linux-kernel - Re: [PATCH] libbpf: Fix is_pow_of

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Date:   Thu, 2 Jun 2022 21:31:07 -0700
From:   Zvi Effron <zeffron@...tgames.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yuze Chi <chiyuze@...gle.com>
Subject: Re: [PATCH] libbpf: Fix is_pow_of_2

On Thu, Jun 2, 2022 at 9:17 PM Ian Rogers <irogers@...gle.com> wrote:
>
> From: Yuze Chi <chiyuze@...gle.com>
>
> There is a missing not. Consider a power of 2 number like 4096:
>
> x && (x & (x - 1))
> 4096 && (4096 & (4096 - 1))
> 4096 && (4096 & 4095)
> 4096 && 0
> 0
>
> with the not this is:
> x && !(x & (x - 1))
> 4096 && !(4096 & (4096 - 1))
> 4096 && !(4096 & 4095)
> 4096 && !0
> 4096 && 1
> 1
>
> Reported-by: Yuze Chi <chiyuze@...gle.com>
> Signed-off-by: Yuze Chi <chiyuze@...gle.com>
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
>  tools/lib/bpf/libbpf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 3f4f18684bd3..fd0414ea00df 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4956,7 +4956,7 @@ static void bpf_map__destroy(struct bpf_map *map);
>
>  static bool is_pow_of_2(size_t x)
>  {
> -       return x && (x & (x - 1));
> +       return x && !(x & (x - 1));

No idea if anyone cares about the consistency, but in linker.c (same directory)
the same static function is defined using == 0 at the end instead of using the
not operator.

Aside from the consistency issue, personally I find the == 0 version a little
bit easier to read and understand because it's a bit less dense (and a "!" next
to a "(" is an easy character to overlook).

>  }
>
>  static size_t adjust_ringbuf_sz(size_t sz)
> --
> 2.36.1.255.ge46751e96f-goog
>