On Wed, Jan 25, 2017 at 12:38:09PM -0500, Frediano Ziglio wrote:
> >
> > On Wed, Jan 25, 2017 at 12:23:35PM -0500, Frediano Ziglio wrote:
> > > >
> > > > When used with QEMU at least, both SPICE and VNC live in the same port
> > > > range (5900+). It is thus not entirely uncommon for a user to mistakenly
> > > > connect to a SPICE server with a VNC client, or vica-verca.
> > > >
> > > > When connecting to VNC server with a SPICE client, you quickly get an
> > > > error. This is because the VNC server sends its short greeting and then
> > > > sees the RedLinkMess from the SPICE client, realizes its garbage and
> > > > drops the connection.
> > > >
> > > > When connecting to a SPICE server with a VNC client though, you get an
> > > > indefinite hang. The VNC client is waiting for the VNC greeting, which
> > > > the SPICE server will never send. The SPICE server is waiting for the
> > > > RedLinkMess which the VNC client will never send.
> > > >
> > > > Since VNC is a server sends first protocol and SPICE is a client sends
> > > > first protocol, it would seem this problem is impossible to fix, but
> > > > I think it is possible to tweak things so it can be more gracefully
> > > > handled.
> > > >
> > > > In VNC the protocol starts with the follow data sent:
> > > >
> > > > Server: "RFB 003.008\n"
> > > > Client: "RFB 003.008\n"
> > > >
> > > > What I'm thinking is that we could easily change the VNC client so that
> > > > it will send some bytes first. eg
> > > >
> > > > Client: "RFB "
> > > > Server: "RFB 003.008\n"
> > > > Client: "003.008\n"
> > > >
> > > > From the VNC server POV, it'll still be receiving the same 12 bytes from
> > > > the client - it just happens that 4 of them might arrive a tiny bit
> > > > earlier than the other 8 of them. IOW nothing should break in the VNC
> > > > server from this change.
> > > >
> > > > I tried this, but we still get a hang in the SPICE server. The problem
> > > > is that the SPICE server code is waiting for the full RedLinkMess
> > > > message to be received before checking its content.
> > > >
> > > > Can we change the SPICE server so that it reads the 'uint32 magic'
> > > > first,
> > > > validates that, and then reads the remainder of RedLinkMess ?
> > > >
> > > > This would solve the problem, as the SPICE server would see 'RFB ' as
> > > > the
> > > > magic, realize it was garbage and so close the connection. The VNC
> > > > client
> > > > which is waiting for the VNC greeting will thus terminate rather than
> > > > hanging
> > > > forever.
> > > >
> > > > Regards,
> > > > Daniel
> > >
> > > Perhaps another solution would be to add a timeout to spice server so
> > > after, say, 2/3 seconds it would close the connection.
> > >
> > > This would work even without changing the VNC client.
> >
> > I'm not a fan of timeouts because this could negatively impact on
> > genuine spice clients which hit a latency spike on the network
> > for whatever reason. The incremental read of RedLinkMess by constrast
> > is guaranteed to not have any chance of negative impact on spice clients.
> >
> > Regards,
> > Daniel
>
> So you would like something like that:
>
>
> diff --git a/server/reds.c b/server/reds.c
> index 8db70ee..50098df 100644
> --- a/server/reds.c
> +++ b/server/reds.c
> @@ -2264,12 +2264,6 @@ static void reds_handle_read_header_done(void *opaque)
> header->minor_version = GUINT32_FROM_LE(header->minor_version);
> header->size = GUINT32_FROM_LE(header->size);
>
> - if (header->magic != SPICE_MAGIC) {
> - reds_send_link_error(link, SPICE_LINK_ERR_INVALID_MAGIC);
> - reds_link_free(link);
> - return;
> - }
> -
> if (header->major_version != SPICE_VERSION_MAJOR) {
> if (header->major_version > 0) {
> reds_send_link_error(link, SPICE_LINK_ERR_VERSION_MISMATCH);
> @@ -2296,13 +2290,31 @@ static void reds_handle_read_header_done(void *opaque)
> link);
> }
>
> +static void reds_handle_read_magic_done(void *opaque)
> +{
> + RedLinkInfo *link = (RedLinkInfo *)opaque;
> + const SpiceLinkHeader *header = &link->link_header;
> +
> + if (header->magic != SPICE_MAGIC) {
> + reds_send_link_error(link, SPICE_LINK_ERR_INVALID_MAGIC);
> + reds_link_free(link);
> + return;
> + }
> +
> + reds_stream_async_read(link->stream,
> + ((uint8_t *)&link->link_header) +
> sizeof(header->magic),
> + sizeof(SpiceLinkHeader) - sizeof(header->magic),
> + reds_handle_read_header_done,
> + link);
> +}
> +
> static void reds_handle_new_link(RedLinkInfo *link)
> {
> reds_stream_set_async_error_handler(link->stream,
> reds_handle_link_error);
> reds_stream_async_read(link->stream,
> (uint8_t *)&link->link_header,
> - sizeof(SpiceLinkHeader),
> - reds_handle_read_header_done,
> + sizeof(link->link_header.magic),
> + reds_handle_read_magic_done,
> link);
> }
>
>
> ?
>
> I don't think the extra code execute is much of a problem.
Looks reasonable - let me give it a test locally to make sure the result
is actually what I expect it to be.
Regards,
Daniel
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