Function if_simloop

freebsd/net/if_loop.c:285–367

* if_simloop() * * This function is to support software emulation of hardware loopback, * i.e., for interfaces with the IFF_SIMPLEX attribute. Since they can't * hear their own broadcasts, we create a copy of the packet that we * would normally receive via a hardware loopback. * * This function expects the packet to include the media header of length hlen. */

 * This function expects the packet to include the media header of length hlen.
 */
int
if_simloop(struct ifnet *ifp, struct mbuf *m, int af, int hlen)
{
	int isr;

	M_ASSERTPKTHDR(m);
	m_tag_delete_nonpersistent(m);
	m->m_pkthdr.rcvif = ifp;

#ifdef MAC
	mac_ifnet_create_mbuf(ifp, m);
#endif

	/*
	 * Let BPF see incoming packet in the following manner:
	 *  - Emulated packet loopback for a simplex interface
	 *    (net/if_ethersubr.c)
	 *	-> passes it to ifp's BPF
	 *  - IPv4/v6 multicast packet loopback (netinet(6)/ip(6)_output.c)
	 *	-> not passes it to any BPF
	 *  - Normal packet loopback from myself to myself (net/if_loop.c)
	 *	-> passes to lo0's BPF (even in case of IPv6, where ifp!=lo0)
	 */
	if (hlen > 0) {
		if (bpf_peers_present(ifp->if_bpf)) {
			bpf_mtap(ifp->if_bpf, m);
		}
	} else {
		if (bpf_peers_present(V_loif->if_bpf)) {
			if ((m->m_flags & M_MCAST) == 0 || V_loif == ifp) {
				/* XXX beware sizeof(af) != 4 */
				u_int32_t af1 = af;

				/*
				 * We need to prepend the address family.
				 */
				bpf_mtap2(V_loif->if_bpf, &af1, sizeof(af1), m);
			}
		}
	}

	/* Strip away media header */
	if (hlen > 0) {
		m_adj(m, hlen);
#ifndef __NO_STRICT_ALIGNMENT
		/*
		 * Some archs do not like unaligned data, so
		 * we move data down in the first mbuf.
		 */
		if (mtod(m, vm_offset_t) & 3) {
			KASSERT(hlen >= 3, ("if_simloop: hlen too small"));
			bcopy(m->m_data,
			    (char *)(mtod(m, vm_offset_t)
				- (mtod(m, vm_offset_t) & 3)),
			    m->m_len);
			m->m_data -= (mtod(m,vm_offset_t) & 3);
		}
#endif