Re: [PATCH bpf-next v3 1/4] xdp: Support specifying expected existing program when attaching XDP

Jakub Kicinski [off-list ref] writes:

On Tue, 24 Mar 2020 17:54:07 -0700 Andrii Nakryiko wrote:

quoted

On Tue, Mar 24, 2020 at 11:13 AM Toke Høiland-Jørgensen [off-list ref] 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_ID, 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.
Setting the new attribute with a negative value means that no program is
expected to be attached, which corresponds to setting the UPDATE_IF_NOEXIST
flag.

A new companion flag, XDP_FLAGS_EXPECT_ID, is also added to explicitly
request checking of the EXPECTED_ID attribute. This is needed for userspace
to discover whether the kernel supports the new attribute.  

Doesn't it feel inconsistent in UAPI that FD is used to specify XDP
program to be attached, but ID is used to specify expected XDP
program? Especially that the same cgroup use case is using
(consistently) prog FDs. Or is it another case where XDP needs its own
special way?

There was a comment during review of v1, I wish you spoke up then.

The prog ID is what dump returns, so the consistency can go either way
(note that this API predates object IDs). Since XDP uses IDs internally
it's just simpler to take prog ID.

But it's a detail, so if you feel strongly I don't really mind.

Using an FD instead of an ID does make this more extensible (such as
supporting bpf_link FDs in the future; see my other reply to Alexei). So
I'll respin this, and switch it back to EXPECTED_FD.

-Toke