Re: [PATCH v2] overflow.h: Add flex_array_size() helper

[Kees Cook]

On Mon, Jun 08, 2020 at 08:22:33PM -0500, Gustavo A. R. Silva wrote:
> Add flex_array_size() helper for the calculation of the size, in bytes,
> of a flexible array member contained within an enclosing structure.
> 
> Example of usage:
> 
> struct something {
> 	size_t count;
> 	struct foo items[];
> };
> 
> struct something *instance;
> 
> instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
> instance->count = count;
> 
> memcpy(instance->items, source, flex_array_size(instance, items, instance->count));
> 
> The helper returns SIZE_MAX on overflow instead of wrapping around.
> 
> (Additionally replace parameter n with count in struct_size() for
> unification).
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
> ---
> Changes in v2:
>  - Add further information to the helper documentation.
>  - Use same code style as struct_size() for consistency.
> 
>  include/linux/overflow.h | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 659045046468f..d2329a914304c 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -304,16 +304,33 @@ static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c)
>   * struct_size() - Calculate size of structure with trailing array.
>   * @p: Pointer to the structure.
>   * @member: Name of the array member.
> - * @n: Number of elements in the array.
> + * @count: Number of elements in the array.
>   *
>   * Calculates size of memory needed for structure @p followed by an
> - * array of @n @member elements.
> + * array of @count @member elements.
>   *
>   * Return: number of bytes needed or SIZE_MAX on overflow.
>   */
> -#define struct_size(p, member, n)					\
> -	__ab_c_size(n,							\
> +#define struct_size(p, member, count)					\
> +	__ab_c_size(count,							\
>  		    sizeof(*(p)->member) + __must_be_array((p)->member),\
>  		    sizeof(*(p)))

I like the "count" change -- "n" can be seen as either count or bytes,
so I like this being distinctly "count".

>  
> +/**
> + * flex_array_size() - Calculate size, in bytes, of a flexible array member
> + * within an enclosing structure. Read on for more details.
> + *
> + * @p: Pointer to the structure.
> + * @member: Name of the flexible array member.
> + * @count: Number of elements in the array.
> + *
> + * Calculates size, in bytes, of a flexible array @member of @count elements
> + * within structure @p.
> + *
> + * Return: number of bytes needed or SIZE_MAX on overflow.
> + */
> +#define flex_array_size(p, member, count)					\
> +	array_size(count,							\
> +		    sizeof(*(p)->member) + __must_be_array((p)->member))
> +
>  #endif /* __LINUX_OVERFLOW_H */

I like it! You mentioned off-list that maybe this could be named
sizeof_flex_array() (like sizeof_field(), etc), and that does seem
attractive. As you also mentioned, it begs the question of renaming
struct_size() to sizeof_struct().

Looking back through the thread[1], it seems the name came from Linus[2],
and was more related to the existing array_size() helper.

So, how about this, as a convention we can use to make a choice:

For things that are strictly constant in size, we can use sizeof_*. For
things that have a dynamic component, we'll use *_size(). So, this patch
is correct as-is.

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

(I wonder who's tree this should go via?)

-Kees

[1] https://lore.kernel.org/lkml/20180507113902.GC18116@xxxxxxxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/lkml/CA+55aFy8DSRoUvtiuu5w+XGOK6tYvtJGBH-i8i-y7aiUD2EGLA@xxxxxxxxxxxxxx/

-- 
Kees Cook