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

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Date:   Mon, 9 Jan 2023 13:23:40 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Sergey Shtylyov <s.shtylyov@....ru>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vsprintf: fix possible NULL pointer deref in vsnprintf()

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