[PATCH v3 2/2] xen/common: Add NUMA node id bounds check to page_alloc.c/node_to_scrub

[Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>]

When building for Power with CONFIG_DEBUG unset, a compiler error gets
raised inside page_alloc.c's node_to_scrub function:

common/page_alloc.c: In function 'node_to_scrub.part.0':
common/page_alloc.c:1217:29: error: array subscript 1 is above array
            bounds of 'long unsigned int[1]' [-Werror=array-bounds]
 1217 |         if ( node_need_scrub[node] )

It appears that this is a false positive, given that in practice
cycle_node should never return a node ID >= MAX_NUMNODES as long as the
architecture's node_online_map is properly defined and initialized, so
this additional bounds check is only to satisfy GCC.

Signed-off-by: Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
v2: Add comment to explain the bounds check.

 xen/common/page_alloc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 35d9a26fa6..9b5df74fdd 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1211,6 +1211,14 @@ static unsigned int node_to_scrub(bool get_node)
         } while ( !cpumask_empty(&node_to_cpumask(node)) &&
                   (node != local_node) );

+        /*
+         * In practice `node` will always be within MAX_NUMNODES, but GCC can't
+         * always see that, so an explicit check is necessary to avoid tripping
+         * its out-of-bounds array access warning (-Warray-bounds).
+         */
+        if ( node >= MAX_NUMNODES )
+            break;
+
         if ( node == local_node )
             break;

--
2.30.2