nbd-client: Refuse read-only client with BDRV_O_RDWR

The NBD spec says that clients should not try to write/trim to
an export advertised as read-only by the server.  But we failed
to check that, and would allow the block layer to use NBD with
BDRV_O_RDWR even when the server is read-only, which meant we
were depending on the server sending a proper EPERM failure for
various commands, and also exposes a leaky abstraction: using
qemu-io in read-write mode would succeed on 'w -z 0 0' because
of local short-circuiting logic, but 'w 0 0' would send a
request over the wire (where it then depends on the server, and
fails at least for qemu-nbd but might pass for other NBD
implementations).

With this patch, a client MUST request read-only mode to access
a server that is doing a read-only export, or else it will get
a message like:

can't open device nbd://localhost:10809/foo: request for write access conflicts with read-only export

It is no longer possible to even attempt writes over the wire
(including the corner case of 0-length writes), because the block
layer enforces the explicit read-only request; this matches the
behavior of qcow2 when backed by a read-only POSIX file.

Fix several iotests to comply with the new behavior (since
qemu-nbd of an internal snapshot, as well as nbd-server-add over QMP,
default to a read-only export, we must tell blockdev-add/qemu-io to
set up a read-only client).

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171108215703.9295-3-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

3 files changed, 7 insertions, 6 deletions

diff --git a/tests/qemu-iotests/058 b/tests/qemu-iotests/058
index 2253c6a6d1..5eb8784669 100755
--- a/tests/qemu-iotests/058
+++ b/tests/qemu-iotests/058
@@ -117,15 +117,15 @@ _export_nbd_snapshot sn1
 
 echo
 echo "== verifying the exported snapshot with patterns, method 1 =="
-$QEMU_IO_NBD -c 'read -P 0xa 0x1000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
-$QEMU_IO_NBD -c 'read -P 0xb 0x2000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
+$QEMU_IO_NBD -r -c 'read -P 0xa 0x1000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
+$QEMU_IO_NBD -r -c 'read -P 0xb 0x2000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
 
 _export_nbd_snapshot1 sn1
 
 echo
 echo "== verifying the exported snapshot with patterns, method 2 =="
-$QEMU_IO_NBD -c 'read -P 0xa 0x1000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
-$QEMU_IO_NBD -c 'read -P 0xb 0x2000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
+$QEMU_IO_NBD -r -c 'read -P 0xa 0x1000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
+$QEMU_IO_NBD -r -c 'read -P 0xb 0x2000 0x1000' "$nbd_snapshot_img" | _filter_qemu_io
 
 $QEMU_IMG convert "$TEST_IMG" -l sn1 -O qcow2 "$converted_image"
 
diff --git a/tests/qemu-iotests/140 b/tests/qemu-iotests/140
index f89d0d6789..a8fc95145c 100755
--- a/tests/qemu-iotests/140
+++ b/tests/qemu-iotests/140
@@ -78,7 +78,7 @@ _send_qemu_cmd $QEMU_HANDLE \
        'arguments': { 'device': 'drv' }}" \
     'return'
 
-$QEMU_IO_PROG -f raw -c 'read -P 42 0 64k' \
+$QEMU_IO_PROG -f raw -r -c 'read -P 42 0 64k' \
     "nbd+unix:///drv?socket=$TEST_DIR/nbd" 2>&1 \
     | _filter_qemu_io | _filter_nbd
 
@@ -87,7 +87,7 @@ _send_qemu_cmd $QEMU_HANDLE \
        'arguments': { 'device': 'drv' }}" \
     'return'
 
-$QEMU_IO_PROG -f raw -c close \
+$QEMU_IO_PROG -f raw -r -c close \
     "nbd+unix:///drv?socket=$TEST_DIR/nbd" 2>&1 \
     | _filter_qemu_io | _filter_nbd
 
diff --git a/tests/qemu-iotests/147 b/tests/qemu-iotests/147
index db34838cd0..90f40ed245 100755
--- a/tests/qemu-iotests/147
+++ b/tests/qemu-iotests/147
@@ -43,6 +43,7 @@ class NBDBlockdevAddBase(iotests.QMPTestCase):
                     'driver': 'raw',
                     'file': {
                         'driver': 'nbd',
+                        'read-only': True,
                         'server': address
                     } }
         if export is not None: