netdev - Re: [PATCH bpf-next] bpf: don't check against device MTU in __bpf_skb_max

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Date:   Wed, 16 Sep 2020 13:37:27 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Maciej Żenczykowski <maze@...gle.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>, bpf <bpf@...r.kernel.org>,
        Linux NetDev <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH bpf-next] bpf: don't check against device MTU in
 __bpf_skb_max_len

Maciej Żenczykowski <maze@...gle.com> writes:

> On Tue, Sep 15, 2020 at 1:47 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> [ just jumping in to answer this bit: ]
>>
>> > Would you happen to know what ebpf startup overhead is?
>> > How big a problem is having two (or more) back to back tc programs
>> > instead of one?
>>
>> With a jit'ed BPF program and the in-kernel dispatcher code (which
>> avoids indirect calls), it's quite close to a native function call.
>
> Hmm, I know we have (had? they're upstream now I think) some CFI vs
> BPF interaction issues.
> We needed to mark the BPF call into JIT'ed code as CFI exempt.
>
> CFI is Code Flow Integrity and is some compiler magic, to quote wikipedia:
> Google has shipped Android with the Linux kernel compiled by Clang
> with link-time optimization (LTO) and CFI since 2018.[12]
> I don't know much more about it.
>
> But we do BPF_JIT_ALWAYS_ON on 64-bit kernels, so it sounds like we
> might be good.

No idea about the CFI thing...

>> > We're running into both verifier performance scaling problems and code
>> > ownership issues with large programs...
>> >
>> > [btw. I understand for XDP we could only use 1 program anyway...]
>>
>> Working on that! See my talk at LPC:
>> https://linuxplumbersconf.org/event/7/contributions/671/
>
> Yes, I'm aware and excited about it!

Great! :)

> Unfortunately, Android S will only support 4.19, 5.4 and 5.10 for
> newly launched devices (and 4.9/4.14 for upgrades).
> (5.10 here means 'whatever is the next 5.x LTS', but that's most likely 5.10)
> I don't (yet) even have real phone hardware running 5.4, and 5.10
> within the next year is even more of a stretch.

Right, I saw your talk at LPC and of course the kernel version thing is
a bit of an issue. I suppose you could do some compile-time magic to
wrap programs and use the tail-call-based chaining for older kernels -
bit of a hassle, though :/

-Toke