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, 2 insertions, 0 deletions

diff --git a/tests/qemu-iotests/177.out b/tests/qemu-iotests/177.out
index e887542..fcfbfa3 100644
--- a/tests/qemu-iotests/177.out
+++ b/tests/qemu-iotests/177.out
@@ -45,5 +45,7 @@ read 30408704/30408704 bytes at offset 80740352
 29 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 23068672/23068672 bytes at offset 111149056
 22 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+Offset          Length          File
+0               0x8000000       json:{"image": {"driver": "IMGFMT", "file": {"driver": "file", "filename": "TEST_DIR/t.IMGFMT"}}, "driver": "blkdebug", "align": "4k"}
 No errors were found on the image.
 *** done