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Date: Sun, 21 Sep 2014 19:56:13 +0400 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] fix a bug in the rand48 family of prng
On Sun, 21 Sep 2014, Szabolcs Nagy wrote:
> * Jens Gustedt <Jens.Gustedt@...ia.fr> [2014-09-21 16:39:34 +0200]:
> >
> > This fixes a bug found by Nadav Har'El, who observed that musl was giving
> > different prn sequences than other systems, even if seeded with the same
> > value.
> >
> > The problem with something like
> >
> > a = lc[0] | lc[1]<<16 | lc[2]+0ULL<<32;
> >
> > where lc[1] is an unsigned short and int is 32bit is the following
> >
> > (1) lc[1] is promoted to int
> > (2) the left shift 16 is performed on int
> >
>
> the fix looks ok, but i'm not clear on why it breaks in practice
UB on shift shouldn't be a problem here; I think the issue is that
"lc[0] | lc[1]<<16" gets sign-extended rather than zero-extended from 32 bit
to 64 bit.
Alexander
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