Re: [PATCH] Boot a Live USB device as read-only after persisting some changes.

[fgrose-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org]

From: Frederick Grose <fgrose-agl9EP5VjmEgsBAKwltoeQ@public.gmane.org>

commit 96ff19a87c0be1eca27860aeef5434513b0e8c61
Author: Frederick Grose <fgrose-agl9EP5VjmEgsBAKwltoeQ@public.gmane.org>

    Allow read-only mounting of the base device filesystem, livedev.
    Use read-only loop devices and non-persistent overlay targets.

---
 modules.d/90dmsquash-live/dmsquash-live-root.sh | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/modules.d/90dmsquash-live/dmsquash-live-root.sh b/modules.d/90dmsquash-live/dmsquash-live-root.sh
index 14e84e1..af0022f 100755
--- a/modules.d/90dmsquash-live/dmsquash-live-root.sh
+++ b/modules.d/90dmsquash-live/dmsquash-live-root.sh
@@ -24,7 +24,7 @@ squash_image=$(getarg rd.live.squashimg)
 
 getargbool 0 rd.live.ram -d -y live_ram && live_ram="yes"
 getargbool 0 rd.live.overlay.reset -d -y reset_overlay && reset_overlay="yes"
-getargbool 0 rd.live.overlay.readonly -d -y readonly_overlay && readonly_overlay="--readonly" || readonly_overlay=""
+getargbool 0 rd.live.overlay.readonly -d -y readonly_overlay && readonly_overlay="-r" || readonly_overlay=""
 overlay=$(getarg rd.live.overlay -d overlay)
 
 # CD/DVD media check
@@ -105,17 +105,23 @@ do_live_overlay() {
     setup=""
     if [ -n "$devspec" -a -n "$pathspec" -a -n "$overlay" ]; then
         mkdir -m 0755 /run/initramfs/overlayfs
-        mount -n -t auto $devspec /run/initramfs/overlayfs || :
-        if [ -f /run/initramfs/overlayfs$pathspec -a -w /run/initramfs/overlayfs$pathspec ]; then
-            losetup $OVERLAY_LOOPDEV /run/initramfs/overlayfs$pathspec
-            if [ -n "$reset_overlay" ]; then
-                dd if=/dev/zero of=$OVERLAY_LOOPDEV bs=64k count=1 conv=fsync 2>/dev/null
+        mount -n -t auto -o $liverw $devspec /run/initramfs/overlayfs || :
+        if [ -f /run/initramfs/overlayfs$pathspec ]; then
+            if [ "$liverw" = ro ]; then
+                readonly_overlay='-r'
+            fi
+            losetup $readonly_overlay $OVERLAY_LOOPDEV /run/initramfs/overlayfs$pathspec
+            if [ -w /run/initramfs/overlayfs$pathspec ]; then
+                if [ -n "$reset_overlay" ]; then
+                    dd if=/dev/zero of=$OVERLAY_LOOPDEV bs=64k count=1 conv=fsync 2>/dev/null
+                fi
             fi
             setup="yes"
         fi
         umount -l /run/initramfs/overlayfs || :
     fi
 
+    persistent='P'
     if [ -z "$setup" -o -n "$readonly_overlay" ]; then
         if [ -n "$setup" ]; then
             warn "Using temporary overlay."
@@ -123,6 +129,7 @@ do_live_overlay() {
             warn "Unable to find persistent overlay; using temporary"
             sleep 5
         fi
+        persistent='N'
 
         dd if=/dev/null of=/overlay bs=1024 count=1 seek=$((512*1024)) 2> /dev/null
         if [ -n "$setup" -a -n "$readonly_overlay" ]; then
@@ -136,14 +143,14 @@ do_live_overlay() {
     # set up the snapshot
     sz=$(blockdev --getsz $BASE_LOOPDEV)
     if [ -n "$readonly_overlay" ]; then
-        echo 0 $sz snapshot $BASE_LOOPDEV $OVERLAY_LOOPDEV p 8 | dmsetup create $readonly_overlay live-ro
+        echo 0 $sz snapshot $BASE_LOOPDEV $OVERLAY_LOOPDEV N 8 | dmsetup create $readonly_overlay live-ro
         base="/dev/mapper/live-ro"
         over=$RO_OVERLAY_LOOPDEV
     else
         base=$BASE_LOOPDEV
         over=$OVERLAY_LOOPDEV
     fi
-    echo 0 $sz snapshot $base $over p 8 | dmsetup create live-rw
+    echo 0 $sz snapshot $base $over $persistent 8 | dmsetup create live-rw
 }
 
 # live cd helper function
@@ -217,7 +224,7 @@ fi
 if [ -b "$OSMIN_LOOPDEV" ]; then
     # set up the devicemapper snapshot device, which will merge
     # the normal live fs image, and the delta, into a minimzied fs image
-    echo "0 $( blockdev --getsz $BASE_LOOPDEV ) snapshot $BASE_LOOPDEV $OSMIN_LOOPDEV p 8" | dmsetup create --readonly live-osimg-min
+    echo "0 $( blockdev --getsz $BASE_LOOPDEV ) snapshot $BASE_LOOPDEV $OSMIN_LOOPDEV N 8" | dmsetup create --readonly live-osimg-min
 fi
 
 ROOTFLAGS="$(getarg rootflags)"
-- 
1.8.3.1