qcow2: Prevent backing file names longer than 1023

We reject backing file names with a length of more than 1023 characters
when opening a qcow2 file, so we should not produce such files
ourselves.

Cc: qemu-stable@nongnu.org
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -1986,6 +1986,10 @@ static int qcow2_change_backing_file(BlockDriverState *bs,
 {
     BDRVQcow2State *s = bs->opaque;
 
+    if (backing_file && strlen(backing_file) > 1023) {
+        return -EINVAL;
+    }
+
     pstrcpy(bs->backing_file, sizeof(bs->backing_file), backing_file ?: "");
     pstrcpy(bs->backing_format, sizeof(bs->backing_format), backing_fmt ?: "");