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

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Date:   Mon, 23 Mar 2020 11:07:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        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>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/4] xdp: Support specifying expected existing
 program when attaching XDP

On Mon, Mar 23, 2020 at 4:25 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > On Fri, Mar 20, 2020 at 1:48 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >>
> >> Jakub Kicinski <kuba@...nel.org> writes:
> >>
> >> > On Thu, 19 Mar 2020 14:13:13 +0100 Toke Høiland-Jørgensen wrote:
> >> >> From: Toke Høiland-Jørgensen <toke@...hat.com>
> >> >>
> >> >> While it is currently possible for userspace to specify that an existing
> >> >> XDP program should not be replaced when attaching to an interface, there is
> >> >> no mechanism to safely replace a specific XDP program with another.
> >> >>
> >> >> This patch adds a new netlink attribute, IFLA_XDP_EXPECTED_FD, which can be
> >> >> set along with IFLA_XDP_FD. If set, the kernel will check that the program
> >> >> currently loaded on the interface matches the expected one, and fail the
> >> >> operation if it does not. This corresponds to a 'cmpxchg' memory operation.
> >> >>
> >> >> A new companion flag, XDP_FLAGS_EXPECT_FD, is also added to explicitly
> >> >> request checking of the EXPECTED_FD attribute. This is needed for userspace
> >> >> to discover whether the kernel supports the new attribute.
> >> >>
> >> >> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> >> >
> >> > I didn't know we wanted to go ahead with this...
> >>
> >> Well, I'm aware of the bpf_link discussion, obviously. Not sure what's
> >> happening with that, though. So since this is a straight-forward
> >> extension of the existing API, that doesn't carry a high implementation
> >> cost, I figured I'd just go ahead with this. Doesn't mean we can't have
> >> something similar in bpf_link as well, of course.
> >>
> >> > 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".
> >
> > For BPF syscall it seems the typical approach when optional FD is
> > needed is to have extra flag (e.g., BPF_F_REPLACE for cgroups) and if
> > it's not specified - enforce zero for that optional fd. That handles
> > backwards compatibility cases well as well.
>
> Never did understand how that is supposed to square with 0 being a valid
> fd number?

You mean a tiny chance that given invalid userspace program behavior
(setting valid FD 0 without specifying BPF_F_REPLACE or not setting
FD, but FD=0 being a valid program FD) it might succeed accidentally?
Sure it's theoretically possible, but highly unlikely and in any case
it's an invalid userspace behavior. So I guess it was deemed
acceptable for the sake of backwards compatibility?

>
> -Toke
>