On Mon, Mar 23, 2020 at 4:25 AM Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > > 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? You mean a tiny chance that given invalid userspace program behavior (setting valid FD 0 without specifying BPF_F_REPLACE or not setting FD, but FD=0 being a valid program FD) it might succeed accidentally? Sure it's theoretically possible, but highly unlikely and in any case it's an invalid userspace behavior. So I guess it was deemed acceptable for the sake of backwards compatibility? > > -Toke >
