[PATCH v2] vsock: keep SOCK_SEQPACKET message boundaries on interrupted send
From: Bartłomiej Dmitruk
Date: Sat Sep 19 2026 - 08:31:57 EST
A credit-limited SOCK_SEQPACKET send transmits fragments as credit becomes
available, and the VIRTIO_VSOCK_SEQ_EOM flag is set only on the fragment
where msg_data_left() reaches 0. If vsock_connectible_sendmsg() exits via
out_err after a partial send -- notably the non-terminal -EINTR path
(signal_pending while blocked for credit), but also sk_err / peer
RCV_SHUTDOWN -- the already-transmitted fragments carry no EOM. The receiver
only advances msg_count / sets msg_ready on an EOM skb, so the orphaned
fragments are silently merged into the next message, violating SOCK_SEQPACKET
atomicity.
Wait until the whole remaining SEQPACKET message fits before enqueuing, so a
message is committed atomically (with its EOM) or not started; an error while
waiting then leaves nothing on the wire. SOCK_STREAM behaviour is unchanged
(min_space == 1 reproduces the old "wait while space == 0").
To avoid blocking forever on a message that can never fit -- a peer can
advertise a small buf_alloc, or the message can simply exceed the transmit
buffer -- reject such a message up front with -EMSGSIZE (the same error the
transport already returns for an oversized message) instead of waiting. This
needs the transport's maximum message size, added as a new optional
seqpacket_max_size() transport op implemented by the virtio/loopback
transports.
Signed-off-by: Bartłomiej Dmitruk <bartlomiej.dmitruk@xxxxxxx>
Assisted-by: Claude (Anthropic)
---
Changes in v2:
- Fix an indefinite wait the v1 approach introduced for oversized
SOCK_SEQPACKET messages (reported by the Sashiko AI review on v1): the
atomic-wait could never be satisfied when len > the transport's max
message size (e.g. a peer advertising a small peer_buf_alloc), so the
sender blocked instead of returning -EMSGSIZE. v2 checks this up front via
the new seqpacket_max_size() op and returns -EMSGSIZE without waiting.
- v1: https://lore.kernel.org/netdev/20260917220101.55744-1-bartlomiej.dmitruk@xxxxxxx/
Testing (vsock_loopback, unprivileged):
- Interrupted partial send (-EINTR) no longer leaks orphan bytes into the
next message (recv() returns only the next message).
- Oversized message and small-peer_buf_alloc cases return -EMSGSIZE promptly
instead of blocking.
- Normal SEQPACKET send/recv unaffected.
diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index f91704731..e96790c03 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -219,6 +219,7 @@ virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
s64 virtio_transport_stream_has_data(struct vsock_sock *vsk);
s64 virtio_transport_stream_has_space(struct vsock_sock *vsk);
u32 virtio_transport_seqpacket_has_data(struct vsock_sock *vsk);
+u32 virtio_transport_seqpacket_max_size(struct vsock_sock *vsk);
ssize_t virtio_transport_unsent_bytes(struct vsock_sock *vsk);
diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index 5549298c1..d94c613ef 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h
@@ -143,6 +143,7 @@ struct vsock_transport {
size_t len);
bool (*seqpacket_allow)(struct vsock_sock *vsk, u32 remote_cid);
u32 (*seqpacket_has_data)(struct vsock_sock *vsk);
+ u32 (*seqpacket_max_size)(struct vsock_sock *vsk);
/* Notification. */
int (*notify_poll_in)(struct vsock_sock *, size_t, bool *);
diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index f840498b5..ac64cb957 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -2250,9 +2250,32 @@ static int vsock_connectible_sendmsg(struct socket *sock, struct msghdr *msg,
while (total_written < len) {
ssize_t written;
+ s64 min_space;
+
+ if (sk->sk_type == SOCK_SEQPACKET) {
+ /* A SEQPACKET message must be delivered atomically, so
+ * wait until the whole remaining message fits before
+ * enqueuing. Otherwise a credit-limited partial send that
+ * later errors out (e.g. -EINTR) leaves EOM-less fragments
+ * that the peer merges into the next message.
+ *
+ * Reject a message that can never fit up front so the wait
+ * below cannot block forever (a peer may advertise a small
+ * buf_alloc); this mirrors the -EMSGSIZE the transport
+ * returns for an oversized message.
+ */
+ if (transport->seqpacket_max_size &&
+ len > transport->seqpacket_max_size(vsk)) {
+ err = -EMSGSIZE;
+ goto out_err;
+ }
+ min_space = len - total_written;
+ } else {
+ min_space = 1;
+ }
add_wait_queue(sk_sleep(sk), &wait);
- while (vsock_stream_has_space(vsk) == 0 &&
+ while (vsock_stream_has_space(vsk) < min_space &&
sk->sk_err == 0 &&
!(sk->sk_shutdown & SEND_SHUTDOWN) &&
!(vsk->peer_shutdown & RCV_SHUTDOWN)) {
diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 4f9aa9c4c..b7d587a20 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -585,6 +585,7 @@ static struct virtio_transport virtio_transport = {
.seqpacket_enqueue = virtio_transport_seqpacket_enqueue,
.seqpacket_allow = virtio_transport_seqpacket_allow,
.seqpacket_has_data = virtio_transport_seqpacket_has_data,
+ .seqpacket_max_size = virtio_transport_seqpacket_max_size,
.msgzerocopy_allow = virtio_transport_msgzerocopy_allow,
diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index f225f53ed..e10e7b958 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -994,6 +994,19 @@ virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
}
EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_enqueue);
+u32 virtio_transport_seqpacket_max_size(struct vsock_sock *vsk)
+{
+ struct virtio_vsock_sock *vvs = vsk->trans;
+ u32 max_size;
+
+ spin_lock_bh(&vvs->tx_lock);
+ max_size = virtio_transport_tx_buf_size(vvs);
+ spin_unlock_bh(&vvs->tx_lock);
+
+ return max_size;
+}
+EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_max_size);
+
int
virtio_transport_dgram_dequeue(struct vsock_sock *vsk,
struct msghdr *msg,
diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
index 8068d1b6e..b9a0cc861 100644
--- a/net/vmw_vsock/vsock_loopback.c
+++ b/net/vmw_vsock/vsock_loopback.c
@@ -90,6 +90,7 @@ static struct virtio_transport loopback_transport = {
.seqpacket_enqueue = virtio_transport_seqpacket_enqueue,
.seqpacket_allow = vsock_loopback_seqpacket_allow,
.seqpacket_has_data = virtio_transport_seqpacket_has_data,
+ .seqpacket_max_size = virtio_transport_seqpacket_max_size,
.msgzerocopy_allow = vsock_loopback_msgzerocopy_allow,