Re: [Qemu-devel] [PATCH v4 04/11] nbd: Improve server handling of bogus commands

[Eric Blake <eblake@redhat.com>]

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[adding nbd list]

On 06/13/2016 06:10 AM, Paolo Bonzini wrote:
> 
> 
> On 12/05/2016 00:39, Eric Blake wrote:
>> - If we report an error to NBD_CMD_READ, we are not writing out
>> any data payload; but the protocol says that a client can expect
>> to read the payload no matter what (and must instead ignore it),
>> which means the client will start reading our next replies as
>> its data payload. Fix by disconnecting (an alternative fix of
>> sending bogus payload would be trickier to implement).
> 
> This is an error in the spec.  The Linux driver doesn't expect to read
> the payload here, and neither does block/nbd-client.c.

That's one of the reasons that there is a proposal to add
STRUCTURED_READ to the spec (although I still haven't had time to
implement that for qemu), so that we have a newer approach that allows
for proper error handling without ambiguity on whether bogus bytes must
be sent on a failed read.  But you'd have to convince me that ALL
existing NBD server and client implementations expect to handle a read
error without read payload, otherwise, I will stick with the notion that
the current spec wording is correct, and that read errors CANNOT be
gracefully recovered from unless BOTH sides transfer (possibly bogus)
bytes along with the error message, and which is why BOTH sides of the
protocol are warned that read errors usually result in a disconnection
rather than clean continuation, without the addition of STRUCTURED_READ.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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