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Re: [Qemu-devel] [PATCH v2 2/7] nbd-client: Refuse read-only client with

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH v2 2/7] nbd-client: Refuse read-only client with BDRV_O_RDWR
Date: Thu, 9 Nov 2017 12:04:38 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

09.11.2017 00:56, Eric Blake wrote:
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

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 iotest 147 to comply with the new behavior (since nbd-server-add
defaults to a read-only export, we must tell blockdev-add to set up a
read-only client).

CC: address@hidden
Signed-off-by: Eric Blake <address@hidden>

Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>

Best regards,

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