Re: [PATCH bpf-next] libbpf: fix error when st-prefix_ops and ops from differ btf

[Martin KaFai Lau <martin.lau@linux.dev>]

On 9/26/25 12:17 AM, D. Wythe wrote:
> When a module registers a struct_ops, the struct_ops type and its
> corresponding map_value type ("bpf_struct_ops_") may reside in different
> btf objects, here are four possible case:
> 
> +--------+---------------+-------------+---------------------------------+
> |        |bpf_struct_ops_| xxx_ops     |                                 |
> +--------+---------------+-------------+---------------------------------+
> | case 0 | btf_vmlinux   | bft_vmlinux | be used and reg only in vmlinux |

s/bft/btf/

> +--------+---------------+-------------+---------------------------------+
> | case 1 | btf_vmlinux   | mod_btf     | INVALID                         |
> +--------+---------------+-------------+---------------------------------+
> | case 2 | mod_btf       | btf_vmlinux | reg in mod but be used both in  |
> |        |               |             | vmlinux and mod.                |
> +--------+---------------+-------------+---------------------------------+
> | case 3 | mod_btf       | mod_btf     | be used and reg only in mod     |
> +--------+---------------+-------------+---------------------------------+
> 
> Currently we figure out the mod_btf by searching with the struct_ops type,
> which makes it impossible to figure out the mod_btf when the struct_ops
> type is in btf_vmlinux while it's corresponding map_value type is in
> mod_btf (case 2).
> 
> The fix is to use the corresponding map_value type ("bpf_struct_ops_")
> as the lookup anchor instead of the struct_ops type to figure out the
> `btf` and `mod_btf` via find_ksym_btf_id(), and then we can locate
> the kern_type_id via btf__find_by_name_kind() with the `btf` we just
> obtained from find_ksym_btf_id().
> 
> With this change the lookup obtains the correct btf and mod_btf for case 2,
> preserves correct behavior for other valid cases, and still fails as
> expected for the invalid scenario (case 1).
> 
> Fixes: 590a00888250 ("bpf: libbpf: Add STRUCT_OPS support")
> Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>   tools/lib/bpf/libbpf.c | 37 ++++++++++++++++++-------------------
>   1 file changed, 18 insertions(+), 19 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 5161c2b39875..a93eed660404 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1018,35 +1018,34 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
>   	const struct btf_member *kern_data_member;
>   	struct btf *btf = NULL;
>   	__s32 kern_vtype_id, kern_type_id;
> -	char tname[256];
> +	char tname[256], stname[256];
>   	__u32 i;
>   
>   	snprintf(tname, sizeof(tname), "%.*s",
>   		 (int)bpf_core_essential_name_len(tname_raw), tname_raw);
>   
> -	kern_type_id = find_ksym_btf_id(obj, tname, BTF_KIND_STRUCT,
> -					&btf, mod_btf);
> -	if (kern_type_id < 0) {
> -		pr_warn("struct_ops init_kern: struct %s is not found in kernel BTF\n",
> -			tname);
> -		return kern_type_id;
> -	}
> -	kern_type = btf__type_by_id(btf, kern_type_id);
> +	snprintf(stname, sizeof(stname), "%s%.*s", STRUCT_OPS_VALUE_PREFIX,
> +		 (int)strlen(tname), tname);

nit. strlen(tname) should not be needed. Others lgtm.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>