Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-snapshot.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 366d9f574c..dac8a778e4 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -444,6 +444,14 @@ int coroutine_fn qcow2_check_read_snapshot_table(BlockDriverState *bs,
     s->snapshots_offset = be64_to_cpu(snapshot_table_pointer.snapshots_offset);
     s->nb_snapshots = be32_to_cpu(snapshot_table_pointer.nb_snapshots);
 
+    if (s->nb_snapshots > QCOW_MAX_SNAPSHOTS && (fix & BDRV_FIX_ERRORS)) {
+        fprintf(stderr, "Discarding %u overhanging snapshots\n",
+                s->nb_snapshots - QCOW_MAX_SNAPSHOTS);
+
+        nb_clusters_reduced += s->nb_snapshots - QCOW_MAX_SNAPSHOTS;
+        s->nb_snapshots = QCOW_MAX_SNAPSHOTS;
+    }
+
     ret = qcow2_validate_table(bs, s->snapshots_offset, s->nb_snapshots,
                                sizeof(QCowSnapshotHeader),
                                sizeof(QCowSnapshotHeader) * QCOW_MAX_SNAPSHOTS,
@@ -452,6 +460,12 @@ int coroutine_fn qcow2_check_read_snapshot_table(BlockDriverState *bs,
         result->check_errors++;
         error_reportf_err(local_err, "ERROR ");
 
+        if (s->nb_snapshots > QCOW_MAX_SNAPSHOTS) {
+            fprintf(stderr, "You can force-remove all %u overhanging snapshots "
+                    "with qemu-img check -r all\n",
+                    s->nb_snapshots - QCOW_MAX_SNAPSHOTS);
+        }
+
         /* We did not read the snapshot table, so invalidate this information */
         s->snapshots_offset = 0;
         s->nb_snapshots = 0;
-- 
2.21.0

On 8/19/19 1:55 PM, Max Reitz wrote:
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---

Short on the reasoning why this isn't a problem in practice.  (Again,
because we only do it via opt-in qemu-img -r; you can already learn if
qemu-img will have problem with your file created externally without
destroying the image, and elect to not have qemu-img clean it if you
don't like the algorithm qemu-img will use).

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

On 19.08.19 21:45, Eric Blake wrote:
> On 8/19/19 1:55 PM, Max Reitz wrote:
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
> 
> Short on the reasoning why this isn't a problem in practice.  (Again,
> because we only do it via opt-in qemu-img -r; you can already learn if
> qemu-img will have problem with your file created externally without
> destroying the image, and elect to not have qemu-img clean it if you
> don't like the algorithm qemu-img will use).

OK.  I’ll add the same message as for patch 12, just with s/64 MB/65536
snapshots/, if that seems good to you.

Max