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Date:   Thu, 17 Nov 2022 13:48:14 -0800
From:   Kees Cook <keescook@chromium.org>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org, tytso@mit.edu, kees@kernel.org,
        linux@armlinux.org.uk, ydroneaud@opteya.com,
        gregkh@linuxfoundation.org, rdunlap@infradead.org
Subject: Re: [PATCH v3] random: add helpers for random numbers with given
 floor or range
Message-ID: <202211171347.92FDE65A6D@keescook>
References: <2b3bc31e-c308-b04c-1759-26bcf8dba6d3@infradead.org>
 <20221117192620.2304613-1-Jason@zx2c4.com>
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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@zx2c4.com>

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

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

-Kees

-- 
Kees Cook