Re: [PATCH v2] minmax: clamp more efficiently by avoiding extra comparison

[Kees Cook]

On Fri, Sep 23, 2022 at 05:40:01PM +0200, Jason A. Donenfeld wrote:
> Currently the clamp algorithm does:
> 
> 	if (val > hi)
> 		val = hi;
> 	if (val < lo)
> 		val = lo;
> 
> But since hi > lo by definition, this can be made more efficient with:
> 
> 	if (val > hi)
> 		val = hi;
> 	else if (val < lo)
> 		val = lo;
> 
> So fix up the clamp and clamp_t functions to do this, adding the same
> argument checking as for min and min_t.
> 
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> ---
>  include/linux/minmax.h | 25 +++++++++++++++++++++++--
>  1 file changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/minmax.h b/include/linux/minmax.h
> index 5433c08fcc68..30e2e2cd0f44 100644
> --- a/include/linux/minmax.h
> +++ b/include/linux/minmax.h
> @@ -37,6 +37,27 @@
>  		__cmp(x, y, op), \
>  		__cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))
>  
> +#define __clamp(val, lo, hi)							\
> +	((val) >= (hi) ? (hi) : ((val) <= (lo) ? (lo) : (val)))
> +
> +#define __clamp_once(val, lo, hi, unique_val, unique_lo, unique_hi) ({		\
> +		typeof(val) unique_val = (val);					\
> +		typeof(lo) unique_lo = (lo);					\
> +		typeof(hi) unique_hi = (hi);					\
> +		__clamp(unique_val, unique_lo, unique_hi); })
> +
> +#define __clamp_input_check(lo, hi)						\
> +        (BUILD_BUG_ON_ZERO(__builtin_choose_expr(				\
> +                __is_constexpr((lo) > (hi)), (lo) > (hi), false)))

Nice. :)

> +
> +#define __careful_clamp(val, lo, hi) ({						\
> +	__clamp_input_check(lo, hi) + 						\
> +	__builtin_choose_expr(__typecheck(val, lo) && __typecheck(val, hi) &&	\
> +			      __typecheck(hi, lo) && __is_constexpr(val) && 	\
> +			      __is_constexpr(lo) && __is_constexpr(hi),		\

I really like it! I might have used:

	__safe_cmp(val, lo) && __safe_cmp(val, hi)

instead of the "open coded" __typecheck()s and __is_constexpr()s, but
it's the same result.

> +		__clamp(val, lo, hi),						\
> +		__clamp_once(val, lo, hi, __UNIQUE_ID(__val), __UNIQUE_ID(__lo), __UNIQUE_ID(__hi))); })

*complaint about line being >100 characters, but I don't really care* If
anyone is really bothered, this looks fine, too:

		__clamp_once(val, lo, hi,					\
			     __UNIQUE_ID(__val), __UNIQUE_ID(__lo), __UNIQUE_ID(__hi))); })

*shrug*

> +
>  /**
>   * min - return minimum of two values of the same or compatible types
>   * @x: first value
> @@ -86,7 +107,7 @@
>   * This macro does strict typechecking of @lo/@hi to make sure they are of the
>   * same type as @val.  See the unnecessary pointer comparisons.
>   */
> -#define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi)
> +#define clamp(val, lo, hi) __careful_clamp(val, lo, hi)
>  
>  /*
>   * ..and if you can't take the strict
> @@ -121,7 +142,7 @@
>   * This macro does no typechecking and uses temporary variables of type
>   * @type to make all the comparisons.
>   */
> -#define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)
> +#define clamp_t(type, val, lo, hi) __careful_clamp((type)(val), (type)(lo), (type)(hi))
>  
>  /**
>   * clamp_val - return a value clamped to a given range using val's type
> -- 
> 2.37.3
> 

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

I can take this unless akpm wants it?

-- 
Kees Cook