Check for allocation overflow in eratosthenes program.

ChangeLog

+2018-07-13  Niels Möller  <nisse@lysator.liu.se>
+
+	* examples/eratosthenes.c (vector_alloc): Add assert related to
+	overflow in the size calculation. Fixes a corner case identified
+	by static analysis.
+
 2018-07-12  Niels Möller  <nisse@lysator.liu.se>
 
 	* examples/eratosthenes.c (main): Don't allocate bitmap storage

examples/eratosthenes.c

@@ -92,8 +92,13 @@ isqrt(unsigned long n)
 static unsigned long *
 vector_alloc(unsigned long size)
 {
-  unsigned long end = (size + BITS_PER_LONG - 1) / BITS_PER_LONG;
-  unsigned long *vector = malloc (end * sizeof(*vector));
+  unsigned long end;
+  unsigned long *vector;
+
+  assert (size <= ULONG_MAX - (BITS_PER_LONG - 1));
+
+  end = (size + BITS_PER_LONG - 1) / BITS_PER_LONG;
+  vector = malloc (end * sizeof(*vector));
 
   if (!vector)
     {