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Date:   Wed, 13 May 2020 15:21:24 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        dccp@...r.kernel.org, kernel-team <kernel-team@...udflare.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Gerrit Renker <gerrit@....abdn.ac.uk>,
        Jakub Kicinski <kuba@...nel.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Marek Majkowski <marek@...udflare.com>
Subject: Re: [PATCH bpf-next v2 05/17] inet: Run SK_LOOKUP BPF program on
 socket lookup

On Tue, 12 May 2020 at 14:52, Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Mon, May 11, 2020 at 10:44 PM CEST, Alexei Starovoitov wrote:
> > On Mon, May 11, 2020 at 08:52:06PM +0200, Jakub Sitnicki wrote:
> >> Run a BPF program before looking up a listening socket on the receive path.
> >> Program selects a listening socket to yield as result of socket lookup by
> >> calling bpf_sk_assign() helper and returning BPF_REDIRECT code.
> >>
> >> Alternatively, program can also fail the lookup by returning with BPF_DROP,
> >> or let the lookup continue as usual with BPF_OK on return.
> >>
> >> This lets the user match packets with listening sockets freely at the last
> >> possible point on the receive path, where we know that packets are destined
> >> for local delivery after undergoing policing, filtering, and routing.
> >>
> >> With BPF code selecting the socket, directing packets destined to an IP
> >> range or to a port range to a single socket becomes possible.
> >>
> >> Suggested-by: Marek Majkowski <marek@...udflare.com>
> >> Reviewed-by: Lorenz Bauer <lmb@...udflare.com>
> >> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> >> ---
> >>  include/net/inet_hashtables.h | 36 +++++++++++++++++++++++++++++++++++
> >>  net/ipv4/inet_hashtables.c    | 15 ++++++++++++++-
> >>  2 files changed, 50 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
> >> index 6072dfbd1078..3fcbc8f66f88 100644
> >> --- a/include/net/inet_hashtables.h
> >> +++ b/include/net/inet_hashtables.h
> >> @@ -422,4 +422,40 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
> >>
> >>  int inet_hash_connect(struct inet_timewait_death_row *death_row,
> >>                    struct sock *sk);
> >> +
> >> +static inline struct sock *bpf_sk_lookup_run(struct net *net,
> >> +                                         struct bpf_sk_lookup_kern *ctx)
> >> +{
> >> +    struct bpf_prog *prog;
> >> +    int ret = BPF_OK;
> >> +
> >> +    rcu_read_lock();
> >> +    prog = rcu_dereference(net->sk_lookup_prog);
> >> +    if (prog)
> >> +            ret = BPF_PROG_RUN(prog, ctx);
> >> +    rcu_read_unlock();
> >> +
> >> +    if (ret == BPF_DROP)
> >> +            return ERR_PTR(-ECONNREFUSED);
> >> +    if (ret == BPF_REDIRECT)
> >> +            return ctx->selected_sk;
> >> +    return NULL;
> >> +}
> >> +
> >> +static inline struct sock *inet_lookup_run_bpf(struct net *net, u8 protocol,
> >> +                                           __be32 saddr, __be16 sport,
> >> +                                           __be32 daddr, u16 dport)
> >> +{
> >> +    struct bpf_sk_lookup_kern ctx = {
> >> +            .family         = AF_INET,
> >> +            .protocol       = protocol,
> >> +            .v4.saddr       = saddr,
> >> +            .v4.daddr       = daddr,
> >> +            .sport          = sport,
> >> +            .dport          = dport,
> >> +    };
> >> +
> >> +    return bpf_sk_lookup_run(net, &ctx);
> >> +}
> >> +
> >>  #endif /* _INET_HASHTABLES_H */
> >> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> >> index ab64834837c8..f4d07285591a 100644
> >> --- a/net/ipv4/inet_hashtables.c
> >> +++ b/net/ipv4/inet_hashtables.c
> >> @@ -307,9 +307,22 @@ struct sock *__inet_lookup_listener(struct net *net,
> >>                                  const int dif, const int sdif)
> >>  {
> >>      struct inet_listen_hashbucket *ilb2;
> >> -    struct sock *result = NULL;
> >> +    struct sock *result, *reuse_sk;
> >>      unsigned int hash2;
> >>
> >> +    /* Lookup redirect from BPF */
> >> +    result = inet_lookup_run_bpf(net, hashinfo->protocol,
> >> +                                 saddr, sport, daddr, hnum);
> >> +    if (IS_ERR(result))
> >> +            return NULL;
> >> +    if (result) {
> >> +            reuse_sk = lookup_reuseport(net, result, skb, doff,
> >> +                                        saddr, sport, daddr, hnum);
> >> +            if (reuse_sk)
> >> +                    result = reuse_sk;
> >> +            goto done;
> >> +    }
> >> +
> >
> > The overhead is too high to do this all the time.
> > The feature has to be static_key-ed.
>
> Static keys is something that Lorenz has also suggested internally, but
> we wanted to keep it simple at first.
>
> Introduction of static keys forces us to decide when non-init_net netns
> are allowed to attach to SK_LOOKUP, as attaching enabling SK_LOOKUP in
> isolated netns will affect the rx path in init_net.
>
> I see two options, which seem sensible:
>
> 1) limit SK_LOOKUP to init_net, which makes testing setup harder, or
>
> 2) allow non-init_net netns to attach to SK_LOOKUP only if static key
>    has been already enabled (via sysctl?).
>
> >
> > Also please add multi-prog support. Adding it later will cause
> > all sorts of compatibility issues. The semantics of multi-prog
> > needs to be thought through right now.
> > For example BPF_DROP or BPF_REDIRECT could terminate the prog_run_array
> > sequence of progs while BPF_OK could continue.
> > It's not ideal, but better than nothing.
>
> I must say this approach is quite appealing because it's simple to
> explain. I would need a custom BPF_PROG_RUN_ARRAY, though.
>
> I'm curious what downside do you see here?
> Is overriding an earlier DROP/REDIRECT verdict useful?
>
> > Another option could be to execute all attached progs regardless
> > of return code, but don't let second prog override selected_sk blindly.
> > bpf_sk_assign() could get smarter.
>
> So if IIUC the rough idea here would be like below?
>
> - 1st program calls
>
>   bpf_sk_assign(ctx, sk1, 0 /*flags*/) -> 0 (OK)
>
> - 2nd program calls
>
>   bpf_sk_assign(ctx, sk2, 0) -> -EBUSY (already selected)
>   bpf_sk_assign(ctx, sk2, BPF_EXIST) -> 0 (OK, replace existing)
>
> In this case the last program to run has the final say, as opposed to
> the semantics where DROP/REDIRECT terminates.

Does sk_assign from TC also gain BPF_EXIST semantics? As you know,
I'm a bit concerned that TC and sk_lookup sk_assign are actually to completely
separate helpers. This is a good way to figure out if its a good idea to
overload the name, imo.

>
> Also, 2nd and subsequent programs would probably need to know if and
> which socket has been already selected. I think the selection could be
> exposed in context as bpf_sock pointer.
>
> I admit, I can't quite see the benefit of running thru all programs in
> array, so I'm tempted to go with terminate of DROP/REDIRECT in v3.
>
> >
> > Also please switch to bpf_link way of attaching. All system wide attachments
> > should be visible and easily debuggable via 'bpftool link show'.
> > Currently we're converting tc and xdp hooks to bpf_link. This new hook
> > should have it from the beginning.
>
> Will do in v3.
>
> Thanks for feedback,
> Jakub



-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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