netdev - Re: [PATCH net 0/2] vsock: don't allow half-closed socket in the host transports

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Date:   Tue, 15 Oct 2019 12:56:37 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Stefano Garzarella <sgarzare@...hat.com>, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Jorgen Hansen <jhansen@...are.com>,
        "David S. Miller" <davem@...emloft.net>,
        Adit Ranadive <aditr@...are.com>,
        Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 0/2] vsock: don't allow half-closed socket in the
 host transports

On Sat, Oct 12, 2019 at 06:38:46PM -0400, Michael S. Tsirkin wrote:
> On Fri, Oct 11, 2019 at 04:34:57PM +0200, Stefano Garzarella wrote:
> > On Fri, Oct 11, 2019 at 10:19:13AM -0400, Michael S. Tsirkin wrote:
> > > On Fri, Oct 11, 2019 at 03:07:56PM +0200, Stefano Garzarella wrote:
> > > > We are implementing a test suite for the VSOCK sockets and we discovered
> > > > that vmci_transport never allowed half-closed socket on the host side.
> > > > 
> > > > As Jorgen explained [1] this is due to the implementation of VMCI.
> > > > 
> > > > Since we want to have the same behaviour across all transports, this
> > > > series adds a section in the "Implementation notes" to exaplain this
> > > > behaviour, and changes the vhost_transport to behave the same way.
> > > > 
> > > > [1] https://patchwork.ozlabs.org/cover/847998/#1831400
> > > 
> > > Half closed sockets are very useful, and lots of
> > > applications use tricks to swap a vsock for a tcp socket,
> > > which might as a result break.
> > 
> > Got it!
> > 
> > > 
> > > If VMCI really cares it can implement an ioctl to
> > > allow applications to detect that half closed sockets aren't supported.
> > > 
> > > It does not look like VMCI wants to bother (users do not read
> > > kernel implementation notes) so it does not really care.
> > > So why do we want to cripple other transports intentionally?
> > 
> > The main reason is that we are developing the test suite and we noticed
> > the miss match. Since we want to make sure that applications behave in
> > the same way on different transports, we thought we would solve it that
> > way.
> > 
> > But what you are saying (also in the reply of the patches) is actually
> > quite right. Not being publicized, applications do not expect this behavior,
> > so please discard this series.
> > 
> > My problem during the tests, was trying to figure out if half-closed
> > sockets were supported or not, so as you say adding an IOCTL or maybe
> > better a getsockopt() could solve the problem.
> > 
> > What do you think?
> > 
> > Thanks,
> > Stefano
> 
> Sure, why not.

The aim is for applications using AF_VSOCK sockets to run on any
transport.  When the semantics differ between transports it creates a
compatibility problem.

That said, I do think keeping the standard sockets behavior is
reasonable.  If applications have problems on VMCI a sockopt may be
necessary :(.

Stefan

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