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

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Date: Wed, 1 Nov 2023 17:17:20 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Kui-Feng Lee <sinquersw@...il.com>
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 5:19 PM, Kui-Feng Lee wrote:
> 
> 
> 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,

Other than bpf_dummy_ops, after reg(), the struct_ops->func() must be used 
somewhere in the kernel or module. Like tcp must be using the tcp_congestion_ops 
after reg(). bpf_dummy_ops is very special and probably should be moved out to 
bpf_testmod somehow but this is for later. Even bpf_dummy_ops does not have an 
issue now. Why it is needed after the kmod support change?

or it is a preemptive addition to be future proof only?

Addition is fine if it is required to work. I am trying to understand why this 
new addition is needed after the kmod support change. The reason why this is 
needed after the kmod support change is not obvious from looking at the code. 
The commit message didn't mention why and what broke after this kmod change. If 
someone wants to clean it up a few months later, we will need to figure out why 
it was added in the first place.

> the module developer will fail to load a struct_ops map of the dummy
> type. This line is added to avoid this awful situation.
>