netdev - Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing program when attaching XDP

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Message-ID: <87imiy6gc5.fsf@toke.dk>
Date:   Fri, 20 Mar 2020 19:17:46 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
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>,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Andrey Ignatov <rdna@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing program when attaching XDP

Jakub Kicinski <kuba@...nel.org> writes:

>> > If we do please run this thru checkpatch, set .strict_start_type,  
>> 
>> Will do.
>> 
>> > and make the expected fd unsigned. A negative expected fd makes no
>> > sense.  
>> 
>> A negative expected_fd corresponds to setting the UPDATE_IF_NOEXIST
>> flag. I guess you could argue that since we have that flag, setting a
>> negative expected_fd is not strictly needed. However, I thought it was
>> weird to have a "this is what I expect" API that did not support
>> expressing "I expect no program to be attached".
>
> I see it now, not entirely unreasonable.
>
> Why did you choose to use the FD rather than passing prog id directly?
> Is the application unlikely to have program ID?

For consistency with other APIs. Seems the pattern is generally that
userspace supplies program FDs, and the kernel returns IDs, no?

-Toke