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Date:   Fri, 20 Mar 2020 11:35:05 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNlbg==?= <toke@redhat.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Lorenz Bauer <lmb@cloudflare.com>,
        Andrey Ignatov <rdna@fb.com>, netdev@vger.kernel.org,
        bpf@vger.kernel.org
Subject: Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing
 program when attaching XDP
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On Fri, 20 Mar 2020 19:17:46 +0100 Toke H=C3=B8iland-J=C3=B8rgensen wrote:
> Jakub Kicinski <kuba@kernel.org> writes:
>=20
> >> > If we do please run this thru checkpatch, set .strict_start_type,   =
=20
> >>=20
> >> Will do.
> >>  =20
> >> > and make the expected fd unsigned. A negative expected fd makes no
> >> > sense.   =20
> >>=20
> >> 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". =20
> >
> > 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? =20
>=20
> For consistency with other APIs. Seems the pattern is generally that
> userspace supplies program FDs, and the kernel returns IDs, no?

This API just predates the IDs. "From kernel" API was added when=20
IDs already existed.

I'd think for cmpxchg it may be easier if user space provides ID
directly, since it's what it get returned.