Subject: Re: [PATCH bpf-next v2 05/17] inet: Run SK_LOOKUP BPF program on socket lookup

From: Jakub Sitnicki <[email-masked]>
Date: Tue, 12 May 2020 15:52:52 +0200

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 <[email-masked]>
>> Reviewed-by: Lorenz Bauer <[email-masked]>
>> Signed-off-by: Jakub Sitnicki <[email-masked]>
>> ---
>>  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