image_signing: avoid using mount -o rw,remount

The comments said remount was avoided due to loop offset usage which,
while true, isn't the only reason.  We can't remount rw without first
calling enable_rw_mount to clear the invalid bits in the fs header.
Update the comment to reflect that and switch to our mount helper as
it will call that for us.

BUG=chromium:938958
TEST=precq passes & signing unittests pass
BRANCH=None

Change-Id: I1063bc84befebddc942a3dec05e8f33ea834db30
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1522089
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>

2 files changed, 9 insertions, 6 deletions

diff --git a/scripts/image_signing/set_chronos_password.sh b/scripts/image_signing/set_chronos_password.sh
index 751f02b3..0b8bce80 100755
--- a/scripts/image_signing/set_chronos_password.sh
+++ b/scripts/image_signing/set_chronos_password.sh
@@ -42,11 +42,12 @@ main() {
       echo "Password is already set [use --force if you'd like to update it]"
       exit 1
     fi
-    # Prepare for remounting read/write.
-    sudo mount -o remount,rw "${rootfs}"
-  else
-    mount_loop_image_partition "${loopdev}" 3 "${rootfs}"
+    # Prepare for remounting read/write.  We can't use `mount -o rw,remount`
+    # because of the bits in the ext4 header we've set to block that.  See
+    # enable_rw_mount for details.
+    sudo umount "${rootfs}"
   fi
+  mount_loop_image_partition "${loopdev}" 3 "${rootfs}"
   change_chronos_password "$rootfs" "$chronos_password"
   touch "$image"  # Updates the image modification time.
   echo "Password Set."
diff --git a/scripts/image_signing/tag_image.sh b/scripts/image_signing/tag_image.sh
index 53afe788..b130fa19 100755
--- a/scripts/image_signing/tag_image.sh
+++ b/scripts/image_signing/tag_image.sh
@@ -206,8 +206,10 @@ process_all_tags "${rootfs}" ${FLAGS_FALSE}
 process_all_lsb_mods "${rootfs}" ${FLAGS_FALSE}
 
 if [ ${g_modified} = ${FLAGS_TRUE} ]; then
-  # Remount as RW.
-  sudo mount -o rw,remount "${rootfs}"
+  # Remount as RW.  We can't use `mount -o rw,remount` because of the bits in
+  # the ext4 header we've set to block that.  See enable_rw_mount for details.
+  sudo umount "${rootfs}"
+  mount_loop_image_partition "${loopdev}" 3 "${rootfs}"
 
   # second round, apply the modification to image.
   process_all_tags "${rootfs}" ${FLAGS_TRUE}