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

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Date:   Wed, 22 Apr 2020 17:39:11 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.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

On Wed, Apr 22, 2020 at 05:51:36PM +0200, Toke Høiland-Jørgensen wrote:
> David Ahern <dsahern@...il.com> writes:
> 
> > On 4/22/20 9:27 AM, Toke Høiland-Jørgensen 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.
A selftest to prove or disprove is necessary, of course.

Thanks Toke for bringing it to my attention.