Re: [PATCH v9 2/8] util_macros: Add exact_type macro to catch type mis-match while compiling

[Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>]

On 8/26/22 2:19 AM, Kees Cook wrote:
> On Wed, Aug 24, 2022 at 05:45:08PM +0900, Gwan-gyeong Mun wrote:
>> It adds exact_type and exactly_pgoff_t macro to catch type mis-match while
>> compiling. The existing typecheck() macro outputs build warnings, but the
>> newly added exact_type() macro uses the BUILD_BUG_ON() macro to generate
>> a build break when the types are different and can be used to detect
>> explicit build errors.
>>
>> v6: Move macro addition location so that it can be used by other than drm
>>      subsystem (Jani, Mauro, Andi)
>>
>> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>> Cc: Matthew Auld <matthew.auld@intel.com>
>> Cc: Nirmoy Das <nirmoy.das@intel.com>
>> Cc: Jani Nikula <jani.nikula@intel.com>
>> Cc: Andi Shyti <andi.shyti@linux.intel.com>
>> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
>> ---
>>   include/linux/util_macros.h | 25 +++++++++++++++++++++++++
>>   1 file changed, 25 insertions(+)
>>
>> diff --git a/include/linux/util_macros.h b/include/linux/util_macros.h
>> index 72299f261b25..b6624b275257 100644
>> --- a/include/linux/util_macros.h
>> +++ b/include/linux/util_macros.h
>> @@ -2,6 +2,9 @@
>>   #ifndef _LINUX_HELPER_MACROS_H_
>>   #define _LINUX_HELPER_MACROS_H_
>>   
>> +#include <linux/types.h>
>> +#include <linux/bug.h>
>> +
>>   #define __find_closest(x, a, as, op)					\
>>   ({									\
>>   	typeof(as) __fc_i, __fc_as = (as) - 1;				\
>> @@ -38,4 +41,26 @@
>>    */
>>   #define find_closest_descending(x, a, as) __find_closest(x, a, as, >=)
>>   
>> +/**
>> + * exact_type - break compile if source type and destination value's type are
>> + * not the same
>> + * @T: Source type
>> + * @n: Destination value
>> + *
>> + * It is a helper macro for a poor man's -Wconversion: only allow variables of
>> + * an exact type. It determines whether the source type and destination value's
>> + * type are the same while compiling, and it breaks compile if two types are
>> + * not the same
>> + */
>> +#define exact_type(T, n) \
>> +	BUILD_BUG_ON(!__builtin_constant_p(n) && !__builtin_types_compatible_p(T, typeof(n)))
> 
> Maybe use __same_type() here instead of open-coded
> __builtin_types_compatible_p()? Also, IIUC, currently coding style
> advise is to use _Static_assert when possible over BUILD_BUG_ON for
> error message readability.
> 
> This macro has a trap-door for literals, yes?
> i.e.  exact_type(pgoff_t, 5) will pass?
> 
yes, I will update in detail comments about trap-door that may occur 
when using constant value.

> I also note that this is very close to the really common (and open-coded)
> test scattered around the kernel already (BUILD_BUG_ON(__same_type(a,
> b))), so I think it's good to get a macro defined for it, though I'm not
> sure about the trap door test. Regardless, I'd like to bikeshed the name
> a bit; I think this should be named something a bit more clear about
> what happens on failure. Perhaps: assert_type()? Or to capture the
> trapdoor idea, assert_typable()?
> 
> #define assert_type(t1, t2)	_Static_assert(__same_type(t1, t2))
> #define assert_typable(t, n)	_Static_assert(__builtin_constant_p(n) ||
> 					       __same_type(t, typeof(n))

The form of the assert_type() / assert_typable() macros you suggested 
looks better to me, so I will add these macros to the header where 
__same_type() is defined and will send a new version of the patch.

many thanks
> 
>> +
>> +/**
>> + * exactly_pgoff_t - helper to check if the type of a value is pgoff_t
>> + * @n: value to compare pgoff_t type
>> + *
>> + * It breaks compile if the argument value's type is not pgoff_t type.
>> + */
>> +#define exactly_pgoff_t(n) exact_type(pgoff_t, n)
> 
> Why specialize this? Just use assert_typable(pgoff_t, n) in the other
> patches? It's almost the same amount to write. :)
>