lkml.org 
[lkml]   [2016]   [May]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH 1/3] random: replace non-blocking pool with a Chacha20-based CRNG
From
> +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

\
 
 \ /
  Last update: 2016-05-04 17:01    [W:0.632 / U:0.444 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site