Re: [PATCH 1/2] tools/lib/bpf/libbpf: Prioritize module kfuncs over vmlinux kfuncs

Re: [PATCH 1/2] tools/lib/bpf/libbpf: Prioritize module kfuncs over vmlinux kfuncs

[Mykyta Yatsenko]

On 4/30/26 6:47 AM, Song Chen wrote:
> Change the kfunc resolution order in find_ksym_btf_id() to search
> module BTFs before vmlinux BTF. This allows kernel modules to override
> vmlinux kfuncs with the same name, enabling a form of live-patching
> for kfuncs.
> 
> Previously, vmlinux kfuncs were always preferred, making it impossible
> for modules to provide enhanced or fixed versions of existing kfuncs.
> With this change, modules can now override kernel kfuncs, while
> programs that don't use module BTFs remain unaffected.
> 
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: Song Chen <chensong_2000@126.com>
> ---
>  tools/lib/bpf/libbpf.c | 34 ++++++++++++++++++----------------
>  1 file changed, 18 insertions(+), 16 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 0be7017800fe..9c308930bddb 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8538,29 +8538,31 @@ static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
>  {
>  	struct module_btf *mod_btf;
>  	struct btf *btf;
> -	int i, id, err;
> +	int i, id = 0, err;

nit: I think you can keep id uninitialized, it's set below 
unconditionally.

>  
> -	btf = obj->btf_vmlinux;
>  	mod_btf = NULL;

nit: It looks like you don't need setting mod_btf to NULL here,
 both code paths set it anyway.

> -	id = btf__find_by_name_kind(btf, ksym_name, kind);
>  
> -	if (id == -ENOENT) {
> -		err = load_module_btfs(obj);
> -		if (err)
> -			return err;
> +	err = load_module_btfs(obj);
> +	if (err)
> +		goto search_vmlinux;
>  
> -		for (i = 0; i < obj->btf_module_cnt; i++) {
> -			/* we assume module_btf's BTF FD is always >0 */
> -			mod_btf = &obj->btf_modules[i];
> -			btf = mod_btf->btf;
> -			id = btf__find_by_name_kind_own(btf, ksym_name, kind);
> -			if (id != -ENOENT)
> -				break;
> -		}
> +	for (i = 0; i < obj->btf_module_cnt; i++) {
> +		/* we assume module_btf's BTF FD is always >0 */
> +		mod_btf = &obj->btf_modules[i];
> +		btf = mod_btf->btf;
> +		id = btf__find_by_name_kind_own(btf, ksym_name, kind);
> +		if (id != -ENOENT)
> +			goto found;

very nit: Perhaps setting *res_btf and *res_mod_btf here inline
and return will make code a bit easier to follow and allow
removing found label.

Overall it looks like the refactoring looks correct, module
btf is processed before the vmlinux btf.

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>

>  	}
> -	if (id <= 0)
> +
> +search_vmlinux:
> +	btf = obj->btf_vmlinux;
> +	mod_btf = NULL;
> +	id = btf__find_by_name_kind(btf, ksym_name, kind);
> +	if (id == -ENOENT)
>  		return -ESRCH;
>  
> +found:
>  	*res_btf = btf;
>  	*res_mod_btf = mod_btf;
>  	return id;