block: Guard against NULL bs->drv

We currently do not guard everywhere against a NULL bs->drv where we
should be doing so.  Most of the places fixed here just do not care
about that case at all.

Some care implicitly, e.g. through a prior function call to
bdrv_getlength() which would always fail for an ejected BDS.  Add an
assert there to make it more obvious.

Other places seem to care, but do so insufficiently: Freeing clusters in
a qcow2 image is an error-free operation, but it may leave the image in
an unusable state anyway.  Giving qcow2_free_clusters() an error code is
not really viable, it is much easier to note that bs->drv may be NULL
even after a successful driver call.  This concerns bdrv_co_flush(), and
the way the check is added to bdrv_co_pdiscard() (in every iteration
instead of only once).

Finally, some places employ at least an assert(bs->drv); somewhere, that
may be reasonable (such as in the reopen code), but in
bdrv_has_zero_init(), it is definitely not.  Returning 0 there in case
of an ejected BDS saves us much headache instead.

Reported-by: R. Nageswara Sastry <nasastry@in.ibm.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1728660
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171110203111.7666-4-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/block/vvfat.c b/block/vvfat.c
index 0841cc4..a690595 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -2947,7 +2947,7 @@ static int do_commit(BDRVVVFATState* s)
         return ret;
     }
 
-    if (s->qcow->bs->drv->bdrv_make_empty) {
+    if (s->qcow->bs->drv && s->qcow->bs->drv->bdrv_make_empty) {
         s->qcow->bs->drv->bdrv_make_empty(s->qcow->bs);
     }