linux-kernel - Re: [PATCH 0/3] add %pX specifier

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Date:   Tue, 10 Oct 2017 20:27:14 -0700
From:   Joe Perches <joe@...ches.com>
To:     "Tobin C. Harding" <me@...in.cc>,
        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>,
        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 Wed, 2017-10-11 at 10:32 +1100, Tobin C. Harding wrote:
> 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.

Another way is using the upper_32_bits() macro.
It's perhaps a bit more readable.