From: Rich Felker <dalias@aerifal.cx>
To: musl@lists.openwall.com
Subject: Re: FreeSec crypt()
Date: Wed, 13 Jun 2012 13:36:31 -0400 [thread overview]
Message-ID: <20120613173631.GG163@brightrain.aerifal.cx> (raw)
In-Reply-To: <20120613173248.GK17860@port70.net>
On Wed, Jun 13, 2012 at 07:32:48PM +0200, Szabolcs Nagy wrote:
> * Solar Designer <solar@openwall.com> [2012-06-13 20:45:46 +0400]:
> > On Wed, Jun 13, 2012 at 10:56:03AM -0400, Rich Felker wrote:
> > > Well if char is signed, (char)0x80 << 1 is -256. If char is unsigned,
> > > (char)0x80 << 1 is 256.
> >
> > Sure, but we had:
> >
> > const char *key;
> > u_char *q;
> > *q++ = *key << 1;
> >
> > so while *key << 1 is either -256 or 256 (promoted to int or unsigned
> > int), those high bits get dropped on the assignment to *q anyway,
> > resulting in the same value there either way. No?
>
> yes the code happens to work whenever -128<<1 is -256
>
> and i assume -256 is what most compilers will give
> usually in case of two's complement int representation
>
> but -128<<1 is UB and should be fixed anyway
Note that x<<1 is always equal to x*2 when both are defined, and the
latter is defined in our case since the range of x is much smaller
than half the range of int. So if the underlying pointer type issue
isn't fixed, just changing <<1 to *2 would work.
In general, portable code wanting to use x<<n with negative values of
x could replace it with x*(1<<n) and hope the compiler is smart enough
to generate code equivalent to the traditional behavior of x<<n (i.e.
hope it can apply the associativity to * and <<).
Rich
next prev parent reply other threads:[~2012-06-13 17:36 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-12 23:51 Solar Designer
2012-06-13 1:18 ` Rich Felker
2012-06-13 6:10 ` Szabolcs Nagy
2012-06-13 12:43 ` Solar Designer
2012-06-13 12:58 ` Rich Felker
2012-06-13 13:18 ` Solar Designer
2012-06-13 14:56 ` Rich Felker
2012-06-13 16:45 ` Solar Designer
2012-06-13 17:27 ` Rich Felker
2012-06-13 17:32 ` Szabolcs Nagy
2012-06-13 17:36 ` Rich Felker [this message]
2012-06-13 12:07 ` Solar Designer
2012-06-13 14:53 ` Rich Felker
2012-06-24 7:21 ` Solar Designer
2012-06-24 7:32 ` Solar Designer
2012-06-25 3:51 ` Rich Felker
2012-06-29 5:25 ` Rich Felker
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