linux-kernel - Re: [PATCH v3 1/2] minmax: sanity check constant bounds when clamping

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Message-ID: <YzGtQnlG560rXQv9@smile.fi.intel.com>
Date:   Mon, 26 Sep 2022 16:46:42 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 1/2] minmax: sanity check constant bounds when clamping

On Mon, Sep 26, 2022 at 03:34:34PM +0200, Jason A. Donenfeld wrote:
> The clamp family of functions only makes sense if hi>=lo. If hi and lo
> are compile-time constants, then raise a build error. Doing so has
> already caught buggy code. This also introduces the infrastructure to
> improve the clamping function in subsequent commits.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Thanks!

> Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  include/linux/minmax.h | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/minmax.h b/include/linux/minmax.h
> index 5433c08fcc68..293a66ad7830 100644
> --- a/include/linux/minmax.h
> +++ b/include/linux/minmax.h
> @@ -37,6 +37,28 @@
>  		__cmp(x, y, op), \
>  		__cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))
>  
> +#define __clamp(val, lo, hi)							\
> +	__cmp(__cmp(val, lo, >), hi, <)
> +
> +#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)))
> +
> +#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),		\
> +		__clamp(val, lo, hi),						\
> +		__clamp_once(val, lo, hi, __UNIQUE_ID(__val),			\
> +			     __UNIQUE_ID(__lo), __UNIQUE_ID(__hi))); })
> +
>  /**
>   * min - return minimum of two values of the same or compatible types
>   * @x: first value
> @@ -86,7 +108,7 @@
>   * 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 +143,7 @@
>   * 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
> 

-- 
With Best Regards,
Andy Shevchenko