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Date:   Wed, 29 Nov 2017 15:20:58 -0800
From:   Andrew Morton <>
To:     "Tobin C. Harding" <>
        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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