linux-kernel - Re: [PATCH bpf-next v11 04/11] libbpf: Optimize type lookup with binary search for sorted BTF

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Message-Id: <4741560dd9cff0cf2643b9ea291154e91a2ce3e57370f020d0b346230a98efb1@mail.kernel.org>
Date: Thu,  8 Jan 2026 03:46:17 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: dolinux.peng@...il.com,ast@...nel.org,andrii.nakryiko@...il.com,eddyz87@...il.com
Cc: zhangxiaoqin@...omi.com,ihor.solodrai@...ux.dev,linux-kernel@...r.kernel.org,bpf@...r.kernel.org,pengdonglin@...omi.com,alan.maguire@...cle.com,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v11 04/11] libbpf: Optimize type lookup with binary search for sorted BTF

> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index bf75f770d29a..60ff8eafea83 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c

[ ... ]

> @@ -897,46 +899,83 @@ int btf__resolve_type(const struct btf *btf, __u32 type_id)
>  	return type_id;
>  }
>
> -__s32 btf__find_by_name(const struct btf *btf, const char *type_name)
> -{
> -	__u32 i, nr_types = btf__type_cnt(btf);
> -
> -	if (!strcmp(type_name, "void"))
> -		return 0;
> -
> -	for (i = 1; i < nr_types; i++) {
                  ^
The original btf__find_by_name() started iteration from ID 1, which meant
it would search through ALL types including those in base BTF for split
BTF scenarios. The btf__type_by_id() call would transparently access
base BTF types when i < btf->start_id.

> -		const struct btf_type *t = btf__type_by_id(btf, i);
> -		const char *name = btf__name_by_offset(btf, t->name_off);
> -
> -		if (name && !strcmp(type_name, name))
> -			return i;
> -	}
> -
> -	return libbpf_err(-ENOENT);
> -}

[ ... ]

> +/* the kind value of -1 indicates that kind matching should be skipped */
> +__s32 btf__find_by_name(const struct btf *btf, const char *type_name)
> +{
> +	return btf_find_by_name_kind(btf, btf->start_id, type_name, -1);
                                        ^^^^^^^^^^^^^^^
Is this intended to change the split BTF lookup behavior? The new
implementation passes btf->start_id as the start_id parameter, which
means for split BTF the search will only cover types in the current BTF
instance, not the base BTF.

In btf_find_by_name_kind(), the base BTF is only searched when
start_id < btf->start_id, but here start_id == btf->start_id so that
condition is never true.

Compare with btf__find_by_name_kind() which still passes 1 as start_id:

  __s32 btf__find_by_name_kind(const struct btf *btf, ...)
  {
      return btf_find_by_name_kind(btf, 1, type_name, kind);
  }

This creates an inconsistency where btf__find_by_name_kind() searches
base BTF but btf__find_by_name() does not. Was this behavior change
intentional? If so, perhaps the commit message should mention it.

> +}
> +
>  __s32 btf__find_by_name_kind_own(const struct btf *btf, const char *type_name,
>  				 __u32 kind)
>  {
>  	return btf_find_by_name_kind(btf, btf->start_id, type_name, kind);
>  }

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20804442135