netdev - RFC: Fixing SK_REUSEPORT from sk_lookup

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Date:   Wed, 15 May 2019 16:11:13 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Networking <netdev@...r.kernel.org>, bpf@...r.kernel.org
Cc:     Joe Stringer <joe@...valent.com>
Subject: RFC: Fixing SK_REUSEPORT from sk_lookup_* helpers

In the BPF-based TPROXY session with Joe Stringer [1], I mentioned
that the sk_lookup_* helpers currently return inconsistent results if
SK_REUSEPORT programs are in play.

SK_REUSEPORT programs are a hook point in inet_lookup. They get access
to the full packet
that triggered the look up. To support this, inet_lookup gained a new
skb argument to provide such context. If skb is NULL, the SK_REUSEPORT
program is skipped and instead the socket is selected by its hash.

The first problem is that not all callers to inet_lookup from BPF have
an skb, e.g. XDP. This means that a look up from XDP gives an
incorrect result. For now that is not a huge problem. However, once we
get sk_assign as proposed by Joe, we can end up circumventing
SK_REUSEPORT.

At the conference, someone suggested using a similar approach to the
work done on the flow dissector by Stanislav: create a dedicated
context sk_reuseport which can either take an skb or a plain pointer.
Patch up load_bytes to deal with both. Pass the context to
inet_lookup.

This is when we hit the second problem: using the skb or XDP context
directly is incorrect, because it assumes that the relevant protocol
headers are at the start of the buffer. In our use case, the correct
headers are at an offset since we're inspecting encapsulated packets.

The best solution I've come up with is to steal 17 bits from the flags
argument to sk_lookup_*, 1 bit for BPF_F_HEADERS_AT_OFFSET, 16bit for
the offset itself.

Thoughts?

1: http://vger.kernel.org/bpfconf2019.html#session-7
-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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