netdev - Re: [PATCH bpf-next v5 4/8] bpf: support attaching freplace programs to multiple attach points

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Date:   Wed, 16 Sep 2020 23:13:18 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 4/8] bpf: support attaching freplace
 programs to multiple attach points


[ will fix all your comments above ]

>> @@ -3924,10 +3983,16 @@ static int tracing_bpf_link_attach(const union bpf_attr *attr, struct bpf_prog *
>>             prog->expected_attach_type == BPF_TRACE_ITER)
>>                 return bpf_iter_link_attach(attr, prog);
>>
>> +       if (attr->link_create.attach_type == BPF_TRACE_FREPLACE &&
>> +           !prog->expected_attach_type)
>> +               return bpf_tracing_prog_attach(prog,
>> +                                              attr->link_create.target_fd,
>> +                                              attr->link_create.target_btf_id);
>
> Hm.. so you added a "fake" BPF_TRACE_FREPLACE attach_type, which is
> not really set with BPF_PROG_TYPE_EXT and is only specified for the
> LINK_CREATE command. Are you just trying to satisfy the link_create
> flow of going from attach_type to program type? If that's the only
> reason, I think we can adjust link_create code to handle this more
> flexibly.
>
> I need to think a bit more whether we want BPF_TRACE_FREPLACE at all,
> but if we do, whether we should make it an expected_attach_type for
> BPF_PROG_TYPE_EXT then...

Yeah, wasn't too sure about this. But attach_type seemed to be the only
way to disambiguate between the different link types in the LINK_CREATE
command, so went with that. Didn't think too much about it, TBH :)

I guess an alternative could be to just enforce attach_type==0 and look
at prog->type? Or if you have any other ideas, I'm all ears!

-Toke