netdev - Re: [PATCH bpf-next 2/6] libbpf: move bpf_helpers.h, bpf

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Date:   Tue, 1 Oct 2019 16:40:01 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 2/6] libbpf: move bpf_helpers.h, bpf_endian.h
 into libbpf

On Tue, Oct 1, 2019 at 4:31 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, Oct 1, 2019 at 3:45 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> >
> > On Mon, Sep 30, 2019 at 11:58:51AM -0700, Andrii Nakryiko wrote:
> > > Make bpf_helpers.h and bpf_endian.h official part of libbpf. Ensure they
> > > are installed along the other libbpf headers.
> > >
> > > Signed-off-by: Andrii Nakryiko <andriin@...com>
> >
> > [...]

[...]

> >
> > As mentioned earlier, the whole helper function description below
> > should get a big cleanup in here when moved into libbpf API.
>
> Right, I just recalled that today, sorry I didn't do it for this version.
>
> There were two things you mentioned on that thread that you wanted to clean up:
> 1. using __u32 instead int and stuff like that
> 2. using macro to hide some of the ugliness of (void *) = BPF_FUNC_xxx
>
> So with 1) I'm concerned that we can't just assume that __u32 is
> always going to be defined. Also we need bpf_helpers.h to be usable
> both with including system headers, as well as auto-generated
> vmlinux.h. In first case, I don't think we can assume that typedef is
> always defined, in latter case we can't really define it on our own.
> So I think we should just keep it as int, unsigned long long, etc.
> Thoughts?
>
> For 2), I'm doing that right now, but it's not that much cleaner, let's see.
>
> Am I missing something else?

Ok, so this doesn't work with just

#define BPF_FUNC(NAME, ...) (*NAME)(__VA_ARGS__)
__attribute__((unused)) = (void *) BPF_FUNC_##NAME

because helper is called bpf_map_update_elem(), but enum value is
BPF_FUNC_map_update_elem (without bpf_ prefix). So one way to do this
would be to have bpf_ prepended to function name by macro:

static int BPF_FUNC(map_update_elem, ....);

But this is super confusing, because this definition becomes basically
un-greppable. I think we should just keep it as is, or go all the way
in for super-verbose

static int BPF_FUNC(BPF_FUNC_map_update_elem, bpf_map_elem, ...);

I hate both, honestly.

>
> >
> > > +static void *(*bpf_map_lookup_elem)(void *map, const void *key) =
> > > +     (void *) BPF_FUNC_map_lookup_elem;
> > > +static int (*bpf_map_update_elem)(void *map, const void *key, const void *value,
> > > +                               unsigned long long flags) =
> > [...]
> > > +

[...]

> > > --
> > > 2.17.1
> > >