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Date:   Thu, 17 Nov 2022 03:05:14 +0100
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     linux-kernel@vger.kernel.org, patches@lists.linux.dev,
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Subject: Re: [PATCH v2 3/3] treewide: use get_random_u32_between() when
 possible
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References: <20221114164558.1180362-1-Jason@zx2c4.com>
 <20221114164558.1180362-4-Jason@zx2c4.com>
 <202211161436.A45AD719A@keescook>
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On Thu, Nov 17, 2022 at 12:55:47AM +0100, Jason A. Donenfeld wrote:
> 1) How/whether to make f(0, UR2_MAX) safe,
>    - without additional 64-bit arithmetic,
>    - minimizing the number of branches.
>    I have a few ideas I'll code golf for a bit.
> I think I can make progress with (1) alone by fiddling around with
> godbolt enough, like usual.

The code gen is definitely worse.

Original half-open interval:

    return floor + get_random_u32_below(ceil - floor);

Suggested fully closed interval:
	
    ceil = ceil - floor + 1;
    return likely(ceil) ? floor + get_random_u32_below(ceil) : get_random_u32();

Is the worse code gen actually worth it? Options:

 a) Decide worse codegen is worth it.
 b) Declare f(0, U32_MAX) undefined and just not handle it.
 c) Stick with original half-open interval that doesn't have this problem.

Jason