On Fri, Mar 20, 2020 at 1:48 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > Jakub Kicinski <kuba@xxxxxxxxxx> writes: > > > On Thu, 19 Mar 2020 14:13:13 +0100 Toke Høiland-Jørgensen wrote: > >> From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> > >> > >> 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@xxxxxxxxxx> > > > > 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 >
