Don't use sudo -v, which was having issues (hanging bots).

enter_chroot.sh

 #!/bin/bash
 
 # Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Script to enter the chroot environment
 
 # --- BEGIN COMMON.SH BOILERPLATE ---
 # Load common CrOS utilities.  Inside the chroot this file is installed in
@@ skipping 109 matching lines @@
 AUTOMOUNT_PREF="/apps/nautilus/preferences/media_automount"
 SAVED_AUTOMOUNT_PREF_FILE="/tmp/.automount_pref"
 
 sudo chmod 0777 "$FLAGS_chroot/var/lock"
 
 LOCKFILE="$FLAGS_chroot/var/lock/enter_chroot"
 
 function setup_env {
   # Validate sudo timestamp before entering the critical section so that we
   # don't stall for a password while we have the lockfile.
-  sudo -v
+  # Don't use sudo -v since that has issues on machines w/ no password.
+  sudo echo "" > /dev/null
 
   (
     flock 200
     echo $$ >> "$LOCKFILE"
 
     info "Mounting chroot environment."
 
     # Mount only if not already mounted
     MOUNTED_PATH="$(readlink -f "$FLAGS_chroot/proc")"
     if [ -z "$(mount | grep -F "on $MOUNTED_PATH ")" ]; then
@@ skipping 90 matching lines @@
         warn "-- Note: outside the chroot."
       fi
     fi
 
   ) 200>>"$LOCKFILE" || die "setup_env failed"
 }
 
 function teardown_env {
   # Validate sudo timestamp before entering the critical section so that we
   # don't stall for a password while we have the lockfile.
-  sudo -v
+  # Don't use sudo -v since that has issues on machines w/ no password.
+  sudo echo "" > /dev/null
 
   # Only teardown if we're the last enter_chroot to die
   (
     flock 200
 
     # check each pid in $LOCKFILE to see if it's died unexpectedly
     TMP_LOCKFILE="$LOCKFILE.tmp"
 
     echo -n > "$TMP_LOCKFILE"  # Erase/reset temp file
     cat "$LOCKFILE" | while read PID; do
@@ skipping 100 matching lines @@
 # Run command or interactive shell.  Also include the non-chrooted path to
 # the source trunk for scripts that may need to print it (e.g.
 # build_image.sh).
 sudo -- chroot "$FLAGS_chroot" sudo -i -u $USER $CHROOT_PASSTHRU \
   EXTERNAL_TRUNK_PATH="${FLAGS_trunk}" LANG=C SSH_AGENT_PID="${SSH_AGENT_PID}" \
   SSH_AUTH_SOCK="${SSH_AUTH_SOCK}" "$@"
 
 # Remove trap and explicitly unmount
 trap - EXIT
 teardown_env