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

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Date:   Mon, 5 Feb 2018 21:58:17 +0100
From:   Adam Borowski <kilobyte@...band.pl>
To:     Kees Cook <keescook@...omium.org>
Cc:     "Tobin C. Harding" <me@...in.cc>, Petr Mladek <pmladek@...e.com>,
        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 06, 2018 at 07:32:32AM +1100, Kees Cook wrote:
> 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:
> >> > 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.
> >
> > 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.

It's easy to paint this bikeshed any color you guys want to: there's an "if"
already.  My preference is also 0000; NULL would be good, too -- I just
don't want (null) as that has a special meaning in usual userspace
implementations; (null) also fits well most other modes of %p as they show
some object the argument points to.  Confusion = wasted debugging time.

This is consistent with what we had before, with %pK special-cased.

> > 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. :)

As in, printing some random (hashed) value?


Let's recap:

Currently:
              not-null              null
%pponies      object's description  (null)
%px           address               (null)
%pK           hash                  hash

I'd propose:
              not-null              null
%pponies      object's description  (null)
%px           address               00000000
%pK           hash                  00000000

The initial patch in this thread changes printk("%px",0) from (null) to
00000000; what Tobin complained about is that printk("%pK",0) prints a
random value.               


Meow!
-- 
⢀⣴⠾⠻⢶⣦⠀ The bill with 3 years prison for mentioning Polish concentration
⣾⠁⢰⠒⠀⣿⡁ camps is back.  What about KL Warschau (operating until 1956)?
⢿⡄⠘⠷⠚⠋⠀ Zgoda?  Łambinowice?  Most ex-German KLs?  If those were "soviet
⠈⠳⣄⠀⠀⠀⠀ puppets", Bereza Kartuska?  Sikorski's camps in UK (thanks Brits!)?