Re: [PATCH bpf-next v9 05/10] libbpf: Verify BTF Sorting

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2025-12-08 at 14:23 +0800, Donglin Peng wrote:
> From: pengdonglin <pengdonglin@xiaomi.com>
> 
> This patch checks whether the BTF is sorted by name in ascending
> order. If sorted, binary search will be used when looking up types.
> 
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Cc: Alan Maguire <alan.maguire@oracle.com>
> Cc: Ihor Solodrai <ihor.solodrai@linux.dev>
> Cc: Xiaoqin Zhang <zhangxiaoqin@xiaomi.com>
> Signed-off-by: pengdonglin <pengdonglin@xiaomi.com>
> ---
>  tools/lib/bpf/btf.c | 46 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 7f150c869bf6..a53d24704857 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -899,6 +899,49 @@ int btf__resolve_type(const struct btf *btf, __u32 type_id)
>  	return type_id;
>  }
>  
> +/*
> + * Assuming that types are sorted by name in ascending order.
> + */
> +static int btf_compare_type_names(const void *a, const void *b, void *priv)

This can be declared as ...(u32 a, u32 b, struct btf *btf).

> +{
> +	struct btf *btf = (struct btf *)priv;
> +	struct btf_type *ta = btf_type_by_id(btf, *(__u32 *)a);
> +	struct btf_type *tb = btf_type_by_id(btf, *(__u32 *)b);
> +	const char *na, *nb;
> +
> +	na = btf__str_by_offset(btf, ta->name_off);
> +	nb = btf__str_by_offset(btf, tb->name_off);
> +	return strcmp(na, nb);
> +}
> +
> +static void btf_check_sorted(struct btf *btf)
> +{
> +	const struct btf_type *t;
> +	int i, k = 0, n;
> +	__u32 sorted_start_id = 0;
> +
> +	if (btf->nr_types < 2)
> +		return;
> +
> +	n = btf__type_cnt(btf) - 1;
> +	for (i = btf->start_id; i < n; i++) {
> +		k = i + 1;
> +		if (btf_compare_type_names(&i, &k, btf) > 0)
> +			return;
> +		t = btf_type_by_id(btf, i);
> +		if (sorted_start_id == 0 &&
> +			!str_is_empty(btf__str_by_offset(btf, t->name_off)))
                ^^^^^^^^
Nit: broken indentation.

> +			sorted_start_id = i;
> +	}
> +
> +	t = btf_type_by_id(btf, k);

Nit: please use 'n' instead of 'k'.
     Maybe just change condition in the loop and avoid the second part?
     E.g.:

       n = btf__type_cnt(btf);
       for (...) {
         ...
         if (k < n && btf_compare_type_names(a: &i, b: &k, priv: btf) > 0)
           return;
         ...
       }

     A bit shorter/simpler this way.

> +	if (sorted_start_id == 0 &&
> +		!str_is_empty(btf__str_by_offset(btf, t->name_off)))
> +		sorted_start_id = k;
> +	if (sorted_start_id)
> +		btf->sorted_start_id = sorted_start_id;
> +}
> +
>  static __s32 btf_find_by_name_bsearch(const struct btf *btf, const char *name,
>  						__s32 start_id, __s32 end_id)
>  {
> @@ -935,7 +978,7 @@ static __s32 btf_find_by_name_kind(const struct btf *btf, int start_id,
>  
>  	if (start_id < btf->start_id) {
>  		idx = btf_find_by_name_kind(btf->base_btf, start_id,
> -			type_name, kind);
> +					    type_name, kind);

Nit: shouldn't be in this patch.

>  		if (idx >= 0)
>  			return idx;
>  		start_id = btf->start_id;
> @@ -1147,6 +1190,7 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf, b
>  	err = err ?: btf_sanity_check(btf);
>  	if (err)
>  		goto done;
> +	btf_check_sorted(btf);
>  
>  done:
>  	if (err) {