linux-kernel - Re: [PATCH v2 2/4] uaccess: always export _copy_[from|to]_user with CONFIG

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Date: Fri,  9 Feb 2024 17:45:05 +0100
From: Valentin Obst <kernel@...entinobst.de>
To: arnd@...db.de
Cc: a.hindborg@...sung.com,
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Subject: Re: [PATCH v2 2/4] uaccess: always export _copy_[from|to]_user with CONFIG_RUST

>>> -#else
>>>  extern __must_check unsigned long
>>>  _copy_from_user(void *, const void __user *, unsigned long);
>>> -#endif
>>
>> This function is now unconditionally declared, but only defined if
>> `!defined(INLINE_COPY_FROM_USER) || defined(CONFIG_RUST)`, i.e., in the
>> common case where it is inlined and Rust is disabled this can lead to
>> link-time problems if someone decides to use it.
>
> Yes, that is intentional.
>
> If someone tries to use it when the declaration is not there,
> they just get a compile-time error, which is not all that
> different from a link-time error in practice.
>
> It's unlikely to make a difference here, but enclosing
> declarations in an #ifdef is annoying when you want to
> reference it from somewhere that is parsed by the compiler
> but not called without the respective options.
>
> The if(IS_ENABLED()) and PTR_IF() constructs in particular
> only work when the unused functions are still declared.

I see, thanks for the explanation.

    - Best Valentin

>
>        Arnd