Jakub Kicinski wrote:

On Fri, 20 Mar 2020 09:48:10 +0100 Toke Høiland-Jørgensen wrote:

quoted

Jakub Kicinski [off-list ref] writes:

quoted

On Thu, 19 Mar 2020 14:13:13 +0100 Toke Høiland-Jørgensen wrote:  

quoted

From: Toke Høiland-Jørgensen <redacted>

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 <redacted>  

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.