linux-kernel - Re: [PATCH] lib/int_sqrt.c: optimize for small argument values

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Date:   Thu, 19 Oct 2017 13:42:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Michael Davidson <md@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        David Miller <davem@...emloft.net>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH] lib/int_sqrt.c: optimize for small argument values

On Thu, Oct 19, 2017 at 1:31 PM, Michael Davidson <md@...gle.com> wrote:
> int_sqrt() currently takes approximately constant time
> regardless of the value of the argument. By using the
> magnitude of the operand to set the initial conditions
> for the calculation the cost becomes proportional to
> log2 of the argument with the worst case behavior not
> being measurably slower than it currently is.

Maybe a stupid question, but is this function ultimately used by any
crypto that expects it to be constant-time for safety?

Also, is there a specific workload that is meaningfully sped up by this change?

-Kees

>
> Signed-off-by: Michael Davidson <md@...gle.com>
> ---
>  lib/int_sqrt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
> index 1ef4cc344977..8394b0dcecd4 100644
> --- a/lib/int_sqrt.c
> +++ b/lib/int_sqrt.c
> @@ -21,7 +21,7 @@ unsigned long int_sqrt(unsigned long x)
>         if (x <= 1)
>                 return x;
>
> -       m = 1UL << (BITS_PER_LONG - 2);
> +       m = 1UL << (__fls(x) & ~1UL);
>         while (m != 0) {
>                 b = y + m;
>                 y >>= 1;
> --
> 2.15.0.rc1.287.g2b38de12cc-goog
>



-- 
Kees Cook
Pixel Security