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

[Song Chen <chensong_2000@126.com>]

Hi Kaitao,

在 2026/5/25 17:18, Kaitao Cheng 写道:
> 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@gmail.com>
>> Suggested-by: Kaitao Cheng <kaitao.cheng@linux.dev>
>> Signed-off-by: Song Chen <chensong_2000@126.com>
>>
>> ---
>> 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@mail.gmail.com/
>> ---
>>   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

I understood the rest part of your comments, but this one "follow the 
kernel coding style", is it an indentation problem? checkpatch didn't 
say anything about it.
> 
>> +			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

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


/Song