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

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Date:   Mon, 21 Oct 2019 19:18:02 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...com, daniel@...earbox.net
Cc:     andrii.nakryiko@...il.com, kernel-team@...com,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf-next] libbpf: make LIBBPF_OPTS macro strictly a variable declaration

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>
> ---
>  tools/bpf/bpftool/prog.c |  6 +++---
>  tools/lib/bpf/libbpf.h   | 13 +++++++------
>  2 files changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> index 27da96a797ab..1a7e8ddf8232 100644
> --- a/tools/bpf/bpftool/prog.c
> +++ b/tools/bpf/bpftool/prog.c
> @@ -1093,6 +1093,9 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
>  {
>  	struct bpf_object_load_attr load_attr = { 0 };
>  	enum bpf_prog_type common_prog_type = BPF_PROG_TYPE_UNSPEC;
> +	LIBBPF_OPTS(bpf_object_open_opts, open_opts,
> +		.relaxed_maps = relaxed_maps,
> +	);
>  	enum bpf_attach_type expected_attach_type;
>  	struct map_replace *map_replace = NULL;
>  	struct bpf_program *prog = NULL, *pos;
> @@ -1106,9 +1109,6 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
>  	const char *file;
>  	int idx, err;
>  
> -	LIBBPF_OPTS(bpf_object_open_opts, open_opts,
> -		.relaxed_maps = relaxed_maps,
> -	);
>  
>  	if (!REQ_ARGS(2))
>  		return -1;
> 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)?

-Toke