block: Guarantee that *file is set on bdrv_get_block_status()

We document that *file is valid if the return is not an error and
includes BDRV_BLOCK_OFFSET_VALID, but forgot to obey this contract
when a driver (such as blkdebug) lacks a callback.  Messed up in
commit 67a0fd2 (v2.6), when we added the file parameter.

Enhance qemu-iotest 177 to cover this, using a sequence that would
print garbage or even SEGV, because it was dererefencing through
uninitialized memory.  [The resulting test output shows that we
have less-than-ideal block status from the blkdebug driver, but
that's a separate fix coming up soon.]

Setting *file on all paths that return BDRV_BLOCK_OFFSET_VALID is
enough to fix the crash, but we can go one step further: always
setting *file, even on error, means that a broken caller that
blindly dereferences file without checking for error is now more
likely to get a reliable SEGV instead of randomly acting on garbage,
making it easier to diagnose such buggy callers.  Adding an
assertion that file is set where expected doesn't hurt either.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/tests/qemu-iotests/177 b/tests/qemu-iotests/177
index 2005c174f2..f8ed8fb86b 100755
--- a/tests/qemu-iotests/177
+++ b/tests/qemu-iotests/177
@@ -43,6 +43,7 @@ _supported_proto file
 CLUSTER_SIZE=1M
 size=128M
 options=driver=blkdebug,image.driver=qcow2
+nested_opts=image.file.driver=file,image.file.filename=$TEST_IMG
 
 echo
 echo "== setting up files =="
@@ -106,6 +107,8 @@ function verify_io()
 }
 
 verify_io | $QEMU_IO -r "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG map --image-opts "$options,$nested_opts,align=4k" \
+    | _filter_qemu_img_map
 
 _check_test_img