Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes: > 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. Never did understand how that is supposed to square with 0 being a valid fd number? -Toke
