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

Andrii Nakryiko
2020-03-20T20:39:15Z

On Fri, Mar 20, 2020 at 1:48 AM Toke Høiland-Jørgensen <toke@redhat.com> 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.
>
> > If we do please run this thru checkpatch, set .strict_start_type,
>
> Will do.
>
> > and make the expected fd unsigned. A negative expected fd makes no
> > sense.
>
> A negative expected_fd corresponds to setting the UPDATE_IF_NOEXIST
> flag. I guess you could argue that since we have that flag, setting a
> negative expected_fd is not strictly needed. However, I thought it was
> weird to have a "this is what I expect" API that did not support
> expressing "I expect no program to be attached".

For BPF syscall it seems the typical approach when optional FD is
needed is to have extra flag (e.g., BPF_F_REPLACE for cgroups) and if
it's not specified - enforce zero for that optional fd. That handles
backwards compatibility cases well as well.

>
> -Toke
>