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

From: tytso
Date: Wed May 04 2016 - 13:49:22 EST

On Wed, May 04, 2016 at 10:40:20AM -0400, Jeffrey Walton wrote:
> > +static inline u32 rotl32(u32 v, u8 n)
> > +{
> > + return (v << n) | (v >> (sizeof(v) * 8 - n));
> > +}
> That's undefined behavior when n=0.

Sure, but it's never called with n = 0; I've double checked and the
compiler seems to do the right thing with the above pattern as well.

Hmm, it looks like there is a "standard" version rotate left and right
defined in include/linux/bitops.h. So I suspect it would make sense
to use rol32 as defined in bitops.h --- and this is probably something
that we should do for the rest of crypto/*.c, where people seem to be
defininig their own version of something like rotl32 (I copied the
contents of crypto/chacha20_generic.c to lib/chacha20, so this pattern
of defining one's own version of rol32 isn't new).

> 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));

Where is this coming from? I don't see this construct in the patch.

- Ted