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

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Date: Thu, 15 Feb 2024 14:47:00 -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 Thu, Feb 15, 2024 at 09:40:40PM +0100, Michal Wajdeczko wrote:
> On 14.02.2024 23:09, Kees Cook wrote:
> > 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?
> 
> note that even FIELD_PREP() is doing silent truncation and these macros
> here could be treated as specialized/simplified variants of FIELD_PREP()
> as alternate implementation can look like:

Also, isn't all of this endian-specific?

-- 
Kees Cook