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@xxxxxxxxxxxxxx>
>> Reviewed-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
>> Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
>> ---
>> 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.
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
