linux-kernel - Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

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Message-ID: <596c917e-3183-d80a-89d4-97e0eb5dc077@infradead.org>
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 Krcmár <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