netdev - Re: [PATCH 1/2] kernel/bpf: change "char *" string form to "char []"

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Message-ID: <bd3d4379-e4aa-79c7-85b8-cc930a04f267@fb.com>
Date:   Thu, 12 May 2022 08:16:21 -0700
From:   Yonghong Song <yhs@...com>
To:     liqiong <liqiong@...china.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, hukun@...china.com,
        qixu@...china.com, yuzhe@...china.com, renyu@...china.com
Subject: Re: [PATCH 1/2] kernel/bpf: change "char *" string form to "char []"



On 5/12/22 7:28 AM, liqiong wrote:
> The string form of "char []" declares a single variable. It is better
> than "char *" which creates two variables.

Could you explain in details about why it is better in generated codes?
It is not clear to me why your patch is better than the original code.

> 
> Signed-off-by: liqiong <liqiong@...china.com>
> ---
>   kernel/bpf/btf.c      | 4 ++--
>   kernel/bpf/verifier.c | 2 +-
>   2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 0918a39279f6..218a8ac73644 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -894,10 +894,10 @@ static const struct btf_type *btf_type_skip_qualifiers(const struct btf *btf,
>   static const char *btf_show_name(struct btf_show *show)
>   {
>   	/* BTF_MAX_ITER array suffixes "[]" */
> -	const char *array_suffixes = "[][][][][][][][][][]";
> +	static const char array_suffixes[] = "[][][][][][][][][][]";
>   	const char *array_suffix = &array_suffixes[strlen(array_suffixes)];
>   	/* BTF_MAX_ITER pointer suffixes "*" */
> -	const char *ptr_suffixes = "**********";
> +	static const char ptr_suffixes[] = "**********";
>   	const char *ptr_suffix = &ptr_suffixes[strlen(ptr_suffixes)];
>   	const char *name = NULL, *prefix = "", *parens = "";
>   	const struct btf_member *m = show->state.member;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index d175b70067b3..78a090fcbc72 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7346,7 +7346,7 @@ static int sanitize_err(struct bpf_verifier_env *env,
>   			const struct bpf_reg_state *off_reg,
>   			const struct bpf_reg_state *dst_reg)
>   {
> -	static const char *err = "pointer arithmetic with it prohibited for !root";
> +	static const char err[] = "pointer arithmetic with it prohibited for !root";
>   	const char *op = BPF_OP(insn->code) == BPF_ADD ? "add" : "sub";
>   	u32 dst = insn->dst_reg, src = insn->src_reg;
>