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

[Sergey Shtylyov <s.shtylyov@omp.ru>]

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.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>

---
This patch is against the 'master' branch of the PRINTK Group's repo...

 lib/vsprintf.c |    9 +++++++++
 1 file changed, 9 insertions(+)

Index: linux/lib/vsprintf.c
===================================================================
--- linux.orig/lib/vsprintf.c
+++ linux/lib/vsprintf.c
@@ -2738,6 +2738,15 @@ int vsnprintf(char *buf, size_t size, co
 	if (WARN_ON_ONCE(size > INT_MAX))
 		return 0;
 
+	/*
+	 * 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;
+
 	str = buf;
 	end = buf + size;