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

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

[Martin KaFai Lau]

Sparse warning:
kernel/bpf/btf.c:1985:34: warning: Variable length array is used.

This patch moves the nr_secs from btf_check_sec_info() to a macro.

Fixes: f80442a4cd18 ("bpf: btf: Change how section is supported in btf_header")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
---
 kernel/bpf/btf.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 9cbeabb5aca3..517296712774 100644
--- 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];
 	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++)
 		secs[i] = *(struct btf_sec_info *)((void *)hdr +
 						   btf_sec_info_offset[i]);
 
-	sort(secs, nr_secs, sizeof(struct btf_sec_info),
+	sort(secs, NR_SECS, sizeof(struct btf_sec_info),
 	     btf_sec_info_cmp, NULL);
 
 	/* Check for gaps and overlap among sections */
 	total = 0;
 	expected_total = btf_data_size - hdr->hdr_len;
-	for (i = 0; i < nr_secs; i++) {
+	for (i = 0; i < NR_SECS; i++) {
 		if (expected_total < secs[i].off) {
 			btf_verifier_log(env, "Invalid section offset");
 			return -EINVAL;
-- 
2.9.5

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

[Joe Perches]

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...