[Qemu-devel] [PULL 1/1] block: mirror - fix full sync mode when target does not support zero init

[Jeff Cody <jcody@redhat.com>]

During mirror, if the target device does not support zero init, a
mirror may result in a corrupted image for sync="full" mode.

This is due to how the initial dirty bitmap is set up prior to copying
data - we did not mark sectors as dirty that are unallocated.  This
means those unallocated sectors are skipped over on the target, and for
a device without zero init, invalid data may reside in those holes.

If both of the following conditions are true, then we will explicitly
mark all sectors as dirty:

    1.) sync = "full"
    2.) bdrv_has_zero_init(target) == false

If the target does support zero init, but a target image is passed in
with data already present (i.e. an "existing" image), it is assumed the
data present in the existing image is valid data for those sectors.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 91ed4bc5bda7e2b09eb508b07c83f4071fe0b3c9.1443705220.git.jcody@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
---
 block/mirror.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/mirror.c b/block/mirror.c
index a258926..87928ab 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -455,6 +455,8 @@ static void coroutine_fn mirror_run(void *opaque)
     if (!s->is_none_mode) {
         /* First part, loop on the sectors and initialize the dirty bitmap.  */
         BlockDriverState *base = s->base;
+        bool mark_all_dirty = s->base == NULL && !bdrv_has_zero_init(s->target);
+
         for (sector_num = 0; sector_num < end; ) {
             /* Just to make sure we are not exceeding int limit. */
             int nb_sectors = MIN(INT_MAX >> BDRV_SECTOR_BITS,
@@ -477,7 +479,7 @@ static void coroutine_fn mirror_run(void *opaque)
             }
 
             assert(n > 0);
-            if (ret == 1) {
+            if (ret == 1 || mark_all_dirty) {
                 bdrv_set_dirty_bitmap(s->dirty_bitmap, sector_num, n);
             }
             sector_num += n;
-- 
1.9.3