linux-kernel - Re: [PATCH v3] random: add helpers for random numbers with given floor or range

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Date:   Thu, 17 Nov 2022 13:48:14 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, tytso@....edu, kees@...nel.org,
        linux@...linux.org.uk, ydroneaud@...eya.com,
        gregkh@...uxfoundation.org, rdunlap@...radead.org
Subject: Re: [PATCH v3] random: add helpers for random numbers with given
 floor or range

On Thu, Nov 17, 2022 at 08:26:20PM +0100, Jason A. Donenfeld wrote:
> Now that we have get_random_u32_below(), it's nearly trivial to make
> inline helpers to compute get_random_u32_above() and
> get_random_u32_inclusive(), which will help clean up open coded loops
> and manual computations throughout the tree.
> 
> One snag is that in order to make get_random_u32_inclusive() operate on
> closed intervals, we have to do some (unlikely) special case handling if
> get_random_u32_interval(0, U32_MAX) is called. The least expensive way
> of doing this is actually to adjust the slowpath of
> get_random_u32_below() to have its undefined 0 result just return the
> output of get_random_u32(). We can make this basically free by calling
> get_random_u32() before the branch, so that the branch latency gets
> interleaved.
> 
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>

I really like these -- unambiguous! :) Thanks for adjusting this API.

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

-- 
Kees Cook