If the target is shorter than the source, mirror would copy data until
it reaches the end of the target and then fail with an I/O error when
trying to write past the end.

If the target is longer than the source, the mirror job would complete
successfully, but the target wouldn't actually be an accurate copy of
the source image (it would contain some additional garbage at the end).

Fix this by checking that both images have the same size when the job
starts.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/mirror.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index aca95c9bc9..201ffa26f9 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -872,6 +872,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
     BlockDriverState *target_bs = blk_bs(s->target);
     bool need_drain = true;
     int64_t length;
+    int64_t target_length;
     BlockDriverInfo bdi;
     char backing_filename[2]; /* we only need 2 characters because we are only
                                  checking for a NULL string */
@@ -887,24 +888,26 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
         goto immediate_exit;
     }
 
+    target_length = blk_getlength(s->target);
+    if (target_length < 0) {
+        ret = target_length;
+        goto immediate_exit;
+    }
+
     /* Active commit must resize the base image if its size differs from the
      * active layer. */
     if (s->base == blk_bs(s->target)) {
-        int64_t base_length;
-
-        base_length = blk_getlength(s->target);
-        if (base_length < 0) {
-            ret = base_length;
-            goto immediate_exit;
-        }
-
-        if (s->bdev_length > base_length) {
+        if (s->bdev_length > target_length) {
             ret = blk_truncate(s->target, s->bdev_length, false,
                                PREALLOC_MODE_OFF, 0, NULL);
             if (ret < 0) {
                 goto immediate_exit;
             }
         }
+    } else if (s->bdev_length != target_length) {
+        error_setg(errp, "Source and target image have different sizes");
+        ret = -EINVAL;
+        goto immediate_exit;
     }
 
     if (s->bdev_length == 0) {
-- 
2.25.3

On 5/7/20 9:52 AM, Kevin Wolf wrote:
> If the target is shorter than the source, mirror would copy data until
> it reaches the end of the target and then fail with an I/O error when
> trying to write past the end.
> 
> If the target is longer than the source, the mirror job would complete
> successfully, but the target wouldn't actually be an accurate copy of
> the source image (it would contain some additional garbage at the end).
> 
> Fix this by checking that both images have the same size when the job
> starts.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   block/mirror.c | 21 ++++++++++++---------
>   1 file changed, 12 insertions(+), 9 deletions(-)

An alternative would be trying to resize the target (like we have to do 
with active commit), but I'm fine with being conservative for now by 
forcing the user to have correct sizing, where we have the option to add 
magic resizing later only if it proves useful and not introducing more 
potential issues.

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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