netdev - Re: [PATCH v3 bpf-next 4/7] libbpf: use static const fmt string in __bpf

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Date:   Mon, 30 Aug 2021 17:37:56 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Dave Marchevsky <davemarchevsky@...com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>, Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 bpf-next 4/7] libbpf: use static const fmt string in __bpf_printk

On Fri, Aug 27, 2021 at 10:20 PM Dave Marchevsky <davemarchevsky@...com> wrote:
>
> The __bpf_printk convenience macro was using a 'char' fmt string holder
> as it predates support for globals in libbpf. Move to more efficient
> 'static const char', but provide a fallback to the old way via
> BPF_NO_GLOBAL_DATA so users on old kernels can still use the macro.
>
> Signed-off-by: Dave Marchevsky <davemarchevsky@...com>
> ---
>  tools/lib/bpf/bpf_helpers.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index 5f087306cdfe..a1d5ec6f285c 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -216,10 +216,16 @@ enum libbpf_tristate {
>                      ___param, sizeof(___param));               \
>  })
>
> +#ifdef BPF_NO_GLOBAL_DATA
> +#define BPF_PRINTK_FMT_TYPE char
> +#else
> +#define BPF_PRINTK_FMT_TYPE static const char
> +#endif
> +
>  /* Helper macro to print out debug messages */
>  #define __bpf_printk(fmt, ...)                         \
>  ({                                                     \
> -       char ____fmt[] = fmt;                           \
> +       BPF_PRINTK_FMT_TYPE ____fmt[] = fmt;            \

personal preferences, of course, but I'd leave char right there (I
think it makes it a bit more obvious what's going on right there), and
s/BPF_PRINTK_FMT_TYPE/BPF_PRINTK_FMT_MOD/ and have it as either "" or
"static const".

>         bpf_trace_printk(____fmt, sizeof(____fmt),      \
>                          ##__VA_ARGS__);                \
>  })
> --
> 2.30.2
>