From 7ec94ee16262eb0da7fb7788347a65162845b1c2 Mon Sep 17 00:00:00 2001
From: Kevin Wolf <kwolf@redhat.com>
Date: Wed, 10 Oct 2018 13:19:57 +0100
Subject: [PATCH 3/5] mirror: Fail gracefully for source == target

RH-Author: Kevin Wolf <kwolf@redhat.com>
Message-id: <20181010131957.23198-2-kwolf@redhat.com>
Patchwork-id: 82564
O-Subject: [RHEL-8 qemu-kvm PATCH 1/1] mirror: Fail gracefully for source == target
Bugzilla: 1637963
RH-Acked-by: John Snow <jsnow@redhat.com>
RH-Acked-by: Fam Zheng <famz@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

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: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 86fae10c64d642256cf019e6829929fa0d259c7a)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 block/mirror.c             | 5 +++++
 tests/qemu-iotests/041     | 6 ++++++
 tests/qemu-iotests/041.out | 4 ++--
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 163b1d4..313e6e9 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1149,6 +1149,11 @@ 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. */
diff --git a/tests/qemu-iotests/041 b/tests/qemu-iotests/041
index c20ac7d..9336ab6 100755
--- a/tests/qemu-iotests/041
+++ b/tests/qemu-iotests/041
@@ -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
diff --git a/tests/qemu-iotests/041.out b/tests/qemu-iotests/041.out
index c28b392..e071d0b 100644
--- a/tests/qemu-iotests/041.out
+++ b/tests/qemu-iotests/041.out
@@ -1,5 +1,5 @@
-.....................................................................................
+........................................................................................
 ----------------------------------------------------------------------
-Ran 85 tests
+Ran 88 tests
 
 OK
-- 
1.8.3.1