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Date: Wed, 29 Nov 2017 15:20:58 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Tobin C. Harding" <me@...in.cc>
Cc: kernel-hardening@...ts.openwall.com, Linus Torvalds
 <torvalds@...ux-foundation.org>, "Jason A. Donenfeld" <Jason@...c4.com>,
 "Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>, 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
 Krčmář <rkrcmar@...hat.com>,
 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>
Subject: Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

On Wed, 29 Nov 2017 13:05:04 +1100 "Tobin C. Harding" <me@...in.cc> wrote:

> printk specifier %p now hashes all addresses before printing. Sometimes
> we need to see the actual unmodified address. This can be achieved using
> %lx but then we face the risk that if in future we want to change the
> way the Kernel handles printing of pointers we will have to grep through
> the already existent 50 000 %lx call sites. 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.
> 
> Add printk specifier %px to print the actual unmodified address.
> 
> ...
>
> +Unmodified Addresses
> +====================
> +
> +::
> +
> +	%px	01234567 or 0123456789abcdef
> +
> +For printing pointers when you _really_ want to print the address. Please
> +consider whether or not you are leaking sensitive information about the
> +Kernel layout in memory before printing pointers with %px. %px is
> +functionally equivalent to %lx. %px is preferred to %lx because it is more
> +uniquely grep'able. If, in the future, we need to modify the way the Kernel
> +handles printing pointers it will be nice to be able to find the call
> +sites.
> +

You might want to add a checkpatch rule which emits a stern
do-you-really-want-to-do-this warning when someone uses %px.

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