[PATCH for 9.2] hw/virtio: fix crash in processing balloon stats

["Daniel P. Berrangé" <berrange@redhat.com>]

balloon_stats_get_all will iterate over guest stats upto the max
VIRTIO_BALLOON_S_NR value, calling visit_type_uint64 to populate
the QObject dict. The dict keys are obtained from the static
array balloon_stat_names which is VIRTIO_BALLOON_S_NR in size.

Unfortunately the way that array is declared results in any
unassigned stats getting a NULL name, which will then cause
visit_type_uint64 to trigger an assert in qobject_output_add_obj.

The balloon_stat_names array was fortunately fully populated with
names until recently:

  commit 0d2eeef77a33315187df8519491a900bde4a3d83
  Author: Bibo Mao <maobibo@loongson.cn>
  Date:   Mon Oct 28 10:38:09 2024 +0800

    linux-headers: Update to Linux v6.12-rc5

pulled a change to include/standard-headers/linux/virtio_balloon.h
which increased VIRTIO_BALLOON_S_NR by 6, and failed to add the new
names to balloon_stat_names.

This commit fills in the missing names, and uses a static assert to
guarantee that any future changes to VIRTIO_BALLOON_S_NR will cause
a build failure until balloon_stat_names is updated.

This problem was detected by the Cockpit Project's automated
integration tests on QEMU 9.2.0-rc1.

Fixes: 0d2eeef77a33315187df8519491a900bde4a3d83
Reported-by: Martin Pitt <mpitt@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---

NB: this is a MUST FIX for 9.2, as a trivial guest triggerable crash
regressing since 9.1

 hw/virtio/virtio-balloon.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 609e39a821..afd2ad6dd6 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -167,19 +167,33 @@ static void balloon_deflate_page(VirtIOBalloon *balloon,
     }
 }
 
+/*
+ * All stats upto VIRTIO_BALLOON_S_NR /must/ have a
+ * non-NULL name declared here, since these are used
+ * as keys for populating the QDict with stats
+ */
 static const char *balloon_stat_names[] = {
    [VIRTIO_BALLOON_S_SWAP_IN] = "stat-swap-in",
    [VIRTIO_BALLOON_S_SWAP_OUT] = "stat-swap-out",
    [VIRTIO_BALLOON_S_MAJFLT] = "stat-major-faults",
    [VIRTIO_BALLOON_S_MINFLT] = "stat-minor-faults",
    [VIRTIO_BALLOON_S_MEMFREE] = "stat-free-memory",
+
    [VIRTIO_BALLOON_S_MEMTOT] = "stat-total-memory",
    [VIRTIO_BALLOON_S_AVAIL] = "stat-available-memory",
    [VIRTIO_BALLOON_S_CACHES] = "stat-disk-caches",
    [VIRTIO_BALLOON_S_HTLB_PGALLOC] = "stat-htlb-pgalloc",
    [VIRTIO_BALLOON_S_HTLB_PGFAIL] = "stat-htlb-pgfail",
-   [VIRTIO_BALLOON_S_NR] = NULL
+
+   [VIRTIO_BALLOON_S_OOM_KILL] = "stat-oom-kills",
+   [VIRTIO_BALLOON_S_ALLOC_STALL] = "stat-alloc-stalls",
+   [VIRTIO_BALLOON_S_ASYNC_SCAN] = "stat-async-scans",
+   [VIRTIO_BALLOON_S_DIRECT_SCAN] = "stat-direct-scans",
+   [VIRTIO_BALLOON_S_ASYNC_RECLAIM] = "stat-async-reclaims",
+
+   [VIRTIO_BALLOON_S_DIRECT_RECLAIM] = "stat-direct-reclaims",
 };
+G_STATIC_ASSERT(G_N_ELEMENTS(balloon_stat_names) == VIRTIO_BALLOON_S_NR);
 
 /*
  * reset_stats - Mark all items in the stats array as unset
-- 
2.46.0