From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au Authentication-Results: passt.top; dkim=pass (2048-bit key; secure) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.a=rsa-sha256 header.s=202510 header.b=fyGwUGBl; dkim-atps=neutral Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id A735E5A026F for ; Mon, 03 Nov 2025 02:10:32 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202510; t=1762132229; bh=xva3NhwHYxQH/QJvksh1gpLUQkhXZGABqDUangqI4Pk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=fyGwUGBlJb4H20EiL5Up0FyWKy6aMqv9iKvlFzzWsHPvHKXOmSkHfrru9StJqfBq+ gq3KDqwBUGFpWRxWG4zEjIJAb0nI5mGDFdmz0Y50MDc1gLwEG0aSY7DYOTDAJQvMZZ SitiPd8OlfZwFE5KhB4bjBHIX9RgjFgLVj5zW6Pp+sQkATiSmCK8Dl3h+qFBR3/fHp niiMmAQqNRhE0tbRLmgwZ/IVHTg+DJ5mJ0y/GuOWpJHroho+zQkjUBxk3PUIKmOY0N nWqlSnCa7SFHV45jE1sSIUhuUIOvXc3l4gmlNgitDIRiVq35PjZSSHEq/oPNKE5v6U lq8JsJ0S64Phg== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4d0D6F2jTSz4wD0; Mon, 03 Nov 2025 12:10:29 +1100 (AEDT) Date: Mon, 3 Nov 2025 12:09:47 +1100 From: David Gibson To: Yumei Huang Subject: Re: [PATCH v7 3/5] tcp: Resend SYN for inbound connections Message-ID: References: <20251031054242.7334-1-yuhuang@redhat.com> <20251031054242.7334-4-yuhuang@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="08/yPAbSyiRNX9Rt" Content-Disposition: inline In-Reply-To: <20251031054242.7334-4-yuhuang@redhat.com> Message-ID-Hash: 2KLWEYVBKISQK3HGA7I7HZ6TVVJ6WWYY X-Message-ID-Hash: 2KLWEYVBKISQK3HGA7I7HZ6TVVJ6WWYY X-MailFrom: dgibson@gandalf.ozlabs.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top, sbrivio@redhat.com X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: --08/yPAbSyiRNX9Rt Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Oct 31, 2025 at 01:42:40PM +0800, Yumei Huang wrote: > If a client connects while guest is not connected or ready yet, > resend SYN instead of just resetting connection after 10 seconds. >=20 > Use the same backoff calculation for the timeout as Linux kernel. >=20 > Link: https://bugs.passt.top/show_bug.cgi?id=3D153 > Signed-off-by: Yumei Huang Reviewed-by: David Gibson There are a couple of tiny style nits and a mostly theoretical bug noted below. > --- > tcp.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++-------- > tcp.h | 4 ++++ > 2 files changed, 53 insertions(+), 8 deletions(-) >=20 > diff --git a/tcp.c b/tcp.c > index 2ec4b0c..bada88a 100644 > --- a/tcp.c > +++ b/tcp.c > @@ -179,9 +179,11 @@ > * > * Timeouts are implemented by means of timerfd timers, set based on fla= gs: > * > - * - SYN_TIMEOUT: if no ACK is received from tap/guest during handshake = (flag > - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, reset= the > - * connection > + * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during hands= hake Nit: I'd maybe say SYN,ACK here to distinguish it from other ACKs. > + * (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this time,= resend > + * SYN. It's the starting timeout for the first SYN retry. Retry for > + * TCP_MAX_RETRIES or (tcp_syn_retries + tcp_syn_linear_timeouts) time= s, > + * reset the connection > * > * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after s= ending > * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data f= rom the > @@ -340,7 +342,7 @@ enum { > #define WINDOW_DEFAULT 14600 /* RFC 6928 */ > =20 > #define ACK_INTERVAL 10 /* ms */ > -#define SYN_TIMEOUT 10 /* s */ > +#define SYN_TIMEOUT_INIT 1 /* s, RFC 6928 */ > #define ACK_TIMEOUT 2 > #define FIN_TIMEOUT 60 > #define ACT_TIMEOUT 7200 > @@ -365,6 +367,12 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUEU= E_MAX]; > =20 > #define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller when above thi= s */ > =20 > +#define TCP_SYN_RETRIES "/proc/sys/net/ipv4/tcp_syn_retries" > +#define TCP_SYN_LINEAR_TIMEOUTS "/proc/sys/net/ipv4/tcp_syn_linear_timeo= uts" > + > +#define TCP_SYN_RETRIES_DEFAULT 6 > +#define TCP_SYN_LINEAR_TIMEOUTS_DEFAULT 4 > + > /* "Extended" data (not stored in the flow table) for TCP flow migration= */ > static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX]; > =20 > @@ -581,8 +589,10 @@ static void tcp_timer_ctl(const struct ctx *c, struc= t tcp_tap_conn *conn) > if (conn->flags & ACK_TO_TAP_DUE) { > it.it_value.tv_nsec =3D (long)ACK_INTERVAL * 1000 * 1000; > } else if (conn->flags & ACK_FROM_TAP_DUE) { > - if (!(conn->events & ESTABLISHED)) > - it.it_value.tv_sec =3D SYN_TIMEOUT; > + if (!(conn->events & ESTABLISHED)) { > + int exp =3D conn->retries - c->tcp.syn_linear_timeouts; > + it.it_value.tv_sec =3D SYN_TIMEOUT_INIT << MAX(exp, 0); > + } > else A non-obvious detail of the style we use is that if one branch of an if uses { }, then the other branch should as well, even if it's one line. So above should be "} else {" and the bit below changed to match. > it.it_value.tv_sec =3D ACK_TIMEOUT; > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > @@ -2409,8 +2419,17 @@ void tcp_timer_handler(const struct ctx *c, union = epoll_ref ref) > tcp_timer_ctl(c, conn); > } else if (conn->flags & ACK_FROM_TAP_DUE) { > if (!(conn->events & ESTABLISHED)) { > - flow_dbg(conn, "handshake timeout"); > - tcp_rst(c, conn); > + if (conn->retries >=3D TCP_MAX_RETRIES || > + conn->retries >=3D (c->tcp.tcp_syn_retries + > + c->tcp.syn_linear_timeouts)) { Here's the theoretical bug. We only clamp tcp_syn_retries and syn_linear_timeouts to a uint8_t, so if the host has these set to strangely large values, this addition could overflow. > + flow_dbg(conn, "handshake timeout"); > + tcp_rst(c, conn); > + } else { > + flow_trace(conn, "SYN timeout, retry"); > + tcp_send_flag(c, conn, SYN); > + conn->retries++; > + tcp_timer_ctl(c, conn); > + } > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > flow_dbg(conn, "FIN timeout"); > tcp_rst(c, conn); > @@ -2766,6 +2785,26 @@ static socklen_t tcp_probe_tcp_info(void) > return sl; > } > =20 > +/** > + * tcp_get_rto_params() - Get host kernel RTO parameters > + * @c: Execution context > +*/ > +void tcp_get_rto_params(struct ctx *c) > +{ > + intmax_t tcp_syn_retries, syn_linear_timeouts; > + > + tcp_syn_retries =3D read_file_integer( > + TCP_SYN_RETRIES, TCP_SYN_RETRIES_DEFAULT); > + syn_linear_timeouts =3D read_file_integer( > + TCP_SYN_LINEAR_TIMEOUTS, TCP_SYN_LINEAR_TIMEOUTS_DEFAULT); > + > + c->tcp.tcp_syn_retries =3D MIN(tcp_syn_retries, UINT8_MAX); > + c->tcp.syn_linear_timeouts =3D MIN(syn_linear_timeouts, UINT8_MAX); > + > + debug("Read sysctl values tcp_syn_retries: %"PRIu8", linear_timeouts: %= "PRIu8, > + c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts); > +} > + > /** > * tcp_init() - Get initial sequence, hash secret, initialise per-socket= data > * @c: Execution context > @@ -2776,6 +2815,8 @@ int tcp_init(struct ctx *c) > { > ASSERT(!c->no_tcp); > =20 > + tcp_get_rto_params(c); > + > tcp_sock_iov_init(c); > =20 > memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)); > diff --git a/tcp.h b/tcp.h > index 234a803..befedde 100644 > --- a/tcp.h > +++ b/tcp.h > @@ -59,12 +59,16 @@ union tcp_listen_epoll_ref { > * @fwd_out: Port forwarding configuration for outbound packets > * @timer_run: Timestamp of most recent timer run > * @pipe_size: Size of pipes for spliced connections > + * @tcp_syn_retries: SYN retries using exponential backoff timeout > + * @syn_linear_timeouts: SYN retries before using exponential backoff ti= meout > */ > struct tcp_ctx { > struct fwd_ports fwd_in; > struct fwd_ports fwd_out; > struct timespec timer_run; > size_t pipe_size; > + uint8_t tcp_syn_retries; > + uint8_t syn_linear_timeouts; > }; > =20 > #endif /* TCP_H */ > --=20 > 2.49.0 >=20 --=20 David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson --08/yPAbSyiRNX9Rt Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmkIANoACgkQzQJF27ox 2GeIYA/7BzBmZSWtUy2HL7q9FaBpvMc5kCB8Ug8xP4dgG33eW14s4RtD8lwvfGBp nj6e3Q1LUZ0VeaqZ+ytiFiBV6T+TkW9QCC0cMsPRcp0/CfDcnhI0cJMtKiSydUH4 SKoZzbBpOUwiXP4EHHRdVw6IfPEQY314VFzJQYh9HKIc1dL+vYVHR8hz0oBAVQdV BJTRwi5Ox0TfD2zrEoSa+/FJuiokcP4WRxaoCN8EhctRr2utMOP8jHhZA+sAVNVs fhTPVASOJAieoC99bwv6/luG20fN317+LdFnLkH+PyjwTcniSsxsILfPUpWoRt0w I4MHmY1vOGYCLgJ83XPQ3/Tnxcb2sl1K7slJ/9UAFlW4xcWMAXNDqk6jL+z/edZA Pva4XqOTP9HOoMgW64VYHhBzrEtTRGg3uA9WgsuKxckg1B1lA9ykRdCjrdYr5TrH NR8NL7oaXJXWRi98UecgOCGRUTgpVcRwE/BFCuNawcH4w8LXbvhQhv7wpx7GVQpF H2TbkrEHhpa6LeUYEIuflUQ1DjEWmsvsxtvGkf5XQpwVJZeSRbzKpXZ+KMQSAaq6 YPPA7RuEijHS6nKBv9MQxk6GWTBymiMsBEctGe6pDPk4LSfiyiXjvUpLQG8jOFes kT4Qeym/ibXNiBC0Y3KVQURUOYCWhoCTmPDgbbe/2lBKJcdBlJs= =uyfw -----END PGP SIGNATURE----- --08/yPAbSyiRNX9Rt--