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

[Andy Shevchenko]

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