[Spice-devel] [PATCH spice v2] reds: Cancel unexpected xfer messages

[Pavel Grunt]

Hi,

On Fri, 2017-06-02 at 11:31 -0400, Frediano Ziglio wrote:
> > 
> > Inform client that transfer will not work, instead of only
> > dropping the agent messages.
> > 
> > The file transfer messages can be disabled using qemu cli option:
> >  "disable-agent-file-xfer"
> > 
> > Resolves:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1373725
> > 
> > Signed-off-by: Pavel Grunt <pgrunt at redhat.com>
> > ---
> >  server/reds.c | 34 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 34 insertions(+)
> > 
> > diff --git a/server/reds.c b/server/reds.c
> > index bafd04bab..fd163250d 100644
> > --- a/server/reds.c
> > +++ b/server/reds.c
> > @@ -1111,6 +1111,39 @@ static void
> > reds_on_main_agent_monitors_config(RedsState *reds,
> >      reds_client_monitors_config_cleanup(reds);
> >  }
> >  
> > +static void agent_data_free(uint8_t *data, G_GNUC_UNUSED void
> > *opaque)
> > +{
> > +    free(data);
> > +}
> > +
> > +/*
> > + * Inform client about discarded file transfer
> > + * https://bugzilla.redhat.com/show_bug.cgi?id=1373725
> > + */
> > +static void agent_xfer_error(MainChannelClient *mcc, const
> > VDAgentMessage
> > *message)
> > +{
> > +    VDAgentMessage *xfer_msg;
> > +    VDAgentFileXferStatusMessage *xfer_status;
> > +    size_t len;
> > +
> > +    if (message->type < VD_AGENT_FILE_XFER_START || message->type 
> > >
> > VD_AGENT_FILE_XFER_DATA) {
> > +        return;
> > +    }
> > +
> > +    len = sizeof(VDAgentMessage) +
> > sizeof(VDAgentFileXferStatusMessage);
> > +    xfer_msg = spice_malloc0(len);
> > +    xfer_msg->protocol = VD_AGENT_PROTOCOL;
> > +    xfer_msg->type = VD_AGENT_FILE_XFER_STATUS;
> > +    xfer_msg->size = sizeof(VDAgentFileXferStatusMessage);
> > +    xfer_status = (VDAgentFileXferStatusMessage *) xfer_msg-
> > >data;
> > +    xfer_status->id = ((VDAgentFileXferStartMessage *) message-
> > >data)->id;
> 
> VDAgentFileXferStartMessage should be const but the real problem is
> this:
> 
> reds.c: In function 'agent_xfer_error':
> reds.c:1139:5: error: dereferencing type-punned pointer will break
> strict-aliasing rules [-Werror=strict-aliasing]
>      xfer_status->id = ((VDAgentFileXferStartMessage *) message-
> >data)->id;
> 
>      ^~~~~~~~~~~
> 
Do you have a clue why I don't get the warning?

my CFLAGS are:
  -Wall -Wextra -g   -Wstrict-aliasing

gcc-7.1.1-1.fc26.x86_64
clang-4.0.0-3.fc26.x86_64

Thanks,
Pavel

> no idea... but if I write (message+1) instead of message->data
> (which is from ABI the same)
> the warning disappears :-( Not counting that data is uint8_t, a type
> that is supposed to
> be ok to be aliases... but maybe I didn't understand something about
> aliasing.
> Also spice_malloc0 is marked as __attribute__((malloc)) which should
> tell compiler
> that xfer_msg (and so xfer_status) cannot alias...
> 
> > +    xfer_status->result = VD_AGENT_FILE_XFER_STATUS_ERROR;
> > +
> > +    spice_debug("canceling file transfer %u", xfer_status->id);
> > +
> > +    main_channel_client_push_agent_data(mcc, (uint8_t *)
> > xfer_msg, len,
> > agent_data_free, NULL);
> > +}
> > +
> >  void reds_on_main_agent_data(RedsState *reds, MainChannelClient
> > *mcc, const
> >  void *message,
> >                               size_t size)
> >  {
> > @@ -1124,6 +1157,7 @@ void reds_on_main_agent_data(RedsState
> > *reds,
> > MainChannelClient *mcc, const void
> >      case AGENT_MSG_FILTER_OK:
> >          break;
> >      case AGENT_MSG_FILTER_DISCARD:
> > +        agent_xfer_error(mcc, message);
> >          return;
> >      case AGENT_MSG_FILTER_MONITORS_CONFIG:
> >          reds_on_main_agent_monitors_config(reds, mcc, message,
> > size);
> 
> Frediano