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Date: Wed, 11 Oct 2017 10:32:09 +1100
From: "Tobin C. Harding" <me@tobin.cc>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "kernel-hardening@lists.openwall.com" 
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Subject: Re: [PATCH 0/3] add %pX specifier
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On Tue, Oct 10, 2017 at 04:15:01PM -0700, Linus Torvalds wrote:
> On Tue, Oct 10, 2017 at 4:09 PM, Tobin C. Harding <me@tobin.cc> wrote:
> >
> > I did not understand the code (specifically why the right shift of 16 twice?)
> 
> It's a traditional trick to get the upper 32 bits.
> 
> So it basically splits the (possibly 64-bit) pointer into the lower 32
> bits and the upper 32 bits for a hash such as "jhash()" that takes
> data that is "unsigned int".
> 
> (NOTE! Using jhash here is not acceptable, since it's not
> cryptographically safe, but think of it as an example of a hash that
> takes 32-bit input).
> 
> Doing ">> 32" is undefined on 32-bit architectures, and wouldn't work.
> 
> But doing >> 16 >> 16 is a fine way to say "shift right by 32 on a
> 64-bit architecture" while also being well-defined on a 32-bit one.
> 
>                Linus

Awesome, thanks.

Tobin.