virtio_balloon: prevent uninitialized variable use

commit f0bb2d50dfcc519f06f901aac88502be6ff1df2c upstream.

The latest gcc-7.0.1 snapshot reports a new warning:

virtio/virtio_balloon.c: In function 'update_balloon_stats':
virtio/virtio_balloon.c:258:26: error: 'events[2]' is used uninitialized in this function [-Werror=uninitialized]
virtio/virtio_balloon.c:260:26: error: 'events[3]' is used uninitialized in this function [-Werror=uninitialized]
virtio/virtio_balloon.c:261:56: error: 'events[18]' is used uninitialized in this function [-Werror=uninitialized]
virtio/virtio_balloon.c:262:56: error: 'events[17]' is used uninitialized in this function [-Werror=uninitialized]

This seems absolutely right, so we should add an extra check to
prevent copying uninitialized stack data into the statistics.
>From all I can tell, this has been broken since the statistics code
was originally added in 2.6.34.

Fixes: 9564e138b1f6 ("virtio: Add memory statistics reporting to the balloon driver (V4)")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 032327e3577adace99800f8461c11c3d00909d8a..0ca4d650dc9bca2ce6b20981e0a6c47b7b770190 100644 (file)
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -218,12 +218,14 @@ static void update_balloon_stats(struct virtio_balloon *vb)
        all_vm_events(events);
        si_meminfo(&i);
 
+#ifdef CONFIG_VM_EVENT_COUNTERS
        update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_IN,
                                pages_to_bytes(events[PSWPIN]));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_OUT,
                                pages_to_bytes(events[PSWPOUT]));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MAJFLT, events[PGMAJFAULT]);
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MINFLT, events[PGFAULT]);
+#endif
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMFREE,
                                pages_to_bytes(i.freeram));
        update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMTOT,