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

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Message-ID: <20171129232603.GO6217@eros>
Date:   Thu, 30 Nov 2017 10:26:03 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Andrew Morton <akpm@...ux-foundation.org>
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, Nov 29, 2017 at 03:20:58PM -0800, Andrew Morton wrote:
> 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.
> 

Oh, nice idea. It has to be a CHECK but right? By stern, you mean use
stern language?

thanks,
Tobin.