linux-kernel - Re: [PATCH] vsprintf: avoid misleading "(null)" for %px

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Message-ID: <CAGXu5j+KSm3rOK46xiYPx8SRaO9T5LaeaXxOA3PSktJv4F__dg@mail.gmail.com>
Date:   Tue, 6 Feb 2018 07:32:32 +1100
From:   Kees Cook <keescook@...omium.org>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     Petr Mladek <pmladek@...e.com>,
        Adam Borowski <kilobyte@...band.pl>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joe Perches <joe@...ches.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Laight <David.Laight@...lab.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] vsprintf: avoid misleading "(null)" for %px

On Tue, Feb 6, 2018 at 7:15 AM, Tobin C. Harding <me@...in.cc> wrote:
> On Tue, Feb 06, 2018 at 05:57:17AM +1100, Kees Cook wrote:
>> On Mon, Feb 5, 2018 at 8:44 PM, Petr Mladek <pmladek@...e.com> wrote:
>> > Hi,
>> >
>> > I add people who actively commented on adding %px modifier,
>> > see the thread starting at
>> > https://lkml.kernel.org/r/1511921105-3647-5-git-send-email-me@tobin.cc
>> >
>> > Just for reference. It seems to be related to the commit 9f36e2c448007b54
>> > ("printk: use %pK for /proc/kallsyms and /proc/modules").
>> >
>> >
>> > On Sun 2018-02-04 18:45:21, Adam Borowski wrote:
>> >> Like %pK already does, print "00000000" instead.
>> >>
>> >> This confused people -- the convention is that "(null)" means you tried to
>> >> dereference a null pointer as opposed to printing the address.
>> >
>> > By other words, this avoids regressions when people convert
>> > %x to %px. Do I get it right, please?
>>
>> Nothing should be converting from %x to %px, it's %p to %px. %p print
>> "(null)" for 0x0, so it would be surprising for a conversion from %p
>> to %px to change that. (Though generally speaking "(null)" is never
>> useful...)
>
> Leaving aside what is converting to %px.  If we consider that using %px
> is meant to convey to us that we _really_ want the address, in hex hence
> the 'x', then it is not surprising that we will get "00000000"'s for a
> null pointer, right?  Yes it is different to before but since we are
> changing the specifier does this not imply that there may be some
> change?

I personally prefer 0000s, but if we're going to change this, we need
to be aware of the difference.

> In what is now to be expected fashion for %p the discussion appears to
> have split into two different things - what to do with %px and what to
> do with %pK :)

I say leave %pK alone. :)

-Kees

-- 
Kees Cook
Pixel Security