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Date: Sun, 21 Sep 2014 18:05:59 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] fix a bug in the rand48 family of prng
Am Sonntag, den 21.09.2014, 17:34 +0200 schrieb Szabolcs Nagy:
> * 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
I was curious, too. The assembler looks a bit different, so it is not
so easy to tell. But it looks to me that the problem comes from a cltq
instruction that is inserted in the faulty code. It probably sets the
high bits of %rax to 1 before the or-ing of the different parts takes
place.
I think the cltq instruction comes handy to zero out the the high
order bits of the %rax register in the defined case.
> (i know it's ub, but gcc used to handle such shifts "as expected"
> the linux kernel is full of them and c++14 allows this and there
> is a dr to change the semantics for c too
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_463.htm
> )
right, so this here can serve as an example why this ain't so easy to
define that behavior.
Jens
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