migration/block-dirty-bitmap: fix larger granularity bitmaps

sectors_per_chunk is a 64 bit integer, but the calculation is done in 32
bits, leading to an overflow for coarse bitmap granularities.

If that results in the value 0, it leads to a hang where no progress is
made but send_bitmap_bits is constantly called with nr_sectors being 0.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Message-Id: <20201021144456.1072-1-s.reiter@proxmox.com>
Fixes: b35ebdf07 migration: add postcopy migration of dirty bitmaps
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: Use correct type for 8ULL, use () to avoid overflow]
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
index 5bef793ac0..98921db772 100644
--- a/migration/block-dirty-bitmap.c
+++ b/migration/block-dirty-bitmap.c
@@ -562,8 +562,9 @@ static int add_bitmaps_to_list(DBMSaveState *s, BlockDriverState *bs,
         dbms->bitmap_alias = g_strdup(bitmap_alias);
         dbms->bitmap = bitmap;
         dbms->total_sectors = bdrv_nb_sectors(bs);
-        dbms->sectors_per_chunk = CHUNK_SIZE * 8 *
-            bdrv_dirty_bitmap_granularity(bitmap) >> BDRV_SECTOR_BITS;
+        dbms->sectors_per_chunk = CHUNK_SIZE * 8LLU *
+            (bdrv_dirty_bitmap_granularity(bitmap) >> BDRV_SECTOR_BITS);
+        assert(dbms->sectors_per_chunk != 0);
         if (bdrv_dirty_bitmap_enabled(bitmap)) {
             dbms->flags |= DIRTY_BITMAP_MIG_START_FLAG_ENABLED;
         }