Re: [PATCH v7 3/5] tcp: Resend SYN for inbound connections

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Nov 03, 2025 at 10:31:21AM +0800, Yumei Huang wrote:
> On Mon, Nov 3, 2025 at 9:10 AM David Gibson <david@gibson.dropbear.id.au> wrote:
> >
> > 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.
> > >
> > > Use the same backoff calculation for the timeout as Linux kernel.
> > >
> > > Link: https://bugs.passt.top/show_bug.cgi?id=153
> > > Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> >
> > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> >
> > 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(-)
> > >
> > > 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 flags:
> > >   *
> > > - * - 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 handshake
> >
> > 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) times,
> > > + *   reset the connection
> > >   *
> > >   * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending
> > >   *   data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the
> > > @@ -340,7 +342,7 @@ enum {
> > >  #define WINDOW_DEFAULT                       14600           /* RFC 6928 */
> > >
> > >  #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_QUEUE_MAX];
> > >
> > >  #define TCP_MIGRATE_RESTORE_CHUNK_MIN        1024 /* Try smaller when above this */
> > >
> > > +#define TCP_SYN_RETRIES              "/proc/sys/net/ipv4/tcp_syn_retries"
> > > +#define TCP_SYN_LINEAR_TIMEOUTS      "/proc/sys/net/ipv4/tcp_syn_linear_timeouts"
> > > +
> > > +#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];
> > >
> > > @@ -581,8 +589,10 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
> > >       if (conn->flags & ACK_TO_TAP_DUE) {
> > >               it.it_value.tv_nsec = (long)ACK_INTERVAL * 1000 * 1000;
> > >       } else if (conn->flags & ACK_FROM_TAP_DUE) {
> > > -             if (!(conn->events & ESTABLISHED))
> > > -                     it.it_value.tv_sec = SYN_TIMEOUT;
> > > +             if (!(conn->events & ESTABLISHED)) {
> > > +                     int exp = conn->retries - c->tcp.syn_linear_timeouts;
> > > +                     it.it_value.tv_sec = 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.
> 
> Oh, I didn't know about this style. Will update it later.

Since it's gone in the next patch, it doesn't really matter.

> > >                       it.it_value.tv_sec = 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 >= TCP_MAX_RETRIES ||
> > > +                         conn->retries >= (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.
> 
> Just checked net/ipv4/sysctl_net_ipv4.c,
> 
>        static int tcp_syn_retries_min = 1;
>         static int tcp_syn_retries_max = MAX_TCP_SYNCNT;
>         static int tcp_syn_linear_timeouts_max = MAX_TCP_SYNCNT;
> 
> And MAX_TCP_SYNCNT is defined as 127:
> 
>         #define MAX_TCP_SYNCNT        127
> 
> they have a max value as 127, so there won't be overflows.

Ok.  That's not at all obvious from within the passt code, though.
Again, this is really only a theoretical problem, but I think ideally
we'd clamp to 127 where we read the values out of the kernel, with a
comment saying that limit is derived from the kernel's own limit.

> 
> >
> > > +                             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;
> > >  }
> > >
> > > +/**
> > > + * 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 = read_file_integer(
> > > +             TCP_SYN_RETRIES, TCP_SYN_RETRIES_DEFAULT);
> > > +     syn_linear_timeouts = read_file_integer(
> > > +             TCP_SYN_LINEAR_TIMEOUTS, TCP_SYN_LINEAR_TIMEOUTS_DEFAULT);
> > > +
> > > +     c->tcp.tcp_syn_retries = MIN(tcp_syn_retries, UINT8_MAX);
> > > +     c->tcp.syn_linear_timeouts = 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);
> > >
> > > +     tcp_get_rto_params(c);
> > > +
> > >       tcp_sock_iov_init(c);
> > >
> > >       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 timeout
> > >   */
> > >  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;
> > >  };
> > >
> > >  #endif /* TCP_H */
> > > --
> > > 2.49.0
> > >
> >
> > --
> > 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
> 
> 
> 
> -- 
> Thanks,
> 
> Yumei Huang
> 

-- 
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

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