linux-kernel - Re: [PATCH 1/2] wordpart.h: Helpers for making u16/u32/u64 values

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Date: Wed, 14 Feb 2024 14:09:34 -0800
From: Kees Cook <keescook@...omium.org>
To: Michal Wajdeczko <michal.wajdeczko@...el.com>
Cc: linux-kernel@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Jani Nikula <jani.nikula@...el.com>
Subject: Re: [PATCH 1/2] wordpart.h: Helpers for making u16/u32/u64 values

On Wed, Feb 14, 2024 at 10:46:53PM +0100, Michal Wajdeczko wrote:
> It is quite common practice to make u16, u32 or u64 values from
> smaller words.  Add simple helpers for that.
> 
> Signed-off-by: Michal Wajdeczko <michal.wajdeczko@...el.com>
> ---
> v2: new macro names due to conflict with crypto/aria.h
>     explicit cast and truncation everywhere (Alexey)
>     moved to wordpart.h (Andy)
> ---
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Cc: Alexey Dobriyan <adobriyan@...il.com>
> Cc: Jani Nikula <jani.nikula@...el.com>
> ---
>  include/linux/wordpart.h | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/include/linux/wordpart.h b/include/linux/wordpart.h
> index f6f8f83b15b0..8c75a5355112 100644
> --- a/include/linux/wordpart.h
> +++ b/include/linux/wordpart.h
> @@ -31,6 +31,38 @@
>   */
>  #define lower_16_bits(n) ((u16)((n) & 0xffff))
>  
> +/**
> + * make_u16_from_u8 - make u16 value from two u8 values
> + * @hi: value representing upper 8 bits
> + * @lo: value representing lower 8 bits
> + */
> +#define make_u16_from_u8(hi, lo) ((u16)((u16)(u8)(hi) << 8 | (u8)(lo)))

Do we want to actually do type validation here? Right now it's just
cast/truncating, which based on the version log is by design. Is silent
truncation the right thing to do?

-- 
Kees Cook