netdev - Re: [PATCH bpf-next 04/16] net: Add BPF_XDP_EGRESS as a bpf_attach

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Date:   Thu, 23 Apr 2020 18:40:20 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...nel.org>,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        prashantbhole.linux@...il.com, jasowang@...hat.com,
        brouer@...hat.com, toshiaki.makita1@...il.com,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com,
        David Ahern <dahern@...italocean.com>
Subject: Re: [PATCH bpf-next 04/16] net: Add BPF_XDP_EGRESS as a bpf_attach_type

Alexei Starovoitov <alexei.starovoitov@...il.com> writes:

> On Wed, Apr 22, 2020 at 05:51:36PM +0200, Toke HÃ¸iland-JÃ¸rgensen wrote:
>> David Ahern <dsahern@...il.com> writes:
>> 
>> > On 4/22/20 9:27 AM, Toke HÃ¸iland-JÃ¸rgensen wrote:
>> >> And as I said in the beginning, I'm perfectly happy to be told why I'm
>> >> wrong; but so far you have just been arguing that I'm out of scope ;)
>> >
>> > you are arguing about a suspected bug with existing code that is no way
>> > touched or modified by this patch set, so yes it is out of scope.
>> 
>> Your patch is relying on the (potentially buggy) behaviour, so I don't
>> think it's out of scope to mention it in this context.
>
> Sorry for slow reply.
> I'm swamped with other things atm.
>
> Looks like there is indeed a bug in prog_type_ext handling code that
> is doing
> env->ops = bpf_verifier_ops[tgt_prog->type];
> I'm not sure whether the verifier can simply add:
> prog->expected_attach_type = tgt_prog->expected_attach_type;
> and be done with it.
> Likely yes, since expected_attach_type must be zero at that point
> that is enforced by bpf_prog_load_check_attach().
> So I suspect it's a single line fix.

Not quite: the check in bpf_tracing_prog_attach() that enforces
prog->expected_attach_type==0 also needs to go. So 5 lines :)

With those two changes, I do seem to get the correct behaviour for XDP
multiprogs in both ingress and egress mode (with David's patch set).

> A selftest to prove or disprove is necessary, of course.

I'll see if I can't hook this into the existing freplace selftest and
send a proper patch. I'm not really sure how to check for any hidden
side effects; in particular I'm a bit twitchy about removing that check
in bpf_tracing_prog_attach() - presumably it is there for a reason? But
I guess I'll send a patch and you can take a look.

> Thanks Toke for bringing it to my attention.

You're quite welcome! And thanks for the hint that the fix was that
simple; I feared something more invasive was needed.

-Toke