Function tunwrite

freebsd/net/if_tuntap.c:1840–1900

* the cdevsw write interface - an atomic write is a packet - or else! */

 * the cdevsw write interface - an atomic write is a packet - or else!
 */
static	int
tunwrite(struct cdev *dev, struct uio *uio, int flag)
{
	struct virtio_net_hdr_mrg_rxbuf vhdr;
	struct tuntap_softc *tp;
	struct ifnet	*ifp;
	struct mbuf	*m;
	uint32_t	mru;
	int		align, vhdrlen, error;
	bool		l2tun;

	tp = dev->si_drv1;
	ifp = TUN2IFP(tp);
	TUNDEBUG(ifp, "tunwrite\n");
	if ((ifp->if_flags & IFF_UP) != IFF_UP)
		/* ignore silently */
		return (0);

	if (uio->uio_resid == 0)
		return (0);

	l2tun = (tp->tun_flags & TUN_L2) != 0;
	mru = l2tun ? TAPMRU : TUNMRU;
	vhdrlen = tp->tun_vhdrlen;
	align = 0;
	if (l2tun) {
		align = ETHER_ALIGN;
		mru += vhdrlen;
	} else if ((tp->tun_flags & TUN_IFHEAD) != 0)
		mru += sizeof(uint32_t);	/* family */
	if (uio->uio_resid < 0 || uio->uio_resid > mru) {
		TUNDEBUG(ifp, "len=%zd!\n", uio->uio_resid);
		return (EIO);
	}

	if (vhdrlen > 0) {
		error = uiomove(&vhdr, vhdrlen, uio);
		if (error != 0)
			return (error);
		TUNDEBUG(ifp, "txvhdr: f %u, gt %u, hl %u, "
		    "gs %u, cs %u, co %u\n", vhdr.hdr.flags,
		    vhdr.hdr.gso_type, vhdr.hdr.hdr_len,
		    vhdr.hdr.gso_size, vhdr.hdr.csum_start,
		    vhdr.hdr.csum_offset);
	}

	if ((m = m_uiotombuf(uio, M_NOWAIT, 0, align, M_PKTHDR)) == NULL) {
		if_inc_counter(ifp, IFCOUNTER_IERRORS, 1);
		return (ENOBUFS);
	}

	m->m_pkthdr.rcvif = ifp;
#ifdef MAC
	mac_ifnet_create_mbuf(ifp, m);
#endif

	if (l2tun)
		return (tunwrite_l2(tp, m, vhdrlen > 0 ? &vhdr : NULL));