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Date: Tue, 5 Dec 2017 13:08:41 -0800
From: Randy Dunlap <rdunlap@...radead.org>
To: David Laight <David.Laight@...LAB.COM>,
 'Kees Cook' <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 "Tobin C. Harding" <me@...in.cc>,
 "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
 "Jason A. Donenfeld" <Jason@...c4.com>, Theodore Ts'o <tytso@....edu>,
 Paolo Bonzini <pbonzini@...hat.com>, Tycho Andersen <tycho@...ho.ws>,
 "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
 <wilal.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>,
 Radim Krcm√°r <rkrcmar@...hat.com>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 Network Development <netdev@...r.kernel.org>,
 David Miller <davem@...emloft.net>, Stephen Rothwell <sfr@...b.auug.org.au>,
 Andrey Ryabinin <aryabinin@...tuozzo.com>,
 Alexander Potapenko <glider@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

On 11/30/2017 02:38 AM, David Laight wrote:
> From: Kees Cook
>> Sent: 29 November 2017 22:28
>> On Wed, Nov 29, 2017 at 2:07 AM, David Laight <David.Laight@...lab.com> wrote:
>>> From: Linus Torvalds
>>>> Sent: 29 November 2017 02:29
>>>>
>>>> On Tue, Nov 28, 2017 at 6:05 PM, Tobin C. Harding <me@...in.cc> wrote:
>>>>>
>>>>>    Let's add specifier %px as a
>>>>> clear, opt-in, way to print a pointer and maintain some level of
>>>>> isolation from all the other hex integer output within the Kernel.
>>>>
>>>> Yes, I like this model. It's easy and it's obvious ("'x' for hex"),
>>>> and it gives people a good way to say "yes, I really want the actual
>>>> address as hex" for if/when the hashed pointer doesn't work for some
>>>> reason.
>>>
>>> Remind me to change every %p to %px on kernels that support it.
>>>
>>> Although the absolute values of pointers may not be useful, knowing
>>> that two pointer differ by a small amount is useful.
>>> It is also useful to know whether pointers are to stack, code, static
>>> data or heap.
>>>
>>> This change to %p is going to make debugging a nightmare.
>>
>> In the future, maybe we could have a knob: unhashed, hashed (default),
>> or zeroed.
> 
> Add a 4th, hashed_page+offset.
> 
> Isn't there already a knob for %pK, bits in the same value could be used.
> That would make it easy to ensure that %pK is more restructive than %p.

(yeah, I'm kind of behind on this thread.)

This kind of option (with default hashed) is what I was just thinking of
after having seen a few unhelpful traces.  But then the knob might not be
changed in time for the traces either. :(


-- 
~Randy

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