Re: [RFC PATCH 3/4] Drivers: hv: vmbus: Introduce vmbus_sendpacket_getid()

[Andrea Parri <parri.andrea@gmail.com>]

> > @@ -354,6 +354,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel,
> >  	}
> >  	desc = hv_get_ring_buffer(outring_info) + old_write;
> >  	desc->trans_id = (rqst_id == VMBUS_NO_RQSTOR) ? requestid : rqst_id;
> > +	if (trans_id)
> > +		*trans_id = desc->trans_id;
> 
> This line should *not* read the trans_id out of the ring buffer, since that
> memory is shared with the Hyper-V host and subject to being maliciously
> changed by the host.  Need to set *trans_id only from local variables, and
> somehow ensure the compiler doesn't generate code that reads the value
> from the ring buffer.  Maybe mark the desc->trans_id field as volatile, or cast
> it as such?  Or does WRITE_ONCE() work when setting it?

I'd stick to WRITE_ONCE() (with a comment).

Good catch!

Thanks,
  Andrea