[PATCH v4 1/2] bpf, sockmap: handle spurious tcp_msg_wait_data() wakeup
From: Nnamdi Onyeyiri <hidden>
Date: 2026-07-15 21:35:45
Also in:
bpf, sashiko-reviews
Subsystem:
bpf [l7 framework] (sockmap), networking [general], networking [tcp], the rest · Maintainers:
John Fastabend, Jakub Sitnicki, Jiayuan Chen, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Neal Cardwell, Linus Torvalds
recvfrom()/recv() are documented as only returning EAGAIN for blocking sockets
when they have a receive timeout configured. However, adding a blocking
ipv4 tcp socket without a receive timeout to a sockmap will cause EAGAIN errors
sporadically. A socket with a receive timeout may return EAGAIN before the
timeout expires.
There are 2 code paths affected by this:
1. tcp_bpf_recvmsg() - Used when the socket has been added to a sockmap
that has no verdict program attached.
2. tcp_bpf_recvmsg_parser() - Used when the socket has been added to a
sockmap that has a verdict program. To reproduce this issue, it is
enough for the verdict program to do nothing but return SK_PASS.
In both cases this happens when tcp_msg_wait_data() wakes spuriously
(returning 0). To fix it, we now loop back to msg_bytes_ready instead
of returning -EAGAIN on spurious wakeup.
To ensure the looping does not cause sockets with a SO_RCVTIMEO set to
wait excessively long, tcp_msg_wait_data() now takes a pointer to timeo,
allowing sk_wait_event() to update it as appropriate.
The logic in tcp_bpf_recvmsg_parser() that allow it to handle signals,
socket errors and closuers in its loop was also added to tcp_bpf_recvmsg().
Signed-off-by: Nnamdi Onyeyiri <redacted>
---
net/ipv4/tcp_bpf.c | 69 ++++++++++++++++++++++++++++++++++++++++------
1 file changed, 60 insertions(+), 9 deletions(-)
diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index cc0bd73f36b6..aa5c5d741599 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c@@ -179,7 +179,7 @@ EXPORT_SYMBOL_GPL(tcp_bpf_sendmsg_redir); #ifdef CONFIG_BPF_SYSCALL static int tcp_msg_wait_data(struct sock *sk, struct sk_psock *psock, - long timeo) + long *timeo) { DEFINE_WAIT_FUNC(wait, woken_wake_function); int ret = 0;
@@ -187,12 +187,12 @@ static int tcp_msg_wait_data(struct sock *sk, struct sk_psock *psock, if (sk->sk_shutdown & RCV_SHUTDOWN) return 1; - if (!timeo) + if (!*timeo) return ret; add_wait_queue(sk_sleep(sk), &wait); sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk); - ret = sk_wait_event(sk, &timeo, + ret = sk_wait_event(sk, timeo, !list_empty(&psock->ingress_msg) || !skb_queue_empty_lockless(&sk->sk_receive_queue), &wait); sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
@@ -229,6 +229,7 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, int copied_from_self = 0; int copied = 0; u32 seq; + long timeo; if (unlikely(flags & MSG_ERRQUEUE)) return inet_recv_error(sk, msg, len);
@@ -262,6 +263,8 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, } } + timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); + msg_bytes_ready: copied = __sk_msg_recvmsg(sk, psock, msg, len, flags, &copied_from_self); /* The typical case for EFAULT is the socket was gracefully
@@ -280,7 +283,6 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, } seq += copied_from_self; if (!copied) { - long timeo; int data; if (sock_flag(sk, SOCK_DONE))
@@ -299,7 +301,6 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, goto out; } - timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); if (!timeo) { copied = -EAGAIN; goto out;
@@ -310,13 +311,15 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, goto out; } - data = tcp_msg_wait_data(sk, psock, timeo); + data = tcp_msg_wait_data(sk, psock, &timeo); if (data < 0) { copied = data; goto unlock; } if (data && !sk_psock_queue_empty(psock)) goto msg_bytes_ready; + if (!data && timeo > 0) + goto msg_bytes_ready; copied = -EAGAIN; } out:
@@ -355,6 +358,7 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, { struct sk_psock *psock; int copied, ret; + long timeo; if (unlikely(flags & MSG_ERRQUEUE)) return inet_recv_error(sk, msg, len);
@@ -371,14 +375,59 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, return tcp_recvmsg(sk, msg, len, flags); } lock_sock(sk); + + timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); + msg_bytes_ready: copied = sk_msg_recvmsg(sk, psock, msg, len, flags); if (!copied) { - long timeo; int data; - timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT); - data = tcp_msg_wait_data(sk, psock, timeo); + if (sock_flag(sk, SOCK_DONE)) { + ret = 0; + goto unlock; + } + + if (sk->sk_err) { + if (!sk_psock_queue_empty(psock)) + goto msg_bytes_ready; + if (!skb_queue_empty(&sk->sk_receive_queue)) { + release_sock(sk); + sk_psock_put(sk, psock); + return tcp_recvmsg(sk, msg, len, flags); + } + ret = sock_error(sk); + goto unlock; + } + + if (sk->sk_shutdown & RCV_SHUTDOWN) { + if (!sk_psock_queue_empty(psock)) + goto msg_bytes_ready; + if (!skb_queue_empty(&sk->sk_receive_queue)) { + release_sock(sk); + sk_psock_put(sk, psock); + return tcp_recvmsg(sk, msg, len, flags); + } + ret = 0; + goto unlock; + } + + if (sk->sk_state == TCP_CLOSE) { + ret = -ENOTCONN; + goto unlock; + } + + if (!timeo) { + ret = -EAGAIN; + goto unlock; + } + + if (signal_pending(current)) { + ret = sock_intr_errno(timeo); + goto unlock; + } + + data = tcp_msg_wait_data(sk, psock, &timeo); if (data < 0) { ret = data; goto unlock;
@@ -390,6 +439,8 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, sk_psock_put(sk, psock); return tcp_recvmsg(sk, msg, len, flags); } + if (!data && timeo > 0) + goto msg_bytes_ready; copied = -EAGAIN; } ret = copied;
--
2.52.0