[PATCH v3] 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>
---
v3:
  - Moved test case into 049 instead of taking up
    298.

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/049     | 4 ++++
 tests/qemu-iotests/049.out | 5 +++++
 3 files changed, 13 insertions(+)

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/049 b/tests/qemu-iotests/049
index 051a1c79e0..82b1e6c202 100755
--- a/tests/qemu-iotests/049
+++ b/tests/qemu-iotests/049
@@ -119,6 +119,10 @@ test_qemu_img create -f $IMGFMT -o 
compat=1.1,lazy_refcounts=on "$TEST_IMG" 64M
 test_qemu_img create -f $IMGFMT -o compat=0.10,lazy_refcounts=off "$TEST_IMG" 
64M
 test_qemu_img create -f $IMGFMT -o compat=0.10,lazy_refcounts=on "$TEST_IMG" 
64M
 
+echo "== Expect error when backing file name is empty string =="
+echo
+test_qemu_img create -f $IMGFMT -b '' $TEST_IMG 1M
+
 # success, all done
 echo "*** done"
 rm -f $seq.full
diff --git a/tests/qemu-iotests/049.out b/tests/qemu-iotests/049.out
index 4c21dc70a5..61ee90b10e 100644
--- a/tests/qemu-iotests/049.out
+++ b/tests/qemu-iotests/049.out
@@ -209,4 +209,9 @@ qemu-img create -f qcow2 -o compat=0.10,lazy_refcounts=on 
TEST_DIR/t.qcow2 64M
 Formatting 'TEST_DIR/t.qcow2', fmt=qcow2 cluster_size=65536 
compression_type=zlib size=67108864 compat=0.10 lazy_refcounts=on 
refcount_bits=16
 qemu-img: TEST_DIR/t.qcow2: Lazy refcounts only supported with compatibility 
level 1.1 and above (use version=v3 or greater)
 
+== Expect error when backing file name is empty string ==
+
+qemu-img create -f qcow2 -b  TEST_DIR/t.qcow2 1M
+qemu-img: TEST_DIR/t.qcow2: Expected backing file name, got empty string
+
 *** done
-- 
2.25.4