linux-kernel - Re: [PATCH] libbpf: extending BTF_KIND_INIT to accommodate some unusual types

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Date: Mon, 22 Apr 2024 10:43:38 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Xin Liu <liuxin350@...wei.com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	martin.lau@...ux.dev, song@...nel.org, yhs@...com, john.fastabend@...il.com, 
	kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org, 
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org, yanan@...wei.com, 
	wuchangye@...wei.com, xiesongyang@...wei.com, kongweibin2@...wei.com, 
	zhangmingyi5@...wei.com, liwei883@...wei.com
Subject: Re: [PATCH] libbpf: extending BTF_KIND_INIT to accommodate some
 unusual types

On Mon, Apr 22, 2024 at 7:46 AM Xin Liu <liuxin350@...wei.com> wrote:
>
> In btf__add_int, the size of the new btf_kind_int type is limited.
> When the size is greater than 16, btf__add_int fails to be added
> and -EINVAL is returned. This is usually effective.
>
> However, when the built-in type __builtin_aarch64_simd_xi in the
> NEON instruction is used in the code in the arm64 system, the value
> of DW_AT_byte_size is 64. This causes btf__add_int to fail to
> properly add btf information to it.
>
> like this:
>   ...
>    <1><cf>: Abbrev Number: 2 (DW_TAG_base_type)
>     <d0>   DW_AT_byte_size   : 64              // over max size 16
>     <d1>   DW_AT_encoding    : 5        (signed)
>     <d2>   DW_AT_name        : (indirect string, offset: 0x53): __builtin_aarch64_simd_xi
>    <1><d6>: Abbrev Number: 0
>   ...
>
> An easier way to solve this problem is to treat it as a base type
> and set byte_size to 64. This patch is modified along these lines.
>
> Fixes: 4a3b33f8579a ("libbpf: Add BTF writing APIs")
> Signed-off-by: Xin Liu <liuxin350@...wei.com>
> ---
>  tools/lib/bpf/btf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 2d0840ef599a..0af121293b65 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -1934,7 +1934,7 @@ int btf__add_int(struct btf *btf, const char *name, size_t byte_sz, int encoding
>         if (!name || !name[0])
>                 return libbpf_err(-EINVAL);
>         /* byte_sz must be power of 2 */
> -       if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 16)
> +       if (!byte_sz || (byte_sz & (byte_sz - 1)) || byte_sz > 64)


maybe we should just remove byte_sz upper limit? We can probably
imagine 256-byte integers at some point, so why bother artificially
restricting it?

pw-bot: cr

>                 return libbpf_err(-EINVAL);
>         if (encoding & ~(BTF_INT_SIGNED | BTF_INT_CHAR | BTF_INT_BOOL))
>                 return libbpf_err(-EINVAL);
> --
> 2.33.0
>