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Re: vnc clipboard support

From: Laszlo Ersek
Subject: Re: vnc clipboard support
Date: Thu, 28 Jan 2021 18:57:24 +0100

On 01/28/21 18:12, Gerd Hoffmann wrote:
>   Hi folks,
> I'm looking for a good way to implement cut+paste support for vnc.
> The vnc core protocol has support for text/plain cut+paste, and there
> is an extension adding support for other formats.  That'll cover one
> part of the problem, exchanging cut+paste data between vnc client and
> qemu vnc server.
> The tricky part is the second: the guest <=> qemu communication.
> I see basically two possible approaches here:
>   (1) Have some guest agent (spice does it that way).
>       Advantage: more flexible, allows more features.
>       Disadvantage: requires agent in the guest.
>   (2) Send text as key events.
>       Advantage: no guest agent needed.
>       Disadvantage: is translated by guests keyboard map, so qemu
>       needs to know the map for proper char -> key event translation.
>       Only works for text/plain and only for chars you can easily
>       type, anything needing input methods (emoji 😊 for example)
>       isn't going to fly.
> I think that (1) is clearly the better way.  Given that the agent
> would need to run in user wayland/xorg session context the existing
> qemu-guest-agent will not work.  Also linking against some UI library
> like gtk3 for clipboard handling is not something we want for the
> qemu-guest-agent.  So we need another one, I'll name it
> qemu-clipboard-agent for the rest of this mail.  And we need a
> communication channel.
> I'd tend to model the qemu-clipboard-agent simliar to the
> qemu-guest-agent, i.e. have some stream as communication path and run
> some stream protocol over it.
> Stream options I see are (in order of personal preference):
>   (1) New virtio-serial port.  virtio-serial likely is there anyway
>       for the qemu-guest-agent ...
>   (2) Have qemu-clipboard-agent and qemu-guest-agent share the agent
>       channel, i.e. qemu-clipboard-agent will proxy everything through
>       qemu-guest-agent (spice does it that way).
> Protocol options I see are (not sure yet which to prefer, need to have
> a closer look at the candidates):
>   (1) Add clipboard commands to QMP and use these.
>   (2) Reuse the clipboard bits of the vnc protocol (forward
>       VNC_MSG_CLIENT_CUT_TEXT messages to the guest agent)
>   (3) Reuse the clipboard bits of the spice-agent protocol.
>   (4) Reuse the clipboard bits of the wayland protocol.
> Once we have sorted the qemu <-> guest communication path it should be
> possible to also hook up other UIs (specifically gtk) without too much
> effort.  Which probably makes (2) a rather poor choice.
> Comments?
> Suggestions?
> Other ideas?

Just a random thought: the guest should not be able to sniff, steal, or
overwrite host-side clipboard (selection) content, without the host user
explicitly requesting a clipboard operation. I understand this is
generally a problem between X11 applications (not so on Wayland, if I
understand correctly), but I assume it becomes graver with
virtualization -- there shouldn't be an unintended channel between
host/guest, or even multiple guests. (Analogy: you can "ssh -X -Y" to
two remote machines at the same time, and assuming "xauth" etc on both
of those remote machines, you can run "gedit" on each of those remote
machines, and cut'n'paste will work between them, *even if* you don't
ask for it -- that's why ssh has a separate option called "-Y".
Cut'n'paste does not work without "-Y". So I believe this is something
to consider here.)


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