Adjust backchannel network setup to require explicit enabling

on the device under test and to use a not-normally-generated interface name, so that use of add-on ethernet devices is not affected in normal operation.

BUG=2962, 2941

(dup of 1810013, git goofiness)

Review URL: http://codereview.chromium.org/1826003

mod_for_test_scripts/100setupTestingInterface

@@ -4,54 +4,63 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-echo "Configuring for eth1 backchannel"
+echo "Configuring for backchannel network device"
+
+testif=eth_test
 
 # Prevent connman from taking control of the backchannel network device.
 ORIG_CONF=${ROOT_FS_DIR}/etc/init/connman.conf
 TEMP_CONF=${ORIG_CONF}.tmp
-sed 's/connmand -W/connmand -I eth1 -W/' ${ORIG_CONF} > ${TEMP_CONF}
+sed "s/connmand -W/connmand -I ${testif} -W/" ${ORIG_CONF} > ${TEMP_CONF}
 mv -f ${TEMP_CONF} ${ORIG_CONF}
 
 # Arrange to run dhclient on the backchannel device but without
 # installing the default route, and stashing said route away for later
 # installation as a host route.
-cat > ${ROOT_FS_DIR}/etc/udev/rules.d/50-backchannel-eth1.rules <<EOF
+cat > ${ROOT_FS_DIR}/etc/udev/rules.d/50-backchannel-network.rules <<EOF
 KERNEL=="eth*", SUBSYSTEMS=="usb", ACTION=="add", RUN+="/sbin/backchannel-setup %k"
-KERNEL=="eth1", SUBSYSTEMS=="usb", ACTION=="remove", RUN+="kill \$(cat /var/run/dhclient-%k.pid)"
+KERNEL=="${testif}", SUBSYSTEMS=="usb", ACTION=="remove", RUN+="kill \$(cat /var/run/dhclient-%k.pid)"
 EOF
 
 cat > ${ROOT_FS_DIR}/sbin/backchannel-setup <<EOF
 #!/bin/sh
 
-if [ -f /var/run/dhclient-eth1.pid ]; then
+testif=${testif}
+
+if [ ! -f /mnt/stateful_partition/etc/enable_backchannel_network ]; then
+    # This mechanism has to be explicitly enabled on the device.
+    exit
+fi
+
+if [ -f /var/run/dhclient-\${testif}.pid ]; then
     # Something else is already going on - perhaps a second
     # USB Ethernet device has been inserted. Let's not mess with it.
     exit
 fi
 
-if [ "\$1" != "eth1" ]; then
+if [ "\$1" != "\${testif}" ]; then
     initctl stop connman
-    # \$1 is the current name of the backchannel device. Swap it with eth1.
-    if ifconfig eth1 > /dev/null 2>&1; then
-        orig_eth1_mac=\$(ifconfig eth1 | awk '/HWaddr/ {print \$5}')
-        ifconfig eth1 down # must be down for nameif to work
-        nameif eth_tmp \${orig_eth1_mac}
+    # \$1 is the current name of the backchannel device. Swap it with testif.
+    if ifconfig \${testif} > /dev/null 2>&1; then
+        orig_mac=\$(ifconfig \${testif} | awk '/HWaddr/ {print \$5}')
+        ifconfig \${testif} down # must be down for nameif to work
+        nameif eth_tmp \${orig_mac}
     fi
     bdev_mac=\$(ifconfig \$1 | awk '/HWaddr/ {print \$5}')
     ifconfig \$1 down # must be down for nameif to work
-    nameif eth1 \${bdev_mac}
-    if [ -n "\${orig_eth1_mac}" ]; then
-        nameif \$1 \${orig_eth1_mac}
+    nameif \${testif} \${bdev_mac}
+    if [ -n "\${orig_mac}" ]; then
+        nameif \$1 \${orig_mac}
     fi
     initctl start connman
 fi
 
 # Bring up the backchannel interface
-dhclient -q -pf /var/run/dhclient-eth1.pid \\
-            -lf /var/run/dhclient-eth1.leases \\
+dhclient -q -pf /var/run/dhclient-\${testif}.pid \\
+            -lf /var/run/dhclient-\${testif}.leases \\
             -cf /etc/dhclient-backchannel.conf \\
             -sf /sbin/dhclient-backchannel-script \\
-         eth1
+         \${testif}
 EOF
 
 chmod +x ${ROOT_FS_DIR}/sbin/backchannel-setup