linux-kernel - Re: [PATCH 1/3] random: replace non-blocking pool with a Chacha20-based CRNG

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Date:	Wed, 4 May 2016 10:40:20 -0400
From:	Jeffrey Walton <noloader@...il.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-kernel@...r.kernel.org,
	Stephan Mueller <smueller@...onox.de>,
	Herbert Xu <herbert@...dor.apana.org.au>, andi@...stfloor.org,
	Sandy Harris <sandyinchina@...il.com>,
	cryptography@...edaemon.net, jsd@...n.com, hpa@...or.com,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH 1/3] random: replace non-blocking pool with a
 Chacha20-based CRNG

> +static inline u32 rotl32(u32 v, u8 n)
> +{
> +       return (v << n) | (v >> (sizeof(v) * 8 - n));
> +}

That's undefined behavior when n=0.

I think the portable way to do a rotate that avoids UB is the
following. GCC, Clang and ICC recognize the pattern, and emit a rotate
instruction.

    static const unsigned int MASK=31;
    return (v<<n)|(v>>(-n&MASK));

You should also avoid the following because its not constant time due
to the branch:

    return n == 0 ? v : (v << n) | (v >> (sizeof(v) * 8 - n));

Jeff