[Spice-devel] [PATCH] spicevmc: Drop unsent data on client disconnection

[Jonathon Jongsma]

On Tue, 2015-09-29 at 12:23 +0200, Victor Toso wrote:
> Hey,
> 
> On Tue, Sep 29, 2015 at 11:53:40AM +0200, Christophe Fergeau wrote:
> > When redirecting a USB webcam over a slow link, it's currently possible
> > to hit an assertion in spice-server by running cheese (application using
> > the webcam), killing the client with ctrl+c and then restarting the
> > client:
> > qemu-kvm: spicevmc.c:324: spicevmc_red_channel_alloc_msg_rcv_buf:
> > Assertion `!state->recv_from_client_buf' failed.
> > 
> > This happens when red_peer_handle_incoming tries to allocate memory for
> > a message using spicevmc:
> > handler->msg = handler->cb->alloc_msg_buf(handler->opaque, msg_type,
> > msg_size);
> > 
> > red_peer_handle_incoming() is called when there is client data to be
> > read, and does
> > - call alloc_msg_buf() to allocate memory for the message
> > - read the message
> > - if the read was partial, return early, the main loop will call again
> >   red_peer_handle_incoming() when there is more data available for that
> >   channel
> > - parse the message
> > - call release_msg_buf() to free the message
> > 
> > For channels based on spicevmc (usbredir and port), alloc_msg_buf()
> > stores message data in SpiceVmcState::recv_from_client_buf and before
> > allocating new memory, it asserts that it's NULL.
> 
> This is a great description of message/memory handling in this channels.
> 
> > 
> > This is what causes this crash in the following scenario:
> > - SpiceVmc::alloc_msg_buf() is called and allocates memory for a new
> >   message in SpiceVmcState::recv_from_client_buf
> > - red_peer_handle_incoming() returns early as all the spicevmc message
> >   data hasn't been received yet
> > - the client gets killed
> > - the main channel notices the disconnect and calls
> >   main_dispatcher_client_disconnect() which will disconnect all the
> >   channels
> > - SpiceVmc::on_disconnect is called
> > - after the new client connects, SpiceVmc::alloc_msg_buf() is called,
> >   notices that SpiceVmcState::recv_from_client_buf is already set, and
> >   asserts()
> > 
> > This commit makes sure the partial SpiceVmcState::recv_from_client_buf
> > data is cleared on disconnect so that the assert does not trigger.
> > 
> > This fixes https://bugzilla.redhat.com/show_bug.cgi?id=1264113
> > ---
> >  server/spicevmc.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/server/spicevmc.c b/server/spicevmc.c
> > index e10f183..80f3aeb 100644
> > --- a/server/spicevmc.c
> > +++ b/server/spicevmc.c
> > @@ -223,6 +223,11 @@ static void
> > spicevmc_red_channel_client_on_disconnect(RedChannelClient *rcc)
> >      sin = state->chardev_sin;
> >      sif = SPICE_CONTAINEROF(sin->base.sif, SpiceCharDeviceInterface, base);
> > 
> > +    if (state->recv_from_client_buf) { /* partial message which wasn't
> > pushed to device */
> > +        spice_char_device_write_buffer_release(state->chardev_st, state
> > ->recv_from_client_buf);
> > +        state->recv_from_client_buf = NULL;
> > +    }
> > +
> >      if (state->chardev_st) {
> >          if (spice_char_device_client_exists(state->chardev_st, rcc
> > ->client)) {
> >              spice_char_device_client_remove(state->chardev_st, rcc
> > ->client);
> > --
> > 2.5.0
> 
> That's make sense. Have you tested this on migration as well?
> IIRC channel is disconnected during migration.
> 
> cheers,
>   toso
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Hi Christophe,

I just ran across this patch while looking through some bugs and noticed that it
doesn't seem to ever have gone upstream. Is it still necessary?

Jonathon