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

Andrii Nakryiko
2020-03-20T20:24:59Z

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.

>
> Would a link_swap() API (proposed by Andrii iirc) resolve this use case as
> well? If not why? If it can it seems like the more general and consistent
> solution. I can imagine swapping links is useful in tracing as well and
> likely other cases I haven't thought about.

Yes, that's the idea. Right now I have implementation for cgroups, but
API itself is generic and should/will be extended to tracing and XDP.