Re: [PATCH 2/3] tcp: Properly propagate tap-side RST to socket side

On Tue, 27 Jan 2026 19:39:52 +1100 David Gibson wrote:

When the guest sends a TCP RST, or on certain error conditions, we want to signal the abnormal termination of a TCP connection to the peer with an RST as well. We attempt to do that by close()ing the socket.

That doesn't work: a close() will usually send a FIN, rather than an RST. The standard method of forcing an RST on a socket is to set the SO_LINGER socket option with a 0 timeout, then close().

Update the tcp_rst() path to do this, so it forces a socket side RST. Update the handling of a guest side RST to use the same path (minus sending a tap side RST) so that we properly propagate guest RSTs to the peer.

Link: https://bugs.passt.top/show_bug.cgi?id=191

Signed-off-by: David Gibson --- tcp.c | 37 +++++++++++++++++++++++++++++++++---- 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/tcp.c b/tcp.c index 45dde5a0..9da37c2f 100644 --- a/tcp.c +++ b/tcp.c @@ -1403,7 +1403,34 @@ static int tcp_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, }

/** - * tcp_rst_do() - Reset a tap connection: send RST segment to tap, close socket + * tcp_sock_rst() - Close TCP connection forcing RST on socket side + * @c: Execution context + * @conn: Connection pointer + */ +static void tcp_sock_rst(const struct ctx *c, struct tcp_tap_conn *conn) +{ + const struct linger linger0 = { + .l_onoff = 1, + .l_linger = 0, + }; + + /* Force RST on socket to inform the peer + * + * We do this by setting SO_LINGER with 0 timeout, which means that + * close() will send an RST (unless the connection is already closed in + * both directions). + */ + if (setsockopt(conn->sock, SOL_SOCKET, + SO_LINGER, &linger0, sizeof(linger0)) < 0) { + flow_dbg_perror(conn, + "SO_LINGER failed, may not send RST to peer"); + } + + conn_event(c, conn, CLOSED); +} + +/** + * tcp_rst_do() - Reset a tap connection: send RST segment on both sides, close * @c: Execution context * @conn: Connection pointer */ @@ -1412,8 +1439,10 @@ void tcp_rst_do(const struct ctx *c, struct tcp_tap_conn *conn) if (conn->events == CLOSED) return;

+ /* Send RST on tap */ tcp_send_flag(c, conn, RST); - conn_event(c, conn, CLOSED); + + tcp_sock_rst(c, conn); }

/** @@ -1884,7 +1913,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn, return -1;

if (th->rst) { - conn_event(c, conn, CLOSED); + tcp_sock_rst(c, conn);

The whole series looks good to me, except for one exceedingly minor aspect: should we do this also in the getsockopt() error handling path of tcp_prepare_flags()? I would be inclined to apply it regardless of that, the fix is critical enough. I'll start the usual test run in a few hours. -- Stefano