netdev - Re: [PATCH bpf-next] libbpf: make LIBBPF_OPTS macro strictly a variable declaration

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Date:   Mon, 21 Oct 2019 10:38:47 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next] libbpf: make LIBBPF_OPTS macro strictly a
 variable declaration

On Mon, Oct 21, 2019 at 10:18 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andriin@...com> writes:
>
> > LIBBPF_OPTS is implemented as a mix of field declaration and memset
> > + assignment. This makes it neither variable declaration nor purely
> > statements, which is a problem, because you can't mix it with either
> > other variable declarations nor other function statements, because C90
> > compiler mode emits warning on mixing all that together.
> >
> > This patch changes LIBBPF_OPTS into a strictly declaration of variable
> > and solves this problem, as can be seen in case of bpftool, which
> > previously would emit compiler warning, if done this way (LIBBPF_OPTS as
> > part of function variables declaration block).
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > ---

[...]

> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 0fdf086beba7..bf105e9e866f 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -77,12 +77,13 @@ struct bpf_object_open_attr {
> >   * bytes, but that's the best way I've found and it seems to work in practice.
> >   */
> >  #define LIBBPF_OPTS(TYPE, NAME, ...)                                     \
> > -     struct TYPE NAME;                                                   \
> > -     memset(&NAME, 0, sizeof(struct TYPE));                              \
> > -     NAME = (struct TYPE) {                                              \
> > -             .sz = sizeof(struct TYPE),                                  \
> > -             __VA_ARGS__                                                 \
> > -     }
> > +     struct TYPE NAME = ({                                               \
> > +             memset(&NAME, 0, sizeof(struct TYPE));                      \
> > +             (struct TYPE) {                                             \
> > +                     .sz = sizeof(struct TYPE),                          \
>
> Wait, you can stick arbitrary code inside a variable initialisation
> block like this? How does that work? Is everything before the (struct
> type) just ignored (and is that a cast)?

Well, you definitely can still arbitrary code into a ({ }) expression
block, that's not that surprising.
The surprising bit that I discovered just recently was that stuff like
this compiles and works correctly, try it:

        void *x = &x;
        printf("%lx == %lx\n", x, &x);

So I'm using the fact that variable address is available inside
variable initialization block.

Beyond that, it's just a fancy, but standard (struct bla){ ...
initializer list ...} syntax (it's not a struct initializer syntax,
mind you, it's a struct assignment from struct literal). Fancy for
sure, but it works and solves problems I mentioned in commit
description.

>
> -Toke
>