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

John Fastabend
2020-03-20T18:30:55Z

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.

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.