Re: [PATCH bpf-next v9 07/10] btf: 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.
> 
> Specifically, vmlinux and kernel module BTFs are always sorted during
> the build phase with anonymous types placed before named types, so we
> only need to identify the starting ID of named 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>
> ---
>  kernel/bpf/btf.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 58 insertions(+)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 842f9c0200e4..925cb524f3a8 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -550,6 +550,60 @@ u32 btf_nr_types(const struct btf *btf)
>  	return total;
>  }
>  
> +/*
> + * Assuming that types are sorted by name in ascending order.
> + */
> +static int btf_compare_type_names(const void *a, const void *b, void *priv)
> +{
> +	struct btf *btf = (struct btf *)priv;
> +	const struct btf_type *ta = btf_type_by_id(btf, *(__u32 *)a);
> +	const struct btf_type *tb = btf_type_by_id(btf, *(__u32 *)b);
> +	const char *na, *nb;
> +
> +	na = btf_name_by_offset(btf, ta->name_off);
> +	nb = btf_name_by_offset(btf, tb->name_off);
> +	return strcmp(na, nb);
> +}
> +
> +/* Note that vmlinux and kernel module BTFs are always sorted
> + * during the building phase.
> + */
> +static void btf_check_sorted(struct btf *btf)
> +{
> +	const struct btf_type *t;
> +	bool skip_cmp = btf_is_kernel(btf);

Nit: maybe just do a separate loop when btf_is_kernel(btf) == true?

  for (i = 0, n = btf_nr_types(btf); i < n, i++) {
    t = btf_type_by_id(btf, i);
    if (t->name_off) {
      btf->sorted_start_id = i;
      return;
    }
  }
  return;

> +	u32 sorted_start_id = 0;
> +	int i, n, k = 0;
> +
> +	if (btf->nr_types < 2)
> +		return;
> +
> +	n = btf_nr_types(btf) - 1;
> +	for (i = btf_start_id(btf); i < n; i++) {
> +		k = i + 1;
> +		if (!skip_cmp &&
> +			btf_compare_type_names(&i, &k, btf) > 0)
> +			return;
> +
> +		if (sorted_start_id == 0) {
> +			t = btf_type_by_id(btf, i);
> +			if (t->name_off) {
> +				sorted_start_id = i;
> +				if (skip_cmp)
> +					break;
> +			}
> +		}
> +	}
> +
> +	if (sorted_start_id == 0) {
> +		t = btf_type_by_id(btf, k);
> +		if (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)
>  {
> @@ -5889,6 +5943,8 @@ static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr, u32 uat
>  	if (err)
>  		goto errout;
>  
> +	btf_check_sorted(btf);
> +
>  	struct_meta_tab = btf_parse_struct_metas(&env->log, btf);
>  	if (IS_ERR(struct_meta_tab)) {
>  		err = PTR_ERR(struct_meta_tab);
> @@ -6296,6 +6352,7 @@ static struct btf *btf_parse_base(struct btf_verifier_env *env, const char *name
>  	if (err)
>  		goto errout;
>  
> +	btf_check_sorted(btf);
>  	refcount_set(&btf->refcnt, 1);
>  
>  	return btf;
> @@ -6430,6 +6487,7 @@ static struct btf *btf_parse_module(const char *module_name, const void *data,
>  	}
>  
>  	btf_verifier_env_free(env);
> +	btf_check_sorted(btf);
>  	refcount_set(&btf->refcnt, 1);
>  	return btf;
>