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

From: Jeffrey Walton
Date: Wed May 04 2016 - 14:22:33 EST

On Wed, May 4, 2016 at 1:49 PM, <tytso@xxxxxxx> wrote:
> 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

bitops.h could work in this case, but its not an ideal solution. GCC
does not optimize the code below as expected under all use cases
because GCC does not recognize it as a rotate (see

return (v << n) | (v >> (sizeof(v) * 8 - n));

And outside of special cases like Salsa, ChaCha and BLAKE2, the code
provided in bitops.h suffers UB on arbitrary data. So I think care
needs to be taken when selecting functions from bitops.h.

> 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).

Yeah, I kind of thought there was some duplication going on.

But I think bitops.h should be fixed. Many folks don't realize the
lurking UB, and many folks don't realize its not always optimized

>> 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.

My bad... It was a general observation. I've seen folks try to correct
the UB by turning to something like that.