Re: [Qemu-block] [Qemu-devel] [PATCH 1/3] nbd: Add option to disallow listing exports

[Eric Blake]

On 04/16/2018 06:00 AM, Daniel P. Berrangé wrote:
> On Mon, Apr 16, 2018 at 11:53:41AM +0100, Richard W.M. Jones wrote:
>> On Mon, Apr 16, 2018 at 11:31:18AM +0100, Daniel P. Berrangé wrote:
>>> Essentially this is abusing the export name as a crude authentication
>>> token. There are NBD servers that expect NBD_OPT_LIST to always succeeed
>>
>> I guess you mean "NBD clients" ...
> 
> Sigh, yes, of course.

qemu 2.10 and older tries to use NBD_OPT_LIST, but gracefully still
tries to connect even if the LIST fails (that is, it's use of
NBD_OPT_LIST was for better error handling than what NBD_OPT_EXPORT_NAME
gives, and not because it actually needed the list).  The recent
introduction in qemu 2.11 for support of NBD_OPT_GO means that modern
qemu is no longer even attempting NBD_OPT_LIST when talking to a new
server.  But cross-implementation compatibility is still a concern, and
there may indeed be non-qemu clients that choke if LIST fails, even
though...

> 
>>> when they detect that the new style protocol is available. I really hate
>>> the idea of making it possible to break the NBD_OPT_LIST functionality
>>> via a command line arg like this.

...the NBD spec suggests that a client that requires LIST to work is not
fully compliant, since NBD_OPT_LIST is an optional feature.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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