On Mon, Oct 20, 2014 at 10:16:35AM -0400, Theodore Ts'o wrote:
> On Mon, Oct 20, 2014 at 04:09:30PM +0200, Daniel Borkmann wrote:
> > >
> > >It's triggering when input_rotate == 0, so UBSan complains about right shift in rol32()
> > >
> > >static inline __u32 rol32(__u32 word, unsigned int shift)
> > >{
> > > return (word << shift) | (word >> (32 - shift));
> > >}
> >
> > So that would be the case when the entropy store's input_rotate calls
> > _mix_pool_bytes() for the very first time ... I don't think it's an
> > issue though.
>
> I'm sure it's not an issue, but it's still true that
>
> return (word << 0) | (word >> 32);
>
> is technically not undefined, and while it would be unfortunate (and
> highly unlikely) if gcc were to say, start nethack, it's technically
> allowed by the C spec. :-)
In fact, n >> 32 == n.
#include <stdio.h>
int main(int argc, char **argv)
{
int i = atoi(argv[1]);
int shift = atoi(argv[2]);
printf("%x\n", i >> shift);
return 0;
}
$ ./shift 5 32
5
On x86 at least the shift ops simply mask out the upper bits and
therefore the 32 == 0.
So you end up OR-ing the same value twice, which is harmless.
So no misbehaviour on the rol32() function.
I think I've ran into this before, in that case I did get fail because I
did indeed expect the 0 and things didn't work out.
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