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

From: "Tobin C. Harding" <me@tobin.cc>
To: Petr Mladek <pmladek@suse.com>
Cc: Adam Borowski <kilobyte@angband.pl>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Joe Perches <joe@perches.com>, Kees Cook <keescook@chromium.org>,
	"Roberts, William C" <william.c.roberts@intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	David Laight <David.Laight@ACULAB.COM>,
	Randy Dunlap <rdunlap@infradead.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>
Subject: Re: [PATCH] vsprintf: avoid misleading "(null)" for %px
Date: Mon, 5 Feb 2018 21:03:05 +1100	[thread overview]
Message-ID: <20180205100305.GO29988@eros> (raw)
In-Reply-To: <20180205094438.pfd7ffymlvklpxe7@pathway.suse.cz>

On Mon, Feb 05, 2018 at 10:44:38AM +0100, Petr Mladek 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?
>
> > Signed-off-by: Adam Borowski <kilobyte@angband.pl>
> > ---
> >  lib/vsprintf.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > 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).

thanks,
Tobin.