Subject: Re: [PATCH v4 2/8] overflow: Add struct_size_with_data() and struct_data_pointer() helpers

From: Kees Cook <[email-masked]>
Date: Wed, 28 Feb 2024 13:37:36 -0800

On Wed, Feb 28, 2024 at 10:41:32PM +0200, Andy Shevchenko wrote:
> Introduce two helper macros to calculate the size of the structure
> with trailing aligned data and to retrieve the pointer to that data.
> 
> Signed-off-by: Andy Shevchenko <[email-masked]>
> ---
>  include/linux/overflow.h | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index bc390f026128..b93bbf1b6aaa 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -2,9 +2,10 @@
>  #ifndef __LINUX_OVERFLOW_H
>  #define __LINUX_OVERFLOW_H
>  
> +#include <linux/align.h>
>  #include <linux/compiler.h>
> -#include <linux/limits.h>
>  #include <linux/const.h>
> +#include <linux/limits.h>
>  
>  /*
>   * We need to compute the minimum and maximum values representable in a given
> @@ -337,6 +338,30 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
>   */
>  #define array3_size(a, b, c)	size_mul(size_mul(a, b), c)
>  
> +/**
> + * struct_size_with_data() - Calculate size of structure with trailing aligned data.
> + * @p: Pointer to the structure.
> + * @a: Alignment in bytes before trailing data.
> + * @s: Data size in bytes (must not be 0).
> + *
> + * Calculates size of memory needed for structure of @p followed by an
> + * aligned data of size @s.
> + *
> + * Return: number of bytes needed or SIZE_MAX on overflow.
> + */
> +#define struct_size_with_data(p, a, s)	size_add(ALIGN(sizeof(*(p)), (a)), (s))

I don't like this -- "p" should have a trailing flexible array. (See
below.)

> +
> +/**
> + * struct_data_pointer - Calculate offset of the trailing data reserved with
> + * struct_size_with_data().
> + * @p: Pointer to the structure.
> + * @a: Alignment in bytes before trailing data.
> + *
> + * Return: offset in bytes to the trailing data reserved with
> + * struct_size_with_data().
> + */
> +#define struct_data_pointer(p, a)	PTR_ALIGN((void *)((p) + 1), (a))

I'm not super excited about propagating the "p + 1" code pattern to find
things after an allocation. This leads to the compiler either being
blind to accesses beyond an allocation, or being too conservative about
accesses beyond an object. Instead of these helpers I would much prefer
that data structures that use this code pattern be converted to using
trailing flexible arrays, at which point the compiler is in a much
better position to reason about sizes.

-- 
Kees Cook