Re: [PATCH bpf] libbpf: fix missing section "sk_skb/skb_verdict"

[Cong Wang <xiyou.wangcong@gmail.com>]

On Tue, Nov 30, 2021 at 8:33 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Nov 30, 2021 at 8:19 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
> >
> > On Tue, Nov 30, 2021 at 3:33 PM Song Liu <song@kernel.org> wrote:
> > >
> > > On Mon, Nov 29, 2021 at 12:51 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
> > > >
> > > > On Fri, Nov 26, 2021 at 04:20:34PM -0800, Song Liu wrote:
> > > > > On Fri, Nov 26, 2021 at 12:45 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
> > > > > >
> > > > > > From: Cong Wang <cong.wang@bytedance.com>
> > > > > >
> > > > > > When BPF_SK_SKB_VERDICT was introduced, I forgot to add
> > > > > > a section mapping for it in libbpf.
> > > > > >
> > > > > > Fixes: a7ba4558e69a ("sock_map: Introduce BPF_SK_SKB_VERDICT")
> > > > > > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > > > > > Cc: John Fastabend <john.fastabend@gmail.com>
> > > > > > Cc: Jakub Sitnicki <jakub@cloudflare.com>
> > > > > > Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> > > > >
> > > > > The patch looks good to me. But seems the selftests are OK without this. So,
> > > > > do we really need this?
> > > > >
> > > >
> > > > Not sure if I understand this question.
> > > >
> > > > At least BPF_SK_SKB_STREAM_PARSER and BPF_SK_SKB_STREAM_VERDICT are already
> > > > there, so either we should remove all of them or add BPF_SK_SKB_VERDICT for
> > > > completeness.
> > > >
> > > > Or are you suggesting we should change it back in selftests too? Note, it was
> > > > changed by Andrii in commit 15669e1dcd75fe6d51e495f8479222b5884665b6:
> > > >
> > > > -SEC("sk_skb/skb_verdict")
> > > > +SEC("sk_skb")
> > >
> > > Yes, I noticed that Andrii made the change, and it seems to work
> > > as-is. Therefore,
> > > I had the question "do we really need it".
> >
> > Same question from me: why still keep sk_skb/stream_parser and
> > sk_skb/stream_verdict? ;) I don't see any reason these two are more
> > special than sk_skb/skb_verdict, therefore we should either keep all
> > of them or remove all of them.
> >
>
> "sk_skb/skb_verdict" was treated by libbpf *exactly* the same way as
> "sk_skb". Which means the attach type was set to BPF_PROG_TYPE_SK_SKB
> and expected_attach_type was 0 (not BPF_SK_SKB_VERDICT!). So that
> program is definitely not a BPF_SK_SKB_VERDICT, libbpf pre-1.0 just
> has a sloppy prefix matching logic.

This is exactly what I meant by "umbrella". ;)

>
> So Song's point is valid, we currently don't have selftests that tests
> BPF_SK_SKB_VERDICT expected attach type, so it would be good to add
> it. Or make sure that existing test that was supposed to test it is
> actually testing it.

Sure, I just noticed we have section name tests a few minutes ago. Will add
it in V2.

>
> > >
> > > If we do need to differentiate skb_verdict from just sk_skb, could you
> >
> > Are you sure sk_skb is a real attach type?? To me, it is an umbrella to
> > catch all of them:
> >
> > SEC_DEF("sk_skb",               SK_SKB, 0, SEC_NONE | SEC_SLOPPY_PFX),
> >
> > whose expected_attach_type is 0. The reason why it works is
> > probably because we don't check BPF_PROG_TYPE_SK_SKB in
> > bpf_prog_load_check_attach().
>
> We don't check expected_attach_type in prog_load, but

I see many checks in bpf_prog_load_check_attach(), for instance:

2084         switch (prog_type) {
2085         case BPF_PROG_TYPE_CGROUP_SOCK:
2086                 switch (expected_attach_type) {
2087                 case BPF_CGROUP_INET_SOCK_CREATE:
2088                 case BPF_CGROUP_INET_SOCK_RELEASE:
2089                 case BPF_CGROUP_INET4_POST_BIND:
2090                 case BPF_CGROUP_INET6_POST_BIND:
2091                         return 0;
2092                 default:
2093                         return -EINVAL;
2094                 }


> sock_map_prog_update in net/core/sock_map.c is checking expected
> attach type and should return -EOPNOTSUPP. But given that no test is
> failing our tests don't even try to attach anything, I assume. Which
> makes them not so great at actually testing anything. Please see if
> you can improve that.

sock_map_prog_update() checks for attach_type, not
expected_attach_type.

>
> >
> > > please add a
> > > case selftest for skb_verdict?
> >
> > Ah, sure, I didn't know we have sec_name_test.
> >
> > >
> > > Also, maybe we can name it as "sk_skb/verdict" to avoid duplication?
> >
> > At least we used to call it sk_skb/skb_verdict before commit 15669e1dcd.
>
> As I mentioned above, it could have been called "sk_skb!dontcare" and

So why commit c6f6851b28ae26000352598f01968b3ff7dcf58 if your point
here is we don't need any name? ;)

> that would still work (and still does if strict mode is not enabled
> for libbpf). For consistency with UAPI expected_attach_type enum it
> should be called "sk_skb/verdict" because BPF_SK_SKB_VERDICT vs
> BPF_SK_SKB_STREAM_VERDICT vs BPF_SK_SKB_STREAM_PARSER.

To me, "verdict" is too broad, it could refer "stream_verdict" or "skb_verdict".
And let me quote commit c6f6851b28ae26000352598f01968b3ff7dcf588:

    "stream_parser" and "stream_verdict" are used instead of simple "parser"
    and "verdict" just to avoid possible confusion in a place where attach
    type is used alone (e.g. in bpftool's show sub-commands) since there is
    another attach point that can be named as "verdict": BPF_SK_MSG_VERDICT.

Thanks.