Re: [PATCH v4] btf: reject to register duplicated kfunc

[Yonghong Song <yonghong.song@linux.dev>]

On 6/2/26 4:07 AM, Song Chen wrote:
> I had an ebpf program which calls a kfunc defined and
> implemented in one of my kernel modules, it was working
> fine in 6.16, but was rejected to run by libbpf in 6.19,
> the error message was:
>
> libbpf: extern (func ksym) 'bpf_strstr': func_proto [5]
> incompatible with vmlinux [94389]
>
> The reason is there is a new added kfunc in kernel 6.19
> which happens to be the same name with my kfunc. However the
> error message is not obvious enough to address problem
> immediately.
>
> Therefore, this patches searches duplicated kfunc in
> both btf_vmlinux and btf_modules list before a kernel module
> attempts to register kfuncs through register_btf_kfunc_id_set,
> it prints clear error message and returns error code if same name
> kfunc has already implemented and registered, then developer
> knows at the first place.
>
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Suggested-by: Kaitao Cheng <kaitao.cheng@linux.dev>
> Reviewed-by: Yonghong Song <yonghong.song@linux.dev>
> Signed-off-by: Song Chen <chensong_2000@126.com>

The subject can be [PATCH bpf v4] bpf: Reject to register duplicated kfunc

>
> ---
> changelog:
> v1 --- v2:
> libbpf has already specified which module this kfunc belongs to as
> ebpf code onwer's expectation, then verifier uses
> find_kallsyms_symbol_value to search kfunc's addr.
>
> v2 --- v3:
> After v2, I tried a new idea of introducing a namespace in libbpf
> to specify kfunc owner in an ebpf program suggested by Kaitao Cheng,
> please see [1]. Alex suggested to go back to report an error during
> kmod load on conflicting kfunc name for now. What's more, v2 only
> searched bpf_vmlinux, v3 also traverses btf_modules list.
>
> v3 --- v4
> Fixed some coding style problems suggested from Kaitao Cheng.
> ---
>   kernel/bpf/btf.c | 38 +++++++++++++++++++++++++++++++++++++-
>   1 file changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 4872d2a6c42d..fe1612677a4a 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -8618,6 +8618,41 @@ static int btf_check_iter_kfuncs(struct btf *btf, const char *func_name,
>   	return 0;
>   }
>   
> +static int btf_check_kfunc_name(struct btf *btf, const char *func_name, u32 kind)
> +{
> +#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> +	struct btf_module *btf_mod, *tmp;
> +#endif
> +	s32 id;

	s32 err = 0, id;

> +
> +	if (!btf_is_module(btf))
> +		return 0;
> +
> +	id = btf_find_by_name_kind(bpf_get_btf_vmlinux(), func_name, kind);
> +	if (id >= 0) {
> +		pr_err("kfunc %s (id: %d) is already present in vmlinux.\n",
> +				func_name, id);
> +		return -EINVAL;
> +	}
> +
> +#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> +	mutex_lock(&btf_module_mutex);
> +	list_for_each_entry_safe(btf_mod, tmp, &btf_modules, list) {
> +		if (btf_mod->btf == btf)
> +			continue;
> +		id = btf_find_by_name_kind(btf_mod->btf, func_name, kind);
> +		if (id >= 0) {
> +			pr_err("kfunc %s (id: %d) is already present in module %s.\n",
> +					func_name, id, btf_mod->module->name);
> +			mutex_unlock(&btf_module_mutex);
> +			return -EINVAL;

Let us avoid the above mutex_unlock and 'return -EINVAL', just do
	err = -EINVAL;
	break;

> +		}
> +	}
> +	mutex_unlock(&btf_module_mutex);
> +#endif
> +	return 0;

	return err;

> +}
> +
>   static int btf_check_kfunc_protos(struct btf *btf, u32 func_id, u32 func_flags)
>   {
>   	const struct btf_type *func;
> @@ -8631,7 +8666,8 @@ static int btf_check_kfunc_protos(struct btf *btf, u32 func_id, u32 func_flags)
>   
>   	/* sanity check kfunc name */
>   	func_name = btf_name_by_offset(btf, func->name_off);
> -	if (!func_name || !func_name[0])
> +	if (!func_name || !func_name[0] ||
> +		btf_check_kfunc_name(btf, func_name, BTF_INFO_KIND(func->info)))

format issue:
	if (!func_name || !func_name[0] ||
	    btf_check_kfunc_name(btf, func_name, BTF_INFO_KIND(func->info)))
	...	

>   		return -EINVAL;
>   
>   	func = btf_type_by_id(btf, func->type);