netdev - Re: [PATCH bpf-next v4 5/8] libbpf: Add helper macro to clear opts structs

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Date: Mon, 10 Jul 2023 21:02:05 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: ast@...nel.org, andrii@...nel.org, martin.lau@...ux.dev, 
	razor@...ckwall.org, sdf@...gle.com, john.fastabend@...il.com, 
	kuba@...nel.org, dxu@...uu.xyz, joe@...ium.io, toke@...nel.org, 
	davem@...emloft.net, bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v4 5/8] libbpf: Add helper macro to clear opts structs

On Mon, Jul 10, 2023 at 1:12 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> Add a small and generic LIBBPF_OPTS_CLEAR() helper macros which clears
> an opts structure and reinitializes its .sz member to place the structure
> size. I found this very useful when developing selftests, but it is also
> generic enough as a macro next to the existing LIBBPF_OPTS() which hides
> the .sz initialization, too.
>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
>  tools/lib/bpf/libbpf_common.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf_common.h b/tools/lib/bpf/libbpf_common.h
> index 9a7937f339df..eb180023aa97 100644
> --- a/tools/lib/bpf/libbpf_common.h
> +++ b/tools/lib/bpf/libbpf_common.h
> @@ -70,4 +70,15 @@
>                 };                                                          \
>         })
>
> +/* Helper macro to clear a libbpf options struct
> + *
> + * Small helper macro to reset all fields and to reinitialize the common
> + * structure size member.
> + */
> +#define LIBBPF_OPTS_CLEAR(NAME)                                                    \
> +       do {                                                                \
> +               memset(&NAME, 0, sizeof(NAME));                             \
> +               NAME.sz = sizeof(NAME);                                     \
> +       } while (0)
> +

This is fine, but I think you can go a half-step further and have
something even more universal and useful. Something like this:


#define LIBBPF_OPTS_RESET(NAME, ...)
    do {
        memset(&NAME, 0, sizeof(NAME));
        NAME = (typeof(NAME)) {
            .sz = sizeof(struct TYPE),
            __VA_ARGS__
        };
     while (0);

I actually haven't tried if that typeof() trick works, but I hope it does :)


Then your LIBBPF_OPTS_CLEAR() is just LIBBPF_OPTS_RESET(x). But you
can also re-initialize:

LIBBPF_OPTS_RESET(x, .flags = 123, .prog_fd = 456);

It's more in line with LIBBPF_OPTS() itself in capabilities, except it
works on existing variable.


>  #endif /* __LIBBPF_LIBBPF_COMMON_H */
> --
> 2.34.1
>