPP;
@@ -1487,6 +1560,12 @@ static int mptcp_getsockopt_v6(struct mptcp_sock *msk, int optname,
 	case IPV6_FREEBIND:
 		return mptcp_put_int_option(msk, optval, optlen,
 					    inet_test_bit(FREEBIND, sk));
+	case IPV6_RECVERR:
+		return mptcp_put_int_option(msk, optval, optlen,
+					    inet6_test_bit(RECVERR6, sk));
+	case IPV6_RECVERR_RFC4884:
+		return mptcp_put_int_option(msk, optval, optlen,
+					    inet6_test_bit(RECVERR6_RFC4884, sk));
 	}
 
 	return -EOPNOTSUPP;
-- 
2.53.0