netdev - Re: [PATCH bpf-next] bpf: btf: Avoid variable length array

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Date:   Wed, 23 May 2018 11:11:14 -0700
From:   Joe Perches <joe@...ches.com>
To:     Martin KaFai Lau <kafai@...com>, netdev@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, kernel-team@...com
Subject: Re: [PATCH bpf-next] bpf: btf: Avoid variable length array

On Wed, 2018-05-23 at 10:46 -0700, Martin KaFai Lau wrote:
> Sparse warning:
> kernel/bpf/btf.c:1985:34: warning: Variable length array is used.

Perhaps use ARRAY_SIZE directly instead of indirectly via a #define

> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
[]
> @@ -1970,6 +1970,8 @@ static const size_t btf_sec_info_offset[] = {
>  	offsetof(struct btf_header, str_off),
>  };
>  
> +#define NR_SECS ARRAY_SIZE(btf_sec_info_offset)
> +
>  static int btf_sec_info_cmp(const void *a, const void *b)
>  {
>  	const struct btf_sec_info *x = a;
> @@ -1981,8 +1983,7 @@ static int btf_sec_info_cmp(const void *a, const void *b)
>  static int btf_check_sec_info(struct btf_verifier_env *env,
>  			      u32 btf_data_size)
>  {
> -	const unsigned int nr_secs = ARRAY_SIZE(btf_sec_info_offset);
> -	struct btf_sec_info secs[nr_secs];
> +	struct btf_sec_info secs[NR_SECS];

	struct btf_sec_info secs[ARRAY_SIZE(btf_sec_info_offset)];

>  	u32 total, expected_total, i;
>  	const struct btf_header *hdr;
>  	const struct btf *btf;
> @@ -1991,17 +1992,17 @@ static int btf_check_sec_info(struct btf_verifier_env *env,
>  	hdr = &btf->hdr;
>  
>  	/* Populate the secs from hdr */
> -	for (i = 0; i < nr_secs; i++)
> +	for (i = 0; i < NR_SECS; i++)

	for (i = 0; i < ARRAY_SIZE(btf_sec_info_offset); i++)

>  		secs[i] = *(struct btf_sec_info *)((void *)hdr +
>  						   btf_sec_info_offset[i]);

which makes this loop more intelligible.

> -	sort(secs, nr_secs, sizeof(struct btf_sec_info),
> +	sort(secs, NR_SECS, sizeof(struct btf_sec_info),
>  	     btf_sec_info_cmp, NULL);

etc...