[net,1/2] tcp: Allow full IP tos/IPv6 tclass to be reflected in L3 header

From: Alexander Duyck <alexanderduyck@fb.com>

An issue was recently found where DCTCP SYN/ACK packets did not have the
ECT bit set in the L3 header. A bit of code review found that the recent
change referenced below had gone though and added a mask that prevented the
ECN bits from being populated in the L3 header.

This patch addresses that by rolling back the mask so that it is only
applied to the flags coming from the incoming TCP request instead of
applying it to the socket tos/tclass field. Doing this the ECT bits were
restored in the SYN/ACK packets in my testing.

One thing that is not addressed by this patch set is the fact that
tcp_reflect_tos appears to be incompatible with ECN based congestion
avoidance algorithms. At a minimum the feature should likely be documented
which it currently isn't.

Fixes: ac8f1710c12b ("tcp: reflect tos value received in SYN to the socket")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
---
 net/ipv4/tcp_ipv4.c |    5 +++--
 net/ipv6/tcp_ipv6.c |    6 +++---
 2 files changed, 6 insertions(+), 5 deletions(-)

On Thu, Nov 19, 2020 at 1:23 PM Alexander Duyck
<alexander.duyck@gmail.com> wrote:
>
> From: Alexander Duyck <alexanderduyck@fb.com>
>
> An issue was recently found where DCTCP SYN/ACK packets did not have the
> ECT bit set in the L3 header. A bit of code review found that the recent
> change referenced below had gone though and added a mask that prevented the
> ECN bits from being populated in the L3 header.
>
> This patch addresses that by rolling back the mask so that it is only
> applied to the flags coming from the incoming TCP request instead of
> applying it to the socket tos/tclass field. Doing this the ECT bits were
> restored in the SYN/ACK packets in my testing.
>
> One thing that is not addressed by this patch set is the fact that
> tcp_reflect_tos appears to be incompatible with ECN based congestion
> avoidance algorithms. At a minimum the feature should likely be documented
> which it currently isn't.
>
> Fixes: ac8f1710c12b ("tcp: reflect tos value received in SYN to the socket")

Acked-by: Wei Wang <weiwan@google.com>

Thanks for catching this. I was under the wrong impression that the
ECT bits were marked in tos after the tcp layer. Upon a closer look,
it seems right now, it only gets marked in inet_sock(sk)->tos from
tcp_init_congestion_control() once.
I will submit a follow-up fix to make sure we include the lower 2 bits
in the reflection case as well.

> Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
> ---
>  net/ipv4/tcp_ipv4.c |    5 +++--
>  net/ipv6/tcp_ipv6.c |    6 +++---
>  2 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index c2d5132c523c..c5f8b686aa82 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -981,7 +981,8 @@ static int tcp_v4_send_synack(const struct sock *sk, struct dst_entry *dst,
>         skb = tcp_make_synack(sk, dst, req, foc, synack_type, syn_skb);
>
>         tos = sock_net(sk)->ipv4.sysctl_tcp_reflect_tos ?
> -                       tcp_rsk(req)->syn_tos : inet_sk(sk)->tos;
> +                       tcp_rsk(req)->syn_tos & ~INET_ECN_MASK :
> +                       inet_sk(sk)->tos;
>
>         if (skb) {
>                 __tcp_v4_send_check(skb, ireq->ir_loc_addr, ireq->ir_rmt_addr);
> @@ -990,7 +991,7 @@ static int tcp_v4_send_synack(const struct sock *sk, struct dst_entry *dst,
>                 err = ip_build_and_send_pkt(skb, sk, ireq->ir_loc_addr,
>                                             ireq->ir_rmt_addr,
>                                             rcu_dereference(ireq->ireq_opt),
> -                                           tos & ~INET_ECN_MASK);
> +                                           tos);
>                 rcu_read_unlock();
>                 err = net_xmit_eval(err);
>         }
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 8db59f4e5f13..3d49e8d0afee 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -530,12 +530,12 @@ static int tcp_v6_send_synack(const struct sock *sk, struct dst_entry *dst,
>                 rcu_read_lock();
>                 opt = ireq->ipv6_opt;
>                 tclass = sock_net(sk)->ipv4.sysctl_tcp_reflect_tos ?
> -                               tcp_rsk(req)->syn_tos : np->tclass;
> +                               tcp_rsk(req)->syn_tos & ~INET_ECN_MASK :
> +                               np->tclass;
>                 if (!opt)
>                         opt = rcu_dereference(np->opt);
>                 err = ip6_xmit(sk, skb, fl6, sk->sk_mark, opt,
> -                              tclass & ~INET_ECN_MASK,
> -                              sk->sk_priority);
> +                              tclass, sk->sk_priority);
>                 rcu_read_unlock();
>                 err = net_xmit_eval(err);
>         }
>
>