Re: [PATCH] vsprintf: fix possible NULL pointer deref in vsnprintf()

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Sergey Shtylyov <s.shtylyov@omp.ru>
Cc: Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vsprintf: fix possible NULL pointer deref in vsnprintf()
Date: Mon, 9 Jan 2023 13:23:40 +0200	[thread overview]
Message-ID: <Y7v5PEcP3LAcfTdk@smile.fi.intel.com> (raw)
In-Reply-To: <1f4d159e-5382-3c75-bd5e-42337ecd8c28@omp.ru>

On Fri, Jan 06, 2023 at 12:16:31AM +0300, Sergey Shtylyov wrote:
> In vsnprintf() etc, C99 allows the 'buf' argument to be NULL when the
> 'size' argument equals 0.  Let us treat NULL passed as if the 'buf'
> argument pointed to a 0-sized buffer, so that we can avoid a NULL pointer
> dereference and still return the # of characters that would be written if
> 'buf' pointed to a valid buffer...
> 
> Found by Linux Verification Center (linuxtesting.org) with the SVACE static
> analysis tool.

...

> +	/*
> +	 * C99 allows @buf to be NULL when @size is 0. We treat such NULL as if
> +	 * @buf pointed to 0-sized buffer, so we can both avoid a NULL pointer
> +	 * dereference and still return # of characters that would be written
> +	 * if @buf pointed to a valid buffer...
> +	 */
> +	if (!buf)
> +		size = 0;

Do we have test cases for that?

And what's wrong to print "(null)" ? Have you checked if your patch makes any
regressions to those cases?

-- 
With Best Regards,
Andy Shevchenko