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

Andy Shevchenko
2022-09-26T15:11:49Z

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@linux.intel.com>
Thanks!

> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.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