Andrey Ignatov <rdna@xxxxxx> writes: > Toke Høiland-Jørgensen <toke@xxxxxxxxxx> [Mon, 2020-03-23 04:25 -0700]: >> 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? > > In BPF_F_REPLACE case (since it was used as an example in this thread) > it's all pretty clear: > > * if the flag is set, use fd from attr.replace_bpf_fd that can be anything > (incl. zero, since indeed it's valid fd) no problem with that; > * if flag is not set, ignore replace_bpf_fd completely. > > It's descirbed in commit log in 7dd68b3279f1: > > ... > > BPF_F_REPLACE is introduced to make the user intent clear, since > replace_bpf_fd alone can't be used for this (its default value, 0, is a > valid fd). BPF_F_REPLACE also makes it possible to extend the API in the > future (e.g. add BPF_F_BEFORE and BPF_F_AFTER if needed). > > ... > > , i.e. flag presense is important, not the fd attribute being zero. > > Hope it clarifies. Yup, it does, thanks! My confusion stemmed from having seen '!= 0' tests for FDs in various places and wondered how that was supposed to work. Didn't realise this was handled by way of an accompanying flag, that does make sense :) -Toke
