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

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Date:   Mon, 5 Feb 2018 16:22:19 +0100
From:   Adam Borowski <kilobyte@...band.pl>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        "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 09:03:05PM +1100, Tobin C. Harding wrote:
> On Mon, Feb 05, 2018 at 10:44:38AM +0100, Petr Mladek 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.
> > 
> > By other words, this avoids regressions when people convert
> > %x to %px. Do I get it right, please?

It's a regression in the sense that it confuses people.  %px never could
dereference a pointer so the information provided doesn't change, merely its
presentation.

> > > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > > index 77ee6ced11b1..d7a708f82559 100644
> > > --- a/lib/vsprintf.c
> > > +++ b/lib/vsprintf.c
> > > @@ -1849,7 +1849,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
> > >  {
> > >  	const int default_width = 2 * sizeof(void *);
> > >  
> > > -	if (!ptr && *fmt != 'K') {
> > > +	if (!ptr && *fmt != 'K' && *fmt != 'x') {
> 
> I don't know if it matters but with this it won't be immediately
> apparent that a null pointer was printed (since zero could hash to
> anything).

My change touches %px only, where your concern doesn't apply.

You're right, though, when it comes to %pK:
    printk("%%pK: %pK, %%px: %px\n", 0, 0);
says
    %pK: 00000000ba8bdc0a, %px: 0000000000000000

So what should we do?  Avoid hashing 0?  Print a special 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!)?