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

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Message-ID: <CAADnVQ+GDR15oArpEGFFOvZ35WthHyL+b97zQUxjXbz-ec5CGw@mail.gmail.com>
Date:   Thu, 23 Apr 2020 09:52:55 -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>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Prashant Bhole <prashantbhole.linux@...il.com>,
        Jason Wang <jasowang@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toshiaki Makita <toshiaki.makita1@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <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 Thu, Apr 23, 2020 at 9:40 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> 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 :)

prog_ext's expected_attach_type needs to stay zero.
It needs to be inherited from tgt prog. Hence one line:
prog->expected_attach_type = tgt_prog->expected_attach_type;