Re: [PATCH 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

From: Joe Perches <joe@perches.com>
To: Jani Nikula <jani.nikula@linux.intel.com>,
	Sakari Ailus <sakari.ailus@linux.intel.com>,
	Petr Mladek <pmladek@suse.com>
Cc: mchehab@kernel.org,
	"Dave Stevenson" <dave.stevenson@raspberrypi.com>,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	hverkuil@xs4all.nl,
	"Sergey Senozhatsky" <sergey.senozhatsky@gmail.com>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	laurent.pinchart@ideasonboard.com,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	linux-media@vger.kernel.org,
	"Ville Syrjälä" <ville.syrjala@linux.intel.com>
Subject: Re: [PATCH 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs
Date: Fri, 03 Apr 2020 06:11:22 -0700	[thread overview]
Message-ID: <a5d0029edb7328691f1a08009d564c576924a3eb.camel@perches.com> (raw)
In-Reply-To: <87zhbtkrc5.fsf@intel.com>

On Fri, 2020-04-03 at 09:37 +0300, Jani Nikula wrote:
> On Thu, 02 Apr 2020, Joe Perches <joe@perches.com> wrote:
> > On Thu, 2020-04-02 at 11:34 +0300, Jani Nikula wrote:
> > > Or could we conceive of a way to make this locally extensible yet safe,
> > > letting callers use something like %{foo}, as well as providing a
> > > locally relevant function to do the conversion?
> > 
> > No.  printf validation would be broken.
> 
> I tossed the idea on a whim, and thinking further I could probably come
> up with a number of challenges, but care to elaborate on what you see as
> the problem in validation?

I understand you to want to add something like

	%<m> where m is a non-standard format specifier

so using using gcc's extension of

__attribute__((__format__(printf, string_index, first_to_check))

could not validate the argument type against use of the %<m>
in the format string.

	printk("%a\n", a);

Compiler bleats.