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mirror: Fail gracefully for source == target

PATCH In review Kevin Wolf 2 participants 2 messages Aug 14, 2018 Block layer

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Kevin Wolf

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PATCH 1/1 mirror: Fail gracefully for source == target

blockdev-mirror with the same node for source and target segfaults
today: A node is in its own backing chain, so mirror_start_job() decides that this is an active commit. When adding the intermediate nodes with block_job_add_bdrv(), it starts the iteration through the subchain with the backing file of source, though, so it never reaches target and instead runs into NULL at the base.

While we could fix that by starting with source itself, there is no point in allowing mirroring a node into itself and I wouldn't be surprised if this caused more problems later.

So just check for this scenario and error out.

Cc [email protected]
Signed-off-by Kevin Wolf <[email protected]>
---
 block/mirror.c             | 5 +++++
 tests/qemu-iotests/041     | 6 ++++++
 tests/qemu-iotests/041.out | 4 ++--
 3 files changed, 13 insertions(+), 2 deletions(-)
block/mirror.c +5 -0
index b48c3f8cf5..dd5ca02b09 100644
@@ -1499,6 +1499,11 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
         buf_size = DEFAULT_MIRROR_BUF_SIZE;
     }
 
+    if (bs == target) {
+        error_setg(errp, "Can't mirror node into itself");
+        return;
+    }
+
     /* In the case of active commit, add dummy driver to provide consistent
      * reads on the top, while disabling it in the intermediate nodes, and make
      * the backing chain writable. */
tests/qemu-iotests/041 +6 -0
index c20ac7da87..9336ab6ff5 100755
@@ -234,6 +234,12 @@ class TestSingleBlockdev(TestSingleDrive):
         result = self.vm.qmp("blockdev-add", **args)
         self.assert_qmp(result, 'return', {})
 
+    def test_mirror_to_self(self):
+        result = self.vm.qmp(self.qmp_cmd, job_id='job0',
+                             device=self.qmp_target, sync='full',
+                             target=self.qmp_target)
+        self.assert_qmp(result, 'error/class', 'GenericError')
+
     test_large_cluster = None
     test_image_not_found = None
     test_small_buffer2 = None
tests/qemu-iotests/041.out +2 -2
index c28b392b87..e071d0b261 100644
@@ -1,5 +1,5 @@
-.....................................................................................
+........................................................................................
 ----------------------------------------------------------------------
-Ran 85 tests
+Ran 88 tests
 
 OK
Signature · 1 line
2.13.6
Reviewed-by on v1 1/1
#1 lore raw
Quoted 13 lines · Kevin Wolf

blockdev-mirror with the same node for source and target segfaults
today: A node is in its own backing chain, so mirror_start_job() decides that this is an active commit. When adding the intermediate nodes with block_job_add_bdrv(), it starts the iteration through the subchain with the backing file of source, though, so it never reaches target and instead runs into NULL at the base.

While we could fix that by starting with source itself, there is no point in allowing mirroring a node into itself and I wouldn't be surprised if this caused more problems later.

So just check for this scenario and error out.

Quoted 8 lines
Cc [email protected]
Signed-off-by Kevin Wolf <[email protected]>
---
  block/mirror.c             | 5 +++++
  tests/qemu-iotests/041     | 6 ++++++
  tests/qemu-iotests/041.out | 4 ++--
  3 files changed, 13 insertions(+), 2 deletions(-)
Reviewed-by Eric Blake <[email protected]>
Signature · 3 lines
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org