netdev - Re: [PATCH bpf-next v8 07/10] bpf, net: switch to dynamic registration

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Date: Tue, 31 Oct 2023 17:19:23 -0700
From: Kui-Feng Lee <sinquersw@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: kuifeng@...a.com, netdev@...r.kernel.org, bpf@...r.kernel.org,
 ast@...nel.org, song@...nel.org, kernel-team@...a.com, andrii@...nel.org,
 thinker.li@...il.com, drosen@...gle.com
Subject: Re: [PATCH bpf-next v8 07/10] bpf, net: switch to dynamic
 registration



On 10/31/23 17:02, Martin KaFai Lau wrote:
> On 10/31/23 4:34 PM, Kui-Feng Lee wrote:
>>>> diff --git a/include/linux/btf.h b/include/linux/btf.h
>>>> index a8813605f2f6..954536431e0b 100644
>>>> --- a/include/linux/btf.h
>>>> +++ b/include/linux/btf.h
>>>> @@ -12,6 +12,8 @@
>>>>   #include <uapi/linux/bpf.h>
>>>>   #define BTF_TYPE_EMIT(type) ((void)(type *)0)
>>>> +#define BTF_STRUCT_OPS_TYPE_EMIT(type) {((void)(struct type *)0);    \
>>>
>>> ((void)(struct type *)0); is new. Why is it needed?
>>
>> This is a trick of BTF to force compiler generate type info for
>> the given type. Without trick, compiler may skip these types if these
>> type are not used at all in the module.  For example, modules usually
>> don't use value types of struct_ops directly.
> It is not the value type and value type emit is understood. It is the 
> struct_ops type itself and it is new addition in this patchset afaict. 
> The value type emit is in the next line which was cut out from the 
> context here.
> 
I mean both of them are required.
In the case of a dummy implementation, struct_ops type itself properly 
never being used, only being declared by the module. Without this line,
the module developer will fail to load a struct_ops map of the dummy
type. This line is added to avoid this awful situation.