fix: Fix upstream xhyve crash (virtio-net + UEFI)

When using virtio-net with an UEFI firmware (-f bootrom),
hyperkit crashes on an assertion. See the related bug on
xhyve: machyve/xhyve#164

The complete details are not clear to me, but it seems
that during boot, the firmware reads on the interface but
doesn't provide big enough read buffers. It seems that
the space for VLAN headers is missing.

This patch drops the related reads, so normal booting can
occur. Network boot probably doesn't work, though.

Signed-off-by: Antoine Martin <amartin@unowhy.com>

Author: Antoine Martin

src/lib/pci_virtio_net_vmnet.c

@@ -320,7 +320,10 @@ vmn_read(struct vmnet_state *vms, struct iovec *iov, int n) {
 		v.vm_pkt_size += iov[i].iov_len;
 	}
 
-	assert(v.vm_pkt_size >= vms->max_packet_size);
+	if (v.vm_pkt_size < vms->max_packet_size) {
+			fprintf(stderr, "Read buffer too small: %ld bytes. vmnet expected at least %d.\n", v.vm_pkt_size, vms->max_packet_size);
+			return (-2);
+	}
 
 	v.vm_pkt_iov = iov;
 	v.vm_pkt_iovcnt = (uint32_t) n;
@@ -539,14 +542,29 @@ pci_vtnet_tap_rx(struct pci_vtnet_softc *sc)
 
 		len = (int) vmn_read(sc->vms, riov, n);
 
-		if (len < 0 && errno == EWOULDBLOCK) {
-			/*
-			 * No more packets, but still some avail ring
-			 * entries.  Interrupt if needed/appropriate.
-			 */
-			vq_retchain(vq);
-			vq_endchains(vq, 0);
-			return;
+		if (len < 0) {
+			if (len == -2) {
+					/*
+						* Buffer passed is too short. The reason is unclear
+						* to me but it seems that at boot some firmwares pass
+						* buffers of the standard 1514 Ethernet length whereas the
+						* size expected by Mac OS is 1518.
+						*/
+					iov[0].iov_base = dummybuf;
+					iov[0].iov_len = sizeof(dummybuf);
+					(void) vmn_read(sc->vms, iov, 1);
+					vq_endchains(vq, 1);
+					return;
+					
+			} else if (errno == EWOULDBLOCK) {
+					/*
+						* No more packets, but still some avail ring
+						* entries.  Interrupt if needed/appropriate.
+						*/
+					vq_retchain(vq);
+					vq_endchains(vq, 0);
+					return;
+			}
 		}
 
 		/*