On 12/14/2015 10:58 AM, H. Peter Anvin wrote:

On 12/11/15 09:52, Daniel Cashman wrote:

quoted

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 844b06d..647fecf 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c

@@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-	/*
-	 *  8 bits of randomness in 32bit mmaps, 20 address space bits
-	 * 28 bits of randomness in 64bit mmaps, 40 address space bits
-	 */
 	if (mmap_is_ia32())
-		rnd = (unsigned long)get_random_int() % (1<<8);
+#ifdef CONFIG_COMPAT
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
+#else
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
+#endif
 	else
-		rnd = (unsigned long)get_random_int() % (1<<28);
+		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
 
 	return rnd << PAGE_SHIFT;
 }

Now, you and I know that both variants can be implemented with a simple
AND, but I have a strong suspicion that once this is turned into a
variable, this will in fact be changed from an AND to a divide.

So I'd prefer to use the
"get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead.

Good point.  Will change in v7 across patch-set.

Thank You,
Dan