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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Mar 24, 2020 at 07:12:53PM +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_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.
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Over the years of defining apis to attach bpf progs to different kernel
components the key mistake we made is that we tried to use corresponding
subsystem way of doing thing and it failed every time. What made the problem
worse that this failure we only learned after many years. Attaching xdp via
netlink felt great back then. Attaching clsbpf via tc felt awesome too. Doing
cgroup-bpf via bpf syscall with three different attaching ways also felt great.
All of these attaching things turned out to be broken because all of them
failed to provide the concept of ownership of the attachment. Which caused 10k
clsbpf progs on one netdev, 64 cgroup-bpf progs in one cgroup, nuked xdp progs.
Hence we have to add the ownership model first. Doing mini extensions to
existing apis is not addressing the giant issue of existing apis.

The idea of this patch is to do atomic replacement of xdp prog. It's a good
idea and useful feature, but I don't want to extend existing broken apis to do
add this feature. atomic replacement has to come with thought through owner
model first.