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

Daniel Borkmann
2020-03-20T20:30:22Z

On 3/20/20 9:48 AM, 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.

Overall series looks okay, but before we go down that road, especially given there is
the new bpf_link object now, I would like us to first elaborate and figure out how XDP
fits into the bpf_link concept, where its limitations are, whether it even fits at all,
and how its semantics should look like realistically given bpf_link is to be generic to
all program types. Then we could extend the atomic replace there generically as well. I
think at the very minimum it might have similarities with what is proposed here, but
from a user experience I would like to avoid having something similar in XDP API and
then again in bpf_link which would just be confusing..

Thanks,
Daniel