[PATCH net-next 0/4] mptcp: support MSG_ERRQUEUE

[PATCH net-next 0/4] mptcp: support MSG_ERRQUEUE

[Matthieu Baerts (NGI0)]

This series lets MPTCP applications use poll(EPOLLERR) and
recvmsg(MSG_ERRQUEUE) on the MPTCP socket to drain TX timestamps
through the standard inet ABI, the same way they would on a plain TCP
socket. ICMP-derived errors stay on the subflow queue: the legacy
RECVERR ABI cannot convey their per-subflow peer identity, and they
are intended for a future MPTCP_RECERR channel.

- Patch 1 splices subflow err-skbs onto the MPTCP's sk_error_queue at
  error-report time. All forwarded events go through sock_queue_err_skb,
  which re-homes skb->sk onto the MPTCP and charges sk_rmem_alloc, so
  the MPTCP's error queue stays bounded by sk_rcvbuf and is dropped under
  rmem pressure, matching tcp's tx-timestamp path and ip_icmp_error() /
  ipv6_icmp_error(). mptcp_recvmsg(MSG_ERRQUEUE) forwards directly to
  inet_recv_error(), and mptcp_poll() advertises EPOLLERR purely on the
  MPTCP's sk_err / sk_error_queue, matching tcp_poll().

- Patch 2 factors the existing inet_flags subflow-propagation hard-coded
  list into a mask, so the next patch can extend it without churn.

- Patch 3 makes IP_RECVERR / IPV6_RECVERR (and the RFC4884 variants)
  propagate to the subflows. The MPTCP stores the bit so MPTCP-aware
  helpers can branch on it.

- Patch 4 is a selftest covering the propagation path.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
David Carlier (4):
      mptcp: support MSG_ERRQUEUE on the parent socket
      mptcp: sockopt: factor inet_flags propagation into a mask
      mptcp: propagate RECVERR sockopts to subflows
      selftests: mptcp: cover IP_RECVERR sockopt propagation

 net/mptcp/protocol.c                              |  55 ++++++--
 net/mptcp/sockopt.c                               | 155 ++++++++++++++++++----
 tools/testing/selftests/net/mptcp/mptcp_sockopt.c |  70 ++++++++++
 3 files changed, 246 insertions(+), 34 deletions(-)
---
base-commit: 4bb9710c6a68d35207f123aef55dcd50e7195ec5
change-id: 20260918-net-next-mptcp-msg_errqueue-0e2049a30061

Best regards,
--  
Matthieu Baerts (NGI0) <matttbe@kernel.org>

[PATCH net-next 1/4] mptcp: support MSG_ERRQUEUE on the parent socket

[Matthieu Baerts (NGI0)]

From: David Carlier <devnexen@gmail.com>

Splice pending err skbs from each subflow's error queue onto the parent
msk's error queue at error-report time, so poll() and recvmsg(MSG_ERRQUEUE)
on the parent socket observe TX timestamps through the standard inet ABI.

The splice filters by SO_EE_ORIGIN: TIMESTAMPING events forward to the
parent because they are tied to user-handed data, not to a specific
path; subflow-level ICMP errors are dropped because the legacy RECVERR
ABI cannot meaningfully convey their per-subflow peer identity to
single-path-aware userspace. Such events will be carried by a future
MPTCP_RECERR channel.

Forwarded events all go through sock_queue_err_skb(), which re-homes
skb->sk onto the parent and charges sk_rmem_alloc, so the parent's error
queue stays bounded by sk_rcvbuf and is dropped under rmem pressure
(sk_rmem_alloc + truesize >= sk_rcvbuf), matching tcp's sk_rcvbuf-gated
tx-timestamp path and ip_icmp_error() / ipv6_icmp_error(). The
MSG_ERRQUEUE branch of mptcp_recvmsg() forwards to inet_recv_error()
directly, and poll() advertises EPOLLERR purely on the parent's sk_err /
sk_error_queue, matching tcp_poll().

Signed-off-by: David Carlier <devnexen@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/protocol.c | 55 +++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 46 insertions(+), 9 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index e89a69ab927c..15ae270ee6fd 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -11,6 +11,7 @@
 #include <linux/netdevice.h>
 #include <linux/sched/signal.h>
 #include <linux/atomic.h>
+#include <linux/errqueue.h>
 #include <net/aligned_data.h>
 #include <net/rps.h>
 #include <net/sock.h>
@@ -919,21 +920,52 @@ static bool __mptcp_ofo_queue(struct mptcp_sock *msk)
 	return moved;
 }
 
+static bool mptcp_errqueue_skb_forwardable(const struct sk_buff *skb)
+{
+	/* Subflow-level ICMP errors are dropped: the legacy RECVERR ABI
+	 * cannot convey their per-subflow peer identity.
+	 */
+	return SKB_EXT_ERR(skb)->ee.ee_origin == SO_EE_ORIGIN_TIMESTAMPING;
+}
+
+static bool __mptcp_subflow_splice_errqueue(struct sock *sk, struct sock *ssk)
+{
+	struct sk_buff *skb;
+	bool moved = false;
+
+	while ((skb = skb_dequeue(&ssk->sk_error_queue))) {
+		/* sock_queue_err_skb() re-homes skb->sk onto the parent and
+		 * charges sk_rmem_alloc, bounding the queue by sk_rcvbuf.
+		 */
+		if (!mptcp_errqueue_skb_forwardable(skb) ||
+		    sock_queue_err_skb(sk, skb)) {
+			kfree_skb(skb);
+			continue;
+		}
+		moved = true;
+	}
+
+	return moved;
+}
+
 static bool __mptcp_subflow_error_report(struct sock *sk, struct sock *ssk)
 {
+	bool propagated = false;
 	int ssk_state;
+	bool report;
 	int err;
 
+	report = __mptcp_subflow_splice_errqueue(sk, ssk);
+
 	/* only propagate errors on fallen-back sockets or
 	 * on MPC connect
 	 */
 	if (sk->sk_state != TCP_SYN_SENT && !__mptcp_check_fallback(mptcp_sk(sk)))
-		return false;
+		goto out;
 
 	err = sock_error(ssk);
 	if (!err)
-		return false;
-
+		goto out;
 	/* We need to propagate only transition to CLOSE state.
 	 * Orphaned socket will see such state change via
 	 * subflow_sched_work_if_closed() and that path will properly
@@ -943,11 +975,16 @@ static bool __mptcp_subflow_error_report(struct sock *sk, struct sock *ssk)
 	if (ssk_state == TCP_CLOSE && !sock_flag(sk, SOCK_DEAD))
 		mptcp_set_state(sk, ssk_state);
 	WRITE_ONCE(sk->sk_err, -err);
+	report = true;
+	propagated = true;
 
-	/* This barrier is coupled with smp_rmb() in mptcp_poll() */
-	smp_wmb();
-	sk_error_report(sk);
-	return true;
+out:
+	if (report) {
+		/* This barrier is coupled with smp_rmb() in mptcp_poll() */
+		smp_wmb();
+		sk_error_report(sk);
+	}
+	return propagated;
 }
 
 void __mptcp_error_report(struct sock *sk)
@@ -2385,7 +2422,6 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 	int target;
 	long timeo;
 
-	/* MSG_ERRQUEUE is really a no-op till we support IP_RECVERR */
 	if (unlikely(flags & MSG_ERRQUEUE))
 		return inet_recv_error(sk, msg, len);
 
@@ -4523,7 +4559,8 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
 
 	/* This barrier is coupled with smp_wmb() in __mptcp_error_report() */
 	smp_rmb();
-	if (READ_ONCE(sk->sk_err))
+	if (READ_ONCE(sk->sk_err) ||
+	    !skb_queue_empty_lockless(&sk->sk_error_queue))
 		mask |= EPOLLERR;
 
 	return mask;

-- 
2.55.0

[PATCH net-next 2/4] mptcp: sockopt: factor inet_flags propagation into a mask

[Matthieu Baerts (NGI0)]

From: David Carlier <devnexen@gmail.com>

Introduce MPTCP_INET_FLAGS_MASK and replace the per-flag
inet_assign_bit() calls in sync_socket_options() with a loop driven
by the mask that calls assign_bit() per set bit, preserving the
per-bit atomicity of the original. Further flags propagated by MPTCP
can be added by extending the mask rather than touching the call
site.

No functional change.

Signed-off-by: David Carlier <devnexen@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/sockopt.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index fcf6feb2a9eb..2ba56832587e 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -16,6 +16,10 @@
 
 #define MIN_INFO_OPTLEN_SIZE		16
 #define MIN_FULL_INFO_OPTLEN_SIZE	40
+#define MPTCP_INET_FLAGS_MASK \
+	(BIT(INET_FLAGS_TRANSPARENT) | \
+	 BIT(INET_FLAGS_FREEBIND) | \
+	 BIT(INET_FLAGS_BIND_ADDRESS_NO_PORT))
 
 static struct sock *__mptcp_tcp_fallback(struct mptcp_sock *msk)
 {
@@ -1550,8 +1554,11 @@ int mptcp_getsockopt(struct sock *sk, int level, int optname,
 static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 {
 	static const unsigned int tx_rx_locks = SOCK_RCVBUF_LOCK | SOCK_SNDBUF_LOCK;
+	unsigned long mask = MPTCP_INET_FLAGS_MASK;
 	struct sock *sk = (struct sock *)msk;
+	unsigned long src;
 	bool keep_open;
+	int b;
 
 	keep_open = sock_flag(sk, SOCK_KEEPOPEN);
 	if (ssk->sk_prot->keepalive)
@@ -1597,9 +1604,11 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 	tcp_sock_set_keepcnt(ssk, msk->keepalive_cnt);
 	tcp_sock_set_maxseg(ssk, msk->maxseg);
 
-	inet_assign_bit(TRANSPARENT, ssk, inet_test_bit(TRANSPARENT, sk));
-	inet_assign_bit(FREEBIND, ssk, inet_test_bit(FREEBIND, sk));
-	inet_assign_bit(BIND_ADDRESS_NO_PORT, ssk, inet_test_bit(BIND_ADDRESS_NO_PORT, sk));
+	src = READ_ONCE(inet_sk(sk)->inet_flags);
+
+	for_each_set_bit(b, &mask, BITS_PER_LONG)
+		assign_bit(b, &inet_sk(ssk)->inet_flags, src & BIT(b));
+
 	WRITE_ONCE(inet_sk(ssk)->local_port_range, READ_ONCE(inet_sk(sk)->local_port_range));
 }
 

-- 
2.55.0

[PATCH net-next 3/4] mptcp: propagate RECVERR sockopts to subflows

[Matthieu Baerts (NGI0)]

From: 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: ipv6_setsockopt() on a sock with sk_family != AF_INET6
returns an error, which would abort the loop and leave the remaining
subflows desynchronised. This branch was unreachable before this
patch (the only caller was TCP_MAXSEG, family-agnostic); it becomes
live with the new IPV6_RECVERR / IPV6_RECVERR_RFC4884 caller and the
v4-subflow-on-AF_INET6-msk case (v4 MP_JOIN, or userspace PM grafting
a v4 subflow onto a v6 msk).

Assisted-by: Codex:gpt-5
Signed-off-by: David Carlier <devnexen@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/sockopt.c | 142 +++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 119 insertions(+), 23 deletions(-)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 2ba56832587e..922f6ae5c80c 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -8,6 +8,7 @@
 
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <net/ipv6.h>
 #include <net/sock.h>
 #include <net/protocol.h>
 #include <net/tcp.h>
@@ -19,7 +20,11 @@
 #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)
 {
@@ -398,6 +403,86 @@ 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);
+		int err;
+
+		/* SOL_IPV6 options on a v4 subflow (v4 MP_JOIN, or userspace PM
+		 * grafting a v4 subflow onto an AF_INET6 msk) would otherwise
+		 * abort the loop with -EAFNOSUPPORT from ipv6_setsockopt().
+		 */
+		if (level == SOL_IPV6 && ssk->sk_family != AF_INET6)
+			continue;
+
+		err = tcp_setsockopt(ssk, level, optname, optval, optlen);
+		if (err < 0 && ret == 0)
+			ret = err;
+	}
+
+	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 = 0, 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) {
+		if (sk->sk_family != AF_INET6)
+			return -ENOPROTOOPT;
+		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
+	}
+
+	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)
 {
@@ -440,6 +525,11 @@ 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;
@@ -785,6 +875,10 @@ 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;
@@ -812,28 +906,6 @@ 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);
-		int err;
-
-		err = tcp_setsockopt(ssk, level, optname, optval, optlen);
-		if (err < 0 && ret == 0)
-			ret = err;
-	}
-
-	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)
 {
@@ -1478,6 +1550,12 @@ 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;
@@ -1498,6 +1576,16 @@ 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:
+		if (sk->sk_family != AF_INET6)
+			return -ENOPROTOOPT;
+		return mptcp_put_int_option(msk, optval, optlen,
+					    inet6_test_bit(RECVERR6, sk));
+	case IPV6_RECVERR_RFC4884:
+		if (sk->sk_family != AF_INET6)
+			return -ENOPROTOOPT;
+		return mptcp_put_int_option(msk, optval, optlen,
+					    inet6_test_bit(RECVERR6_RFC4884, sk));
 	}
 
 	return -EOPNOTSUPP;
@@ -1606,6 +1694,14 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 
 	src = READ_ONCE(inet_sk(sk)->inet_flags);
 
+	/* RECVERR6 bits are only read on AF_INET6 sockets; copying them onto a
+	 * v4 subflow is dead state and diverges from the SOL_IPV6 skip in
+	 * mptcp_setsockopt_all_sf().
+	 */
+	if (ssk->sk_family != AF_INET6)
+		mask &= ~(BIT(INET_FLAGS_RECVERR6) |
+			BIT(INET_FLAGS_RECVERR6_RFC4884));
+
 	for_each_set_bit(b, &mask, BITS_PER_LONG)
 		assign_bit(b, &inet_sk(ssk)->inet_flags, src & BIT(b));
 

-- 
2.55.0

[PATCH net-next 4/4] selftests: mptcp: cover IP_RECVERR sockopt propagation

[Matthieu Baerts (NGI0)]

From: David Carlier <devnexen@gmail.com>

Exercise setsockopt/getsockopt of IP_RECVERR and IPV6_RECVERR on the
MPTCP parent socket, including the empty-errqueue EAGAIN contract on
MSG_ERRQUEUE|MSG_DONTWAIT.

End-to-end errqueue delivery (ICMP, TX timestamps, zerocopy) depends on
subflow-side producers that are out of scope for this series and will be
covered by follow-up work.

Assisted-by: Codex:gpt-5
Signed-off-by: David Carlier <devnexen@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
To: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
---
 tools/testing/selftests/net/mptcp/mptcp_sockopt.c | 70 +++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
index b6e58d936ebe..d68515b7903b 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_sockopt.c
@@ -183,6 +183,13 @@ static void xgetaddrinfo(const char *node, const char *service,
 	}
 }
 
+static bool expect_all_features(void)
+{
+	char *env = getenv("SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES");
+
+	return env && strcmp(env, "1") == 0;
+}
+
 static int sock_listen_mptcp(const char * const listenaddr,
 			     const char * const port)
 {
@@ -769,6 +776,68 @@ static void test_ip_tos_sockopt(int fd)
 		xerror("expect socklen_t == -1");
 }
 
+static void test_ip_recverr_sockopt(int fd)
+{
+	struct iovec iov = {
+		.iov_base = &(char){ 0 },
+		.iov_len = 1,
+	};
+	struct msghdr msg = {
+		.msg_iov = &iov,
+		.msg_iovlen = 1,
+	};
+	int one = 1, zero = 0, val = -1;
+	socklen_t s = sizeof(val);
+	int level, optname, r;
+
+	switch (pf) {
+	case AF_INET:
+		level = SOL_IP;
+		optname = IP_RECVERR;
+		break;
+	case AF_INET6:
+		level = SOL_IPV6;
+		optname = IPV6_RECVERR;
+		break;
+	default:
+		xerror("Unknown pf %d\n", pf);
+	}
+
+	r = setsockopt(fd, level, optname, &one, sizeof(one));
+	if (r) {
+		/* For older kernels not supporting IP(V6)_RECVERR yet */
+		if (errno == EOPNOTSUPP && !expect_all_features()) {
+			fprintf(stderr, "IP(V6)_RECVERR not supported, SKIP\n");
+			return;
+		}
+
+		die_perror("setsockopt IP(V6)_RECVERR on");
+	}
+
+	r = getsockopt(fd, level, optname, &val, &s);
+	if (r)
+		die_perror("getsockopt IP(V6)_RECVERR on");
+	if (s != sizeof(val) || val != one)
+		xerror("IP(V6)_RECVERR on mismatch val=%d len=%u", val, s);
+
+	r = recvmsg(fd, &msg, MSG_ERRQUEUE | MSG_DONTWAIT);
+	if (r != -1 || errno != EAGAIN)
+		xerror("expected empty errqueue to return EAGAIN, r=%d err=%d",
+		       r, errno);
+
+	r = setsockopt(fd, level, optname, &zero, sizeof(zero));
+	if (r)
+		die_perror("setsockopt IP(V6)_RECVERR off");
+
+	val = -1;
+	s = sizeof(val);
+	r = getsockopt(fd, level, optname, &val, &s);
+	if (r)
+		die_perror("getsockopt IP(V6)_RECVERR off");
+	if (s != sizeof(val) || val != zero)
+		xerror("IP(V6)_RECVERR off mismatch val=%d len=%u", val, s);
+}
+
 static int client(int pipefd)
 {
 	int fd = -1;
@@ -787,6 +856,7 @@ static int client(int pipefd)
 	}
 
 	test_ip_tos_sockopt(fd);
+	test_ip_recverr_sockopt(fd);
 
 	connect_one_server(fd, pipefd);
 

-- 
2.55.0