linux-kernel - Re: [PATCH bpf-next v9 01/10] libbpf: Add BTF permutation support for type reordering

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Message-ID: <3777b2f096877a9965a0fa6905fbabb06826d13f.camel@gmail.com>
Date: Tue, 16 Dec 2025 14:00:21 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Donglin Peng <dolinux.peng@...il.com>, ast@...nel.org, 
	andrii.nakryiko@...il.com
Cc: zhangxiaoqin@...omi.com, ihor.solodrai@...ux.dev, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, pengdonglin
	 <pengdonglin@...omi.com>, Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCH bpf-next v9 01/10] libbpf: Add BTF permutation support
 for type reordering

On Mon, 2025-12-08 at 14:23 +0800, Donglin Peng wrote:
> From: pengdonglin <pengdonglin@...omi.com>
> 
> Introduce btf__permute() API to allow in-place rearrangement of BTF types.
> This function reorganizes BTF type order according to a provided array of
> type IDs, updating all type references to maintain consistency.
> 
> 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: Ihor Solodrai <ihor.solodrai@...ux.dev>
> Cc: Xiaoqin Zhang <zhangxiaoqin@...omi.com>
> Signed-off-by: pengdonglin <pengdonglin@...omi.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@...il.com>

[...]

> diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> index cc01494d6210..ba67e5457e3a 100644
> --- a/tools/lib/bpf/btf.h
> +++ b/tools/lib/bpf/btf.h
> @@ -281,6 +281,42 @@ LIBBPF_API int btf__dedup(struct btf *btf, const struct btf_dedup_opts *opts);
>   */
>  LIBBPF_API int btf__relocate(struct btf *btf, const struct btf *base_btf);
>  
> +struct btf_permute_opts {
> +	size_t sz;
> +	/* optional .BTF.ext info along the main BTF info */
> +	struct btf_ext *btf_ext;
> +	size_t :0;
> +};
> +#define btf_permute_opts__last_field btf_ext
> +
> +/**
> + * @brief **btf__permute()** performs in-place BTF type rearrangement
> + * @param btf BTF object to permute
> + * @param id_map Array mapping original type IDs to new IDs
> + * @param id_map_cnt Number of elements in @id_map
> + * @param opts Optional parameters for BTF extension updates
> + * @return 0 on success, negative error code on failure
> + *
> + * **btf__permute()** rearranges BTF types according to the specified ID mapping.
> + * The @id_map array defines the new type ID for each original type ID.
> + *
> + * For **base BTF**:
> + * - @id_map must include all types from ID 1 to `btf__type_cnt(btf)-1`
> + * - @id_map_cnt should be `btf__type_cnt(btf) - 1`
> + * - Mapping uses `id_map[original_id - 1] = new_id`
> + *
> + * For **split BTF**:
> + * - @id_map should cover only split types
> + * - @id_map_cnt should be `btf__type_cnt(btf) - btf__type_cnt(btf__base_btf(btf))`
> + * - Mapping uses `id_map[original_id - btf__type_cnt(btf__base_btf(btf))] = new_id`

Nit: internally the rule does not have special cases:

       id_map[original_id - start_id] = new_id

     So, maybe there is no need split these cases in the docstring?
     Otherwise it is not immediately clear that both cases are handled
     uniformly.

> + *
> + * On error, returns negative error code and sets errno:
> + *   - `-EINVAL`: Invalid parameters or ID mapping (duplicates, out-of-range)
> + *   - `-ENOMEM`: Memory allocation failure
> + */
> +LIBBPF_API int btf__permute(struct btf *btf, __u32 *id_map, __u32 id_map_cnt,
> +			    const struct btf_permute_opts *opts);
> +
>  struct btf_dump;
>  
>  struct btf_dump_opts {
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 8ed8749907d4..b778e5a5d0a8 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -451,4 +451,5 @@ LIBBPF_1.7.0 {
>  	global:
>  		bpf_map__set_exclusive_program;
>  		bpf_map__exclusive_program;
> +		btf__permute;
>  } LIBBPF_1.6.0;