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Date: Wed, 11 Oct 2017 10:32:09 +1100
From: "Tobin C. Harding" <me@...in.cc>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
KVM list <kvm@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Kees Cook <keescook@...omium.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Tycho Andersen <tycho@...ker.com>,
"Roberts, William C" <william.c.roberts@...el.com>,
Tejun Heo <tj@...nel.org>,
Jordan Glover <Golden_Miller83@...tonmail.ch>,
Greg KH <gregkh@...uxfoundation.org>,
Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
Ian Campbell <ijc@...lion.org.uk>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com>,
Daniel Micay <danielmicay@...il.com>,
Djalal Harouni <tixxdz@...il.com>
Subject: Re: [PATCH 0/3] add %pX specifier
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@...in.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.
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