Re: [Qemu-devel] [Nbd] [PATCH 1/3] NBD proto: forbid TRIM command without negotiation

[Wouter Verhelst <w@uter.be>]

On Mon, Mar 28, 2016 at 07:00:17AM -0600, Eric Blake wrote:
> On 03/28/2016 04:43 AM, Denis V. Lunev wrote:
> > From: Pavel Borzenkov <pborzenkov@virtuozzo.com>
> > 
> > There is a loophole in the protocol that allows a client to send TRIM
> > request even if support for it wasn't negotiated with the server. State
> > explicitly that the client MUST NOT send such command without prior
> > successful negotiation.
> > 
> > Signed-off-by: Pavel Borzenkov <pborzenkov@virtuozzo.com>
> > Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
> > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > CC: Wouter Verhelst <w@uter.be>
> > CC: Eric Blake <eblake@redhat.com>
> > CC: Alex Bligh <alex@alex.org.uk>
> > ---
> >  doc/proto.md | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/doc/proto.md b/doc/proto.md
> > index 6d1cb34..d54ed19 100644
> > --- a/doc/proto.md
> > +++ b/doc/proto.md
> > @@ -471,6 +471,9 @@ The following request types exist:
> >      about the contents of the export affected by this command, until
> >      overwriting it again with `NBD_CMD_WRITE`.
> >  
> > +    A client MUST NOT send a trim request unless `NBD_FLAG_SEND_TRIM`
> > +    was set in the export flags field.
> > +
> 
> Do we also want to mention that the server SHOULD fail with EINVAL if
> the client sends it anyway, and similarly if NBD_CMD_FLUSH was sent
> without the appropriate export flag (but that the client should not rely
> on that particular failure)?

I think the protocol should mention that the server MAY fail with
EINVAL, rather than SHOULD. Rationale: the robusness principle -- if you
didn't negotiate it, you may end up with a server who doesn't know about
the feature; but if it just so happens that the server does know about it even
though you didn't negotiate it, there is little harm in it following up on the
request.

> But as this is a strict improvement,
> Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
< 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