Re: [PATCH] overflow: Provide constant expression struct_size

[Gustavo A. R. Silva]

On Wed, Feb 09, 2022 at 04:43:26PM -0800, Kees Cook wrote:
> There have been cases where struct_size() (or flex_array_size()) needs
> to be calculated for an initializer, which requires it be a constant
> expression. This is possible when the "count" argument is a constant
> expression, so provide this ability for the helpers.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Reviewed-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
Tested-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Thanks!
--
Gustavo

> ---
>  include/linux/overflow.h | 10 +++++++---
>  lib/test_overflow.c      |  7 +++++++
>  2 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 59d7228104d0..f1221d11f8e5 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -4,6 +4,7 @@
>  
>  #include <linux/compiler.h>
>  #include <linux/limits.h>
> +#include <linux/const.h>
>  
>  /*
>   * We need to compute the minimum and maximum values representable in a given
> @@ -221,8 +222,9 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
>   * Return: number of bytes needed or SIZE_MAX on overflow.
>   */
>  #define flex_array_size(p, member, count)				\
> -	size_mul(count,							\
> -		 sizeof(*(p)->member) + __must_be_array((p)->member))
> +	__builtin_choose_expr(__is_constexpr(count),			\
> +		(count) * sizeof(*(p)->member) + __must_be_array((p)->member),	\
> +		size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
>  
>  /**
>   * struct_size() - Calculate size of structure with trailing flexible array.
> @@ -237,6 +239,8 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
>   * Return: number of bytes needed or SIZE_MAX on overflow.
>   */
>  #define struct_size(p, member, count)					\
> -	size_add(sizeof(*(p)), flex_array_size(p, member, count))
> +	__builtin_choose_expr(__is_constexpr(count),			\
> +		sizeof(*(p)) + flex_array_size(p, member, count),	\
> +		size_add(sizeof(*(p)), flex_array_size(p, member, count)))
>  
>  #endif /* __LINUX_OVERFLOW_H */
> diff --git a/lib/test_overflow.c b/lib/test_overflow.c
> index 712fb2351c27..d488ffaf56be 100644
> --- a/lib/test_overflow.c
> +++ b/lib/test_overflow.c
> @@ -602,11 +602,18 @@ struct __test_flex_array {
>  
>  static int __init test_overflow_size_helpers(void)
>  {
> +	/* Make sure struct_size() can be used in a constant expression. */
> +	u8 ce_array[struct_size((struct __test_flex_array *)0, data, 55)];
>  	struct __test_flex_array *obj;
>  	int count = 0;
>  	int err = 0;
>  	int var;
>  
> +	/* Verify constant expression against runtime version. */
> +	var = 55;
> +	OPTIMIZER_HIDE_VAR(var);
> +	err |= sizeof(ce_array) != struct_size(obj, data, var);
> +
>  #define check_one_size_helper(expected, func, args...)	({	\
>  	bool __failure = false;					\
>  	size_t _r;						\
> -- 
> 2.30.2
>