rds_sendmsg() reuses the connection cached in rs->rs_conn when its peer address and ToS match the request. The interface index is part of a connection's identity as well: rds_conn_create_outgoing() passes the request's scope_id down as dev_if, and rds_conn_lookup() compares c_dev_if, so sends to the same link-local address through two interfaces are two different connections. The cache-hit test never looked at it. A socket bound to a non-link-local address has rs_bound_scope_id 0, and the scope check at the top of rds_sendmsg() accepts any non-zero destination scope for such a socket. So after a send to fe80::x%ifA, a send to fe80::x%ifB hits the cached ifA connection and the datagram leaves through ifA, to whichever peer answers to that address there. Compare c_dev_if with the request's scope_id in the cache test, so that such a send takes the lookup path and finds, or creates, the ifB connection - the same key the lookup uses. With the previous patch a send without a destination carries the connected peer's scope, so the comparison is exact for it too. Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support") Assisted-by: Claude-Code:claude-fable-5 Signed-off-by: Allison Henderson --- net/rds/send.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/net/rds/send.c b/net/rds/send.c index 9380b67675bd..26a937a9df6a 100644 --- a/net/rds/send.c +++ b/net/rds/send.c @@ -1342,7 +1342,8 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len) /* rds_conn_create has a spinlock that runs with IRQ off. * Caching the conn in the socket helps a lot. */ if (rs->rs_conn && ipv6_addr_equal(&rs->rs_conn->c_faddr, &daddr) && - rs->rs_tos == rs->rs_conn->c_tos) { + rs->rs_tos == rs->rs_conn->c_tos && + rs->rs_conn->c_dev_if == scope_id) { conn = rs->rs_conn; } else { conn = rds_conn_create_outgoing(sock_net(sock->sk), -- 2.25.1