[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <1507692434.3552.41.camel@perches.com>
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.
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
