linux-kernel - Re: [RFC v3 1/5] cleanup: Fix discarded const warning when defining lock guard

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Date: Mon, 18 Mar 2024 16:13:12 +0100
From: Christian Brauner <brauner@...nel.org>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc: amir73il@...il.com, hu1.chen@...el.com, miklos@...redi.hu, 
	malini.bhandaru@...el.com, tim.c.chen@...el.com, mikko.ylinen@...el.com, 
	lizhen.you@...el.com, linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC v3 1/5] cleanup: Fix discarded const warning when defining
 lock guard

On Thu, Feb 15, 2024 at 09:16:36PM -0800, Vinicius Costa Gomes wrote:
> Fix the following warning when defining a cleanup guard for a "const"
> pointer type:
> 
> ./include/linux/cleanup.h:211:18: warning: return discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
>   211 |         return _T->lock;                                                \
>       |                ~~^~~~~~
> ./include/linux/cleanup.h:233:1: note: in expansion of macro ‘__DEFINE_UNLOCK_GUARD’
>   233 | __DEFINE_UNLOCK_GUARD(_name, _type, _unlock, __VA_ARGS__)               \
>       | ^~~~~~~~~~~~~~~~~~~~~
> ./include/linux/cred.h:193:1: note: in expansion of macro ‘DEFINE_LOCK_GUARD_1’
>   193 | DEFINE_LOCK_GUARD_1(cred, const struct cred, _T->lock = override_creds_light(_T->lock),
>       | ^~~~~~~~~~~~~~~~~~~
> 
> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>
> ---
>  include/linux/cleanup.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index c2d09bc4f976..085482ef46c8 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -208,7 +208,7 @@ static inline void class_##_name##_destructor(class_##_name##_t *_T)	\
>  									\
>  static inline void *class_##_name##_lock_ptr(class_##_name##_t *_T)	\
>  {									\
> -	return _T->lock;						\
> +	return (void *)_T->lock;					\
>  }

I think both of these patches are a bit ugly as we burden the generic
cleanup code with casting to void which could cause actual issues.

Casting from const to non-const is rather specific to the cred code so I
would rather like to put the burden on the cred code instead of the
generic code if possible.