Re: [Intel-gfx] [PATCH v2 1/7] drm: Move and add a few utility macros into drm util header

[Mauro Carvalho Chehab <mauro.chehab@linux.intel.com>]

On Tue,  5 Jul 2022 15:24:49 +0300
Gwan-gyeong Mun <gwan-gyeong.mun@intel.com> wrote:

> It moves overflows_type utility macro into drm util header from i915_utils
> header. The overflows_type can be used to catch the truncation between data
> types. And it adds safe_conversion() macro which performs a type conversion
> (cast) of an source value into a new variable, checking that the
> destination is large enough to hold the source value.
> And it adds exact_type and exactly_pgoff_t macro to catch type mis-match
> while compiling.
> 
> 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>
> ---
>  drivers/gpu/drm/i915/i915_utils.h |  5 +--
>  include/drm/drm_util.h            | 54 +++++++++++++++++++++++++++++++
>  2 files changed, 55 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
> index c10d68cdc3ca..345e5b2dc1cd 100644
> --- a/drivers/gpu/drm/i915/i915_utils.h
> +++ b/drivers/gpu/drm/i915/i915_utils.h
> @@ -32,6 +32,7 @@
>  #include <linux/types.h>
>  #include <linux/workqueue.h>
>  #include <linux/sched/clock.h>
> +#include <drm/drm_util.h>
>  
>  #ifdef CONFIG_X86
>  #include <asm/hypervisor.h>
> @@ -111,10 +112,6 @@ bool i915_error_injected(void);
>  #define range_overflows_end_t(type, start, size, max) \
>  	range_overflows_end((type)(start), (type)(size), (type)(max))
>  
> -/* Note we don't consider signbits :| */
> -#define overflows_type(x, T) \
> -	(sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T))
> -
>  #define ptr_mask_bits(ptr, n) ({					\
>  	unsigned long __v = (unsigned long)(ptr);			\
>  	(typeof(ptr))(__v & -BIT(n));					\
> diff --git a/include/drm/drm_util.h b/include/drm/drm_util.h
> index 79952d8c4bba..c56230e39e37 100644
> --- a/include/drm/drm_util.h
> +++ b/include/drm/drm_util.h
> @@ -62,6 +62,60 @@
>   */
>  #define for_each_if(condition) if (!(condition)) {} else
>  
> +/**
> + * overflows_type - helper for checking the truncation between data types
> + * @x: Source for overflow type comparison
> + * @T: Destination for overflow type comparison
> + *
> + * It compares the values and size of each data type between the first and
> + * second argument to check whether truncation can occur when assigning the
> + * first argument to the variable of the second argument.
> + * It does't consider signbits.
> + *
> + * Returns:
> + * True if truncation can occur, false otherwise.
> + */
> +#define overflows_type(x, T) \
> +	(sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T))

As pointed on its description, this macro only works if both types
are either signed or unsigned. However, the macro itself doesn't check
it.

It probably worth adding something there to ensure that both types are 
either signed or unsigned. I would add this ancillary macro probably on
on a generic kernel header - as this can be useful outside drm:

	#define sign_matches(x, y) \
	        (!((typeof(x))-1 >= 0) ^ ((typeof(y))-1 >= 0))

And then include use it at overflows_type:

	BUILD_BUG_ON(!sign_matches(x, T))

> +
> +/**
> + * 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)))
> +
> +/**
> + * 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)
> +
> +/*
> + * safe_conversion - perform a type conversion (cast) of an source value into
> + * a new variable, checking that the destination is large enough to hold the
> + * source value.
> + * @ptr: Destination pointer address
> + * @value: Source value
> + *
> + * Returns:
> + * If the value would overflow the destination, it returns false.
> + */
> +#define safe_conversion(ptr, value) ({ \
> +	typeof(value) __v = (value); \
> +	typeof(ptr) __ptr = (ptr); \
> +	overflows_type(__v, *__ptr) ? 0 : (*__ptr = (typeof(*__ptr))__v), 1; \
> +})
> +
>  /**
>   * drm_can_sleep - returns true if currently okay to sleep
>   *