Re: [PATCH bpf-next 2/5] libbpf: support BPF_PROG_TYPE_USER programs

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Aug 4, 2020 at 8:59 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Aug 4, 2020, at 6:38 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >
> > On Mon, Aug 3, 2020 at 6:18 PM Song Liu <songliubraving@fb.com> wrote:
> >>
> >>
> >>
> >>> On Aug 2, 2020, at 6:40 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >>>
> >>> On Sat, Aug 1, 2020 at 1:50 AM Song Liu <songliubraving@fb.com> wrote:
> >>>>
> >>
> >> [...]
> >>
> >>>
> >>>> };
> >>>>
> >>>> LIBBPF_API int bpf_prog_test_run_xattr(struct bpf_prog_test_run_attr *test_attr);
> >>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >>>> index b9f11f854985b..9ce175a486214 100644
> >>>> --- a/tools/lib/bpf/libbpf.c
> >>>> +++ b/tools/lib/bpf/libbpf.c
> >>>> @@ -6922,6 +6922,7 @@ static const struct bpf_sec_def section_defs[] = {
> >>>>       BPF_PROG_SEC("lwt_out",                 BPF_PROG_TYPE_LWT_OUT),
> >>>>       BPF_PROG_SEC("lwt_xmit",                BPF_PROG_TYPE_LWT_XMIT),
> >>>>       BPF_PROG_SEC("lwt_seg6local",           BPF_PROG_TYPE_LWT_SEG6LOCAL),
> >>>> +       BPF_PROG_SEC("user",                    BPF_PROG_TYPE_USER),
> >>>
> >>> let's do "user/" for consistency with most other prog types (and nice
> >>> separation between prog type and custom user name)
> >>
> >> About "user" vs. "user/", I still think "user" is better.
> >>
> >> Unlike kprobe and tracepoint, user prog doesn't use the part after "/".
> >> This is similar to "perf_event" for BPF_PROG_TYPE_PERF_EVENT, "xdl" for
> >> BPF_PROG_TYPE_XDP, etc. If we specify "user" here, "user/" and "user/xxx"
> >> would also work. However, if we specify "user/" here, programs that used
> >> "user" by accident will fail to load, with a message like:
> >>
> >>        libbpf: failed to load program 'user'
> >>
> >> which is confusing.
> >
> > xdp, perf_event and a bunch of others don't enforce it, that's true,
> > they are a bit of a legacy,
>
> I don't see w/o "/" is a legacy thing. BPF_PROG_TYPE_STRUCT_OPS just uses
> "struct_ops".
>
> > unfortunately. But all the recent ones do,
> > and we explicitly did that for xdp_dev/xdp_cpu, for instance.
> > Specifying just "user" in the spec would allow something nonsensical
> > like "userargh", for instance, due to this being treated as a prefix.
> > There is no harm to require users to do "user/my_prog", though.
>
> I don't see why allowing "userargh" is a problem. Failing "user" is
> more confusing. We can probably improve that by a hint like:
>
>     libbpf: failed to load program 'user', do you mean "user/"?
>
> But it is pretty silly. "user/something_never_used" also looks weird.

"userargh" is terrible, IMO. It's a different identifier that just
happens to have the first 4 letters matching "user" program type.
There must be either a standardized separator (which happens to be
'/') or none. See the suggestion below.
>
> > Alternatively, we could introduce a new convention in the spec,
> > something like "user?", which would accept either "user" or
> > "user/something", but not "user/" nor "userblah". We can try that as
> > well.
>
> Again, I don't really understand why allowing "userblah" is a problem.
> We already have "xdp", "xdp_devmap/", and "xdp_cpumap/", they all work
> fine so far.

Right, we have "xdp_devmap/" and "xdp_cpumap/", as you say. I haven't
seen so much pushback against trailing forward slash with those ;)

But anyways, as part of deprecating APIs and preparing libbpf for 1.0
release over this half, I think I'm going to emit warnings for names
like "prog_type_whatever" or "prog_typeevenworse", etc. And asking
users to normalize section names to either "prog_type" or
"prog_type/something/here", whichever makes sense for a specific
program type. Right now libbpf doesn't allow two separate BPF programs
with the same section name, so enforcing strict "user" is limiting to
users. We are going to lift that restriction pretty soon, though. But
for now, please stick with what we've been doing lately and mark it as
"user/", later we'll allow just "user" as well.

>
> Thanks,
> Song