Re: [PATCH v4 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

From: Sakari Ailus <sakari.ailus@linux.intel.com>
To: Joe Perches <joe@perches.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
	Petr Mladek <pmladek@suse.com>,
	Dave Stevenson <dave.stevenson@raspberrypi.com>,
	dri-devel@lists.freedesktop.org, hverkuil@xs4all.nl,
	laurent.pinchart@ideasonboard.com, mchehab@kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH v4 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs
Date: Fri, 13 Nov 2020 12:54:18 +0200	[thread overview]
Message-ID: <20201113105418.GB15897@paasikivi.fi.intel.com> (raw)
In-Reply-To: <b389f6991ede1f8ae89a0dbaa8deab06aecc6146.camel@perches.com>

Hi Joe,

On Tue, Nov 03, 2020 at 08:49:36AM -0800, Joe Perches wrote:
> On Tue, 2020-11-03 at 16:56 +0200, Sakari Ailus wrote:
> > On Tue, Nov 03, 2020 at 04:47:47PM +0200, Andy Shevchenko wrote:
> > > On Tue, Nov 03, 2020 at 03:34:00PM +0200, Sakari Ailus wrote:
> > > > Add a printk modifier %p4cc (for pixel format) for printing V4L2 and DRM
> > > > pixel formats denoted by fourccs. The fourcc encoding is the same for both
> > > > so the same implementation can be used.
> > > 
> > > ...
> > > 
> > > > +static noinline_for_stack
> > > > +char *fourcc_string(char *buf, char *end, const u32 *fourcc,
> > > > +		    struct printf_spec spec, const char *fmt)
> > > > +{
> > > > +	char output[sizeof("(xx)(xx)(xx)(xx) little-endian (0x01234567)")];
> > > 
> > > I would add a comment that there is another possibility, i.e. big-endian, but
> > > it occupies less space.
> 
> I think it's unnecessary as it's obvious and similarly done in other
> <foo>_string type functions.
> 
> > > > +	p = special_hex_number(p, output + sizeof(output) - 2, *fourcc,
> > > > +			       sizeof(u32));
> > > 
> > > I would go with one line here.
> > 
> > It's wrapped since the result would be over 80 otherwise.
> 
> Perhaps simpler as
> 
> 	p = special_hex_number(p, p + 10, *fourcc, sizeof(u32));

Yes. But having bugs elsewhere would have a magnified effect. I wouldn't be
afraid of a newline here.

> 
> > > The (theoretical) problem is here that the case when buffer size is not enough
> > > to print a value will be like '(0xabc)' but should be rather '(0xabcd' like
> > > snprintf() does in general.
> 
> Isn't the stack buffer known to be large enough?

Yes. If there are no bugs, that is.

> 
> > > > +	*p++ = ')';
> > > > +	*p = '\0';
> > > > +
> > > > +	return string(buf, end, output, spec);
> 
> Isn't the actual output buffer used here truncating output?
> 
> If the general problem is someone using a limited length pointer
> output like %10p4cc, then all the output is getting truncated no?

Correct. But I'd also be surprised if anyone ever used this feature.

-- 
Kind regards,

Sakari Ailus