Re: [PATCH v11.5] overflow: Introduce overflows_type() and __castable_to_type()

[Gwan-gyeong Mun]

On 9/26/22 3:37 AM, Kees Cook wrote:
> Add overflows_type() to test if a variable or constant value would
> overflow another variable or type. This can be used as a constant
> expression for static_assert() (which requires a constant
> expression[1][2]) when used on constant values. This must be constructed
> manually, since __builtin_add_overflow() does not produce a constant
> expression[3].
>
> Additionally adds __castable_to_type(), similar to __same_type(), for
> checking if a constant value will fit in a given type (i.e. it could
> be cast to the type without overflow).
>
> Add unit tests for overflows_type(), __same_type(), and
> __castable_to_type() to the existing KUnit "overflow" test.
>
> [1] https://en.cppreference.com/w/c/language/_Static_assert
> [2] C11 standard (ISO/IEC 9899:2011): 6.7.10 Static assertions
> [3] https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html
>      6.56 Built-in Functions to Perform Arithmetic with Overflow Checking
>      Built-in Function: bool __builtin_add_overflow (type1 a, type2 b,
>                                                      type3 *res)
>
> Cc: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Cc: Tom Rix <trix@xxxxxxxxxx>
> Cc: Daniel Latypov <dlatypov@xxxxxxxxxx>
> Cc: Vitor Massaru Iha <vitor@xxxxxxxxxxx>
> Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> Cc: llvm@xxxxxxxxxxxxxxx
> Co-developed-by: Gwan-gyeong Mun <gwan-gyeong.mun@xxxxxxxxx>
> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@xxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>   include/linux/compiler.h |   1 +
>   include/linux/overflow.h |  48 +++++
>   lib/overflow_kunit.c     | 393 ++++++++++++++++++++++++++++++++++++++-
>   3 files changed, 441 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 7713d7bcdaea..c631107e93b1 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -244,6 +244,7 @@ static inline void *offset_to_ptr(const int *off)
>    * bool and also pointer types.
>    */
>   #define is_signed_type(type) (((type)(-1)) < (__force type)1)
> +#define is_unsigned_type(type) (!is_signed_type(type))
>   
>   /*
>    * This is needed in functions which generate the stack canary, see
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 19dfdd74835e..c8cbeae5f4f8 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -127,6 +127,54 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>   	(*_d >> _to_shift) != _a);					\
>   }))
>   
> +#define __overflows_type_constexpr(x, T) (			\
> +	is_unsigned_type(typeof(x)) ?				\
> +		(x) > type_max(typeof(T)) ? 1 : 0		\
> +	: is_unsigned_type(typeof(T)) ?				\
> +		(x) < 0 || (x) > type_max(typeof(T)) ? 1 : 0	\
> +		: (x) < type_min(typeof(T)) ||			\
> +		  (x) > type_max(typeof(T)) ? 1 : 0 )
> +
> +#define __overflows_type(x, T)		({	\
> +	typeof(T) v = 0;			\
> +	check_add_overflow((x), v, &v);		\
> +})
> +
> +/**
> + * overflows_type - helper for checking the overflows between value, variables,
> + *		    or data type
> + *
> + * @n: source constant value or variable to be checked
> + * @T: destination variable or data type proposed to store @x
> + *
> + * Compares the @x expression for whether or not it can safely fit in
> + * the storage of the type in @T. @x and @T can have different types.
> + * If @x is a conxtant expression, this will also resolve to a constant
> + * expression.
> + *
> + * Returns: true if overflow can occur, false otherwise.
> + */
> +#define overflows_type(n, T)					\
> +	__builtin_choose_expr(__is_constexpr(n),		\
> +			      __overflows_type_constexpr(n, T),	\
> +			      __overflows_type(n, T))
> +
> +/**
> + * __castable_to_type - like __same_type(), but also allows for casted literals
> + *
> + * @n: variable or constant value
> + * @T: data type or variable
> + *
> + * Unlike the __same_type() macro, this allows a constant value as the
> + * first argument. If this value would not overflow into an assignment
> + * of the second argument's type, it returns true. Otherwise, this falls
> + * back to __same_type().
> + */
> +#define __castable_to_type(n, T)					\
> +	__builtin_choose_expr(__is_constexpr(n),			\
> +			      !__overflows_type_constexpr(n, T),	\
> +			      __same_type(n, T))
> +
This name is fine, but I prefer the __same_typable you suggested as a 
comment in the previous patch better, what do you think?
( __castable_to_type(n, T); The macro name seems to handle if type 
casting is possible to the second argument type from the first argument 
variable. )

G.G.