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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