mirror: Fix access of uninitialised fields during start

bdrv_mirror_top_pwritev() accesses the job object when active mirroring
is enabled. It disables this code during early initialisation while
s->job isn't set yet.

However, s->job is still set way too early when the job object isn't
fully initialised. For example, &s->ops_in_flight isn't initialised yet
and the in_flight bitmap doesn't exist yet. This causes crashes when a
write request comes in too early.

Move the assignment of s->job to when the mirror job is actually fully
initialised to make sure that the mirror_top driver doesn't access it
too early.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230203152202.49054-3-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/mirror.c

@@ -896,6 +896,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
     BlockDriverState *bs = s->mirror_top_bs->backing->bs;
+    MirrorBDSOpaque *mirror_top_opaque = s->mirror_top_bs->opaque;
     BlockDriverState *target_bs = blk_bs(s->target);
     bool need_drain = true;
     BlockDeviceIoStatus iostatus;
@@ -985,6 +986,12 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
         }
     }
 
+    /*
+     * Only now the job is fully initialised and mirror_top_bs should start
+     * accessing it.
+     */
+    mirror_top_opaque->job = s;
+
     assert(!s->dbi);
     s->dbi = bdrv_dirty_iter_new(s->dirty_bitmap);
     for (;;) {
@@ -1704,7 +1711,6 @@ static BlockJob *mirror_start_job(
     if (!s) {
         goto fail;
     }
-    bs_opaque->job = s;
 
     /* The block job now has a reference to this node */
     bdrv_unref(mirror_top_bs);