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

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Message-ID: <875xxjp5n3.fsf@intel.com>
Date: Mon, 18 Mar 2024 14:54:56 -0700
From: Vinicius Costa Gomes <vinicius.gomes@...el.com>
To: Christian Brauner <brauner@...nel.org>
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

Christian Brauner <brauner@...nel.org> writes:

> 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.

Fair point.

>
> 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.

For what it's worth, I liked your changes, will remove these two "fixes"
from the series and use your suggestions in the next version.


Thank you,
-- 
Vinicius