BPF: Start with the EOF marker set

This will allow us to abort any BPF read loop if it's reset.
While here, delay restarting DHCP slightly to allow privsep builds
to recover the same error.

Fixes #481.

Author: rsmarples

src/bpf.c

@@ -170,6 +170,7 @@ bpf_open(const struct interface *ifp,
 	if (bpf == NULL)
 		return NULL;
 	bpf->bpf_ifp = ifp;
+	bpf->bpf_flags = BPF_EOF;
 
 	/* /dev/bpf is a cloner on modern kernels */
 	bpf->bpf_fd = open("/dev/bpf", BPF_OPEN_FLAGS);
@@ -218,10 +219,12 @@ bpf_open(const struct interface *ifp,
 	/* Get the required BPF buffer length from the kernel. */
 	if (ioctl(bpf->bpf_fd, BIOCGBLEN, &ibuf_len) == -1)
 		goto eexit;
+
 	bpf->bpf_size = (size_t)ibuf_len;
 	bpf->bpf_buffer = malloc(bpf->bpf_size);
 	if (bpf->bpf_buffer == NULL)
 		goto eexit;
+
 	return bpf;
 
 eexit:

src/dhcp.c

@@ -2236,12 +2236,18 @@ dhcp_arp_defend_failed(struct arp_state *astate)
 {
 	struct interface *ifp = astate->iface;
 	struct dhcp_state *state = D_STATE(ifp);
+	unsigned int delay;
 
 	if (!(ifp->options->options & (DHCPCD_INFORM | DHCPCD_STATIC)))
 		dhcp_decline(ifp);
 	dhcp_drop(ifp, "EXPIRED");
 	dhcp_unlink(ifp->ctx, state->leasefile);
-	dhcp_start1(ifp);
+
+	// Delay restarting to give time for the BPF ARP process to exit
+	// as we may spawn a new one with a different filter fairly quickly
+	delay = MSEC_PER_SEC +
+		(arc4random_uniform(MSEC_PER_SEC * 2) - MSEC_PER_SEC);
+	eloop_timeout_add_msec(ifp->ctx->eloop, delay, dhcp_start1, ifp);
 }
 #endif
 

src/if-linux.c

@@ -1850,6 +1850,7 @@ bpf_open(const struct interface *ifp,
 	if (bpf == NULL)
 		return NULL;
 	bpf->bpf_ifp = ifp;
+	bpf->bpf_flags = BPF_EOF;
 
 	/* Allocate a suitably large buffer for a single packet. */
 	bpf->bpf_size = ETH_FRAME_LEN;