netdev - Re: [PATCH v4 bpf-next 5/9] libbpf: use static const fmt string in __bpf

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Date:   Thu, 2 Sep 2021 16:09:19 -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 v4 bpf-next 5/9] libbpf: use static const fmt string in __bpf_printk

On Thu, Sep 2, 2021 at 10:23 AM 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>
> ---

Great, thanks!

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>  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 a7e73be6dac4..7df7d9a23099 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_MOD
> +#else
> +#define BPF_PRINTK_FMT_MOD static const
> +#endif
> +
>  /* Helper macro to print out debug messages */
>  #define __bpf_printk(fmt, ...)                         \
>  ({                                                     \
> -       char ____fmt[] = fmt;                           \
> +       BPF_PRINTK_FMT_MOD char ____fmt[] = fmt;        \
>         bpf_trace_printk(____fmt, sizeof(____fmt),      \
>                          ##__VA_ARGS__);                \
>  })
> --
> 2.30.2
>