Re: [Qemu-devel] [Nbd] [PATCH 3/1] doc: Propose Structured Replies extension

[Wouter Verhelst <w@uter.be>]

On Tue, Mar 29, 2016 at 11:45:45AM -0600, Eric Blake wrote:
> On 03/29/2016 11:34 AM, Alex Bligh wrote:
> > I would agree. I think if it supports the structured reply semantics,
> > it should also support 'DF'. So if you know the server supports
> > structured replies, you know you can set DF on them without any
> > further requirements.
> 
> Supporting DF merely transfers the burden of collection between server
> and client.  I suspect that there are cases where the server does NOT
> want to support DF (because it would require the server to allocate
> memory to collect the data before sending a single structured read
> reply),

There are other ways to handle that; e.g., the server could have a
"request too large for non-fragmented read" error message. The spec
should give a minimum size that the server MUST support (which should be
reasonably large), and should state that a server MAY reply to any
request with DF set for a block larger than that minimum, with that
error.

Otherwise the client could conceivably send out heaps of requests for
(UINT32_MAX - 8) bytes with DF set and basically DoS the server.

> just as you have stated that there are cases where the client
> has an additional burden if DF is not supported.  So for v2, I'm going
> to explicitly document a DF export flag, and recommend (but not require)
> that the server support it.

I'd prefer not to see that.

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12