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

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Date:   Tue, 6 Feb 2018 18:14:03 +0000
From:   "Roberts, William C" <william.c.roberts@...el.com>
To:     "Tobin C. Harding" <me@...in.cc>,
        Adam Borowski <kilobyte@...band.pl>
CC:     Kees Cook <keescook@...omium.org>, 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>,
        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



> -----Original Message-----
> From: Tobin C. Harding [mailto:me@...in.cc]
> Sent: Monday, February 5, 2018 2:23 PM
> To: Adam Borowski <kilobyte@...band.pl>
> Cc: Kees Cook <keescook@...omium.org>; 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 Mon, Feb 05, 2018 at 09:58:17PM +0100, Adam Borowski wrote:
> > 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.
> 
> Epic fail on my behalf, my first comment was _wrong_ and brought %pK into the
> discussion - bad Tobin, please crawl back under your rock.
> 
> The original patch is good IMO and I AFAICT in everyone else's.
Nod.
> 
> 	Tobin