[PATCH v1] mm/mempolicy.c: Fix get_nodes() off-by-one error.

[Luis Felipe Sandoval Castro]

set_mempolicy() and mbind() take as argument a pointer to a bit mask
(nodemask) and the number of bits in the mask the kernel will use
(maxnode), among others.  For instace on a system with 2 NUMA nodes valid
masks are: 0b00, 0b01, 0b10 and 0b11 it's clear maxnode=2, however an
off-by-one error in get_nodes() the function that copies the node mask from
user space requires users to pass maxnode = 3 in this example and maxnode =
actual_maxnode + 1 in the general case. This patch fixes such error.

Signed-off-by: Luis Felipe Sandoval Castro <luis.felipe.sandoval.castro@xxxxxxxxx>
---
 mm/mempolicy.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index d911fa5..5274e9d2 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1208,11 +1208,10 @@ static int get_nodes(nodemask_t *nodes, const unsigned long __user *nmask,
 	unsigned long nlongs;
 	unsigned long endmask;
 
-	--maxnode;
 	nodes_clear(*nodes);
-	if (maxnode == 0 || !nmask)
+	if (maxnode == 1 || !nmask)
 		return 0;
-	if (maxnode > PAGE_SIZE*BITS_PER_BYTE)
+	if (maxnode - 1 > PAGE_SIZE * BITS_PER_BYTE)
 		return -EINVAL;
 
 	nlongs = BITS_TO_LONGS(maxnode);
-- 
1.8.3.1