linux-kernel - Re: [RFC PATCH v7 5/7] libbpf: Implement BTF type sorting validation for binary search optimization

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Message-ID: <CAEf4BzYQfHKHUdxv7W7mET1xBXuokvx9v=69HNAkhg_CAPCm-g@mail.gmail.com>
Date: Wed, 19 Nov 2025 11:50:27 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Donglin Peng <dolinux.peng@...il.com>
Cc: ast@...nel.org, eddyz87@...il.com, zhangxiaoqin@...omi.com, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	Donglin Peng <pengdonglin@...omi.com>, Alan Maguire <alan.maguire@...cle.com>, 
	Song Liu <song@...nel.org>
Subject: Re: [RFC PATCH v7 5/7] libbpf: Implement BTF type sorting validation
 for binary search optimization

On Tue, Nov 18, 2025 at 7:21 PM Donglin Peng <dolinux.peng@...il.com> wrote:
>
> From: Donglin Peng <pengdonglin@...omi.com>
>
> This patch adds validation to verify BTF type name sorting, enabling
> binary search optimization for lookups.
>
> Cc: Eduard Zingerman <eddyz87@...il.com>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> Cc: Alan Maguire <alan.maguire@...cle.com>
> Cc: Song Liu <song@...nel.org>
> Cc: Xiaoqin Zhang <zhangxiaoqin@...omi.com>
> Signed-off-by: Donglin Peng <pengdonglin@...omi.com>
> ---
>  tools/lib/bpf/btf.c | 59 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 1d19d95da1d0..d872abff42e1 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -903,6 +903,64 @@ int btf__resolve_type(const struct btf *btf, __u32 type_id)
>         return type_id;
>  }
>
> +/* Anonymous types (with empty names) are considered greater than named types
> + * and are sorted after them. Two anonymous types are considered equal. Named
> + * types are compared lexicographically.
> + */
> +static int btf_compare_type_names(const void *a, const void *b, void *priv)
> +{
> +       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;
> +       bool anon_a, anon_b;
> +
> +       na = btf__str_by_offset(btf, ta->name_off);
> +       nb = btf__str_by_offset(btf, tb->name_off);
> +       anon_a = str_is_empty(na);
> +       anon_b = str_is_empty(nb);
> +
> +       if (anon_a && !anon_b)
> +               return 1;
> +       if (!anon_a && anon_b)
> +               return -1;
> +       if (anon_a && anon_b)
> +               return 0;

any reason to hard-code that anonymous types should come *after* named
ones? That requires custom comparison logic here and resolve_btfids,
instead of just relying on btf__str_by_offset() returning valid empty
string for name_off == 0 and then sorting anon types before named
ones, following normal lexicographical sorting rules?

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