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

Andrii Nakryiko
2020-03-23T18:14:59Z

On Mon, Mar 23, 2020 at 4:24 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>
> > On Fri, Mar 20, 2020 at 11:31 AM John Fastabend
> > <john.fastabend@gmail.com> wrote:
> >>
> >> Jakub Kicinski wrote:
> >> > On Fri, 20 Mar 2020 09:48:10 +0100 Toke Høiland-Jørgensen wrote:
> >> > > Jakub Kicinski <kuba@kernel.org> writes:
> >> > > > On Thu, 19 Mar 2020 14:13:13 +0100 Toke Høiland-Jørgensen wrote:
> >> > > >> From: Toke Høiland-Jørgensen <toke@redhat.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@redhat.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.
> >> >
> >> > I'm not really in the loop, but from what I overheard - I think the
> >> > bpf_link may be targeting something non-networking first.
> >>
> >> My preference is to avoid building two different APIs one for XDP and another
> >> for everything else. If we have userlands that already understand links and
> >> pinning support is on the way imo lets use these APIs for networking as well.
> >
> > I agree here. And yes, I've been working on extending bpf_link into
> > cgroup and then to XDP. We are still discussing some cgroup-specific
> > details, but the patch is ready. I'm going to post it as an RFC to get
> > the discussion started, before we do this for XDP.
>
> Well, my reason for being skeptic about bpf_link and proposing the
> netlink-based API is actually exactly this, but in reverse: With
> bpf_link we will be in the situation that everything related to a netdev
> is configured over netlink *except* XDP.

One can argue that everything related to use of BPF is going to be
uniform and done through BPF syscall? Given variety of possible BPF
hooks/targets, using custom ways to attach for all those many cases is
really bad as well, so having a unifying concept and single entry to
do this is good, no?

>
> Other than that, I don't see any reason why the bpf_link API won't work.
> So I guess that if no one else has any problem with BPF insisting on
> being a special snowflake, I guess I can live with it as well... *shrugs* :)

Apart from derogatory remark, BPF is a bit special here, because it
requires every potential BPF hook (be it cgroups, xdp, perf_event,
etc) to be aware of BPF program(s) and execute them with special
macro. So like it or not, it is special and each driver supporting BPF
needs to implement this BPF wiring.

>
> -Toke
>