[pve-devel] [PATCH qemu 1/4] migration/block-dirty-bitmap: fix larger granularity bitmaps

[Stefan Reiter <s.reiter@proxmox.com>]

sectors_per_chunk is a 64 bit integer, but the calculation would be 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.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---

See also this thread for the upstream discussion:
https://lists.gnu.org/archive/html/qemu-devel/2020-10/msg06145.html

 migration/block-dirty-bitmap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
index 784330ebe1..5bf0d9fbc6 100644
--- a/migration/block-dirty-bitmap.c
+++ b/migration/block-dirty-bitmap.c
@@ -334,8 +334,9 @@ static int add_bitmaps_to_list(DBMSaveState *s, BlockDriverState *bs,
         dbms->node_name = bs_name;
         dbms->bitmap = bitmap;
         dbms->total_sectors = bdrv_nb_sectors(bs);
-        dbms->sectors_per_chunk = CHUNK_SIZE * 8 *
+        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;
         }
-- 
2.20.1