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Date: Wed, 29 Nov 2017 15:20:58 -0800
From: Andrew Morton <>
To: "Tobin C. Harding" <>
Cc:, Linus Torvalds
 <>, "Jason A. Donenfeld" <>,
 "Theodore Ts'o" <>, Kees Cook <>, Paolo
 Bonzini <>, Tycho Andersen <>,
 "Roberts, William C" <>, Tejun Heo
 <>, Jordan Glover <>, Greg KH
 <>, Petr Mladek <>, Joe Perches
 <>, Ian Campbell <>, Sergey Senozhatsky
 <>, Catalin Marinas <>,
 Will Deacon <>, Steven Rostedt <>,
 Chris Fries <>, Dave Weinstein <>, Daniel
 Micay <>, Djalal Harouni <>, Radim
 Krčmář <>,, Network Development <>,
 David Miller <>, Stephen Rothwell
 <>, Andrey Ryabinin <>,
 Alexander Potapenko <>, Dmitry Vyukov <>
Subject: Re: [PATCH V11 4/5] vsprintf: add printk specifier %px

On Wed, 29 Nov 2017 13:05:04 +1100 "Tobin C. Harding" <> 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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