[PATCH v2] block: Raise an error when backing file parameter is an empty string

[Connor Kuehl]

Providing an empty string for the backing file parameter like so:

        qemu-img create -f qcow2 -b '' /tmp/foo

allows the flow of control to reach and subsequently fail an assert
statement because passing an empty string to

        bdrv_get_full_backing_filename_from_filename()

simply results in NULL being returned without an error being raised.

To fix this, let's check for an empty string when getting the value from
the opts list.

Reported-by: Attila Fazekas <afazekas@redhat.com>
Fixes: https://bugzilla.redhat.com/1809553
Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
---
v2:
  - Removed 4 spaces to resolve pylint warning
  - Updated format to be 'iotests.imgfmt' instead
    of hardcoding 'qcow2'
  - Use temporary file instead of '/tmp/foo'
  - Give a size parameter to qemu-img
  - Run test for qcow2, qcow, qed and *not* raw

 block.c                    |  4 ++++
 tests/qemu-iotests/298     | 49 ++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/298.out |  5 ++++
 tests/qemu-iotests/group   |  1 +
 4 files changed, 59 insertions(+)
 create mode 100755 tests/qemu-iotests/298
 create mode 100644 tests/qemu-iotests/298.out

diff --git a/block.c b/block.c
index d9ac0e07eb..1f72275b87 100644
--- a/block.c
+++ b/block.c
@@ -6117,6 +6117,10 @@ void bdrv_img_create(const char *filename, const char 
*fmt,
                              "same filename as the backing file");
             goto out;
         }
+        if (backing_file[0] == '\0') {
+            error_setg(errp, "Expected backing file name, got empty string");
+            goto out;
+        }
     }
 
     backing_fmt = qemu_opt_get(opts, BLOCK_OPT_BACKING_FMT);
diff --git a/tests/qemu-iotests/298 b/tests/qemu-iotests/298
new file mode 100755
index 0000000000..879dae2d8e
--- /dev/null
+++ b/tests/qemu-iotests/298
@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2020 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+
+# Regression test for avoiding an assertion when the 'backing file'
+# parameter (-b) is set to an empty string for qemu-img create
+#
+#   qemu-img create -f qcow2 -b '' /tmp/foo
+#
+# This ensures the invalid parameter is handled with a user-
+# friendly message instead of a failed assertion.
+
+import iotests
+
+class TestEmptyBackingFilename(iotests.QMPTestCase):
+
+
+    def test_empty_backing_file_name(self):
+        with iotests.FilePath('test.img') as img_path:
+            actual = iotests.qemu_img_pipe(
+                'create',
+                '-f', iotests.imgfmt,
+                '-b', '',
+                img_path,
+                '1M'
+            )
+            expected = f'qemu-img: {img_path}: Expected backing file name,' \
+                       ' got empty string'
+
+            self.assertEqual(actual.strip(), expected.strip())
+
+
+if __name__ == '__main__':
+    iotests.main(supported_fmts=['qed', 'qcow', 'qcow2'])
diff --git a/tests/qemu-iotests/298.out b/tests/qemu-iotests/298.out
new file mode 100644
index 0000000000..ae1213e6f8
--- /dev/null
+++ b/tests/qemu-iotests/298.out
@@ -0,0 +1,5 @@
+.
+----------------------------------------------------------------------
+Ran 1 tests
+
+OK
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index 025ed5238d..6f80c884a1 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -306,6 +306,7 @@
 295 rw
 296 rw
 297 meta
+298 img auto quick
 299 auto quick
 301 backing quick
 302 quick
-- 
2.25.4