Re: [PATCH v3] kernel/bpf/btf.c: reject to register duplicated kfunc

[Kaitao Cheng]

在 2026/5/24 16:29, Song Chen 写道:
> 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@xxxxxxxxx>
> Suggested-by: Kaitao Cheng <kaitao.cheng@xxxxxxxxx>
> Signed-off-by: Song Chen <chensong_2000@xxxxxxx>
> 
> ---
> 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.
> 
> [1]:https://lore.kernel.org/all/CAADnVQ+jYGMjAC9aNygmhyppUO9foWN4z9cjSpwCEXAFHpRVJQ@xxxxxxxxxxxxxx/
> ---
>  kernel/bpf/btf.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 43 insertions(+), 1 deletion(-)
>

btf: reject to register duplicated kfunc
 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 4872d2a6c42d..a14ad3720872 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -8618,6 +8618,47 @@ 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;
> +	int ret = 0;

This ret variable may be unnecessary.

> +
> +	if (!btf_is_module(btf))
> +		goto out;
> +
> +	id = btf_find_by_name_kind(bpf_get_btf_vmlinux(),
> +					func_name, kind);

It seems unnecessary to split this call across multiple lines. Also,
some of the continuation-line indentation elsewhere does not appear
to follow the kernel coding style.

> +	if (id >= 0) {
> +		pr_err("kfunc %s (id: %d) is already present in vmlinux.\n",
> +					func_name, id);
> +		ret = -EINVAL;
> +		goto out;

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);

follow the kernel coding style

> +			ret = -EINVAL;
> +			break;

mutex_unlock(&btf_module_mutex);
return -EINVAL;

> +		}
> +	}
> +	mutex_unlock(&btf_module_mutex);
> +#endif
> +
> +out:
> +	return ret;
> +}
> +
>  static int btf_check_kfunc_protos(struct btf *btf, u32 func_id, u32 func_flags)
>  {
>  	const struct btf_type *func;
> @@ -8631,7 +8672,8 @@ 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)))

follow the kernel coding style

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

-- 
Thanks
Kaitao Cheng