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

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Message-ID: <CAGXu5j+rWhT7_DH5Z96_7naPou96-6736x4mjq0r_8EG4iiQ_g@mail.gmail.com>
Date:   Tue, 6 Feb 2018 05:57:17 +1100
From:   Kees Cook <keescook@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Adam Borowski <kilobyte@...band.pl>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Tobin C. Harding" <me@...in.cc>,
        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 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...)

-Kees

-- 
Kees Cook
Pixel Security