Adding support for read/write filesystem images

A user can provide a filesystem image (rootfs.img) inside a compressed
tarball and that filesystem image will be mounted read/write.  This provides
some benefits over a device mapper snapshot overlay, especially when the
live system becomes full.  The boot command line simple needs
"rd.writable.fsimg" added to utilize this feature.

Additional documentation for this option as well as other live boot
options is included.

Signed-off-by: Major Hayden <major@mhtx.net>

dracut.cmdline.7.asc

@@ -733,10 +733,90 @@ rd.znet=qeth,0.0.0600,0.0.0601,0.0.0602,layer2=1,portname=foo
 rd.znet=ctc,0.0.0600,0.0.0601,protocol=bar
 --
 
+Booting live images
+~~~~~~~~~~~~~~~~~~~
+Dracut offers multiple options for live booted images:
+
+=====================
+squashfs with read-only filesystem image::: The system will boot with a read
+only filesystem from the squashfs and apply a writable device mapper snapshot
+over the read only filesystem.  Using this method ensures a relatively fast
+boot and lower RAM usage. Users **must be careful** to avoid writing too many
+blocks to the snapshot volume.  Once the blocks of the snapshot are exhaused,
+the live filesystem becomes unusable and requires a reboot.
++
+The filesystem structure is expected to be:
++
+[listing]
+--
+squashfs.img          |  Squashfs downloaded via network
+   !(mount)
+   /LiveOS
+       |- ext3fs.img  |  Filesystem image to mount read-only
+            !(mount)
+            /bin      |  Live filesystem
+            /boot     |
+            /dev      |
+            ...       |
+--
++
+Dracut uses this method of live booting by default.  No additional command line
+options are required other than **root=live:<URL>** to specify the location
+of your squashed filesystem.
++
+writable filesystem image::: The system will retrieve a compressed filesystem
+image, connect it to a loopback device, and mount it as a writable volume.  More
+RAM is required during boot but the live filesystem is easier to manage if it
+becomes full.  Users can make a filesystem image of any size and that size will
+be maintained when the system boots.
++
+The filesystem structure is expected to be:
++
+[listing]
+--
+rootfs.tgz            |  Compressed tarball containing fileystem image
+   !(unpack)
+   /rootfs.img        |  Filesystem image
+      !(mount)
+      /bin            |  Live filesystem
+      /boot           |
+      /dev            |
+      ...             |
+--
++
+To use this boot option, ensure that **rd.writable_fsimg=1** is in your kernel
+command line and add the **root=live:<URL>** to specify the location
+of your compressed filesystem image tarball.
+=====================
+
+**root=**live:__<url>__::
+Boots a live image retrieved from __<url>__.  Valid handlers: __http, httpd, ftp, tftp__.
++
+[listing]
+.Example
+--
+root=live:http://example.com/liveboot.img
+root=live:ftp://ftp.example.com/liveboot.img
+--
+
+**rd.live.debug=**1::
+Enables debug output from the live boot process.
+
+**rd.live.dir=**__<path>__::
+Specifies the directory within the squashfs where the ext3fs.img or rootfs.img
+can be found.  By default, this is __LiveOS__.
+
+**rd.writable.fsimg=**1::
+Enables writable filesystem support.  The system will boot with a fully 
+writable filesystem without snapshots __(see notes above about available live boot options)__.
+You can use the **rootflags** option to set mount options for the live
+filesystem as well __(see documentation about rootflags in the **Standard** section above)__.
+
+
 Plymouth Boot Splash
 ~~~~~~~~~~~~~~~~~~~~
 **plymouth.enable=0**::
-    disable the plymouth bootsplash completly.
+    disable the plymouth bootsplash completely.
 
 **rd.plymouth=0**::
     disable the plymouth bootsplash only for the initramfs.

modules.d/90dmsquash-live/dmsquash-live-root.sh

@@ -4,6 +4,8 @@
 
 type getarg >/dev/null 2>&1 || . /lib/dracut-lib.sh
 
+command -v unpack_archive >/dev/null || . /lib/img-lib.sh
+
 PATH=/usr/sbin:/usr/bin:/sbin:/bin
 
 if getargbool 0 rd.live.debug -n -y rdlivedebug; then
@@ -26,6 +28,7 @@ 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=""
 overlay=$(getarg rd.live.overlay -d overlay)
+getargbool 0 rd.writable.fsimg -d -y writable_fsimg && writable_fsimg="yes"
 
 # CD/DVD media check
 [ -b $livedev ] && fs=$(blkid -s TYPE -o value $livedev)
@@ -180,9 +183,18 @@ fi
 
 if [ -n "$FSIMG" ] ; then
     BASE_LOOPDEV=$( losetup -f )
-    losetup -r $BASE_LOOPDEV $FSIMG
 
-    do_live_from_base_loop
+    if [ -n "$writable_fsimg" ] ; then
+        # mount the provided fileysstem read/write
+        echo "Unpacking live filesystem (may take some time)"
+        unpack_archive $FSIMG /tmp/fsimg/
+        losetup $BASE_LOOPDEV /tmp/fsimg/rootfs.img
+        echo "0 $( blockdev --getsize $BASE_LOOPDEV ) linear $BASE_LOOPDEV 0" | dmsetup create live-rw
+    else
+        # mount the filesystem read-only and add a dm snapshot for writes
+        losetup -r $BASE_LOOPDEV $FSIMG
+        do_live_from_base_loop
+    fi
 fi
 
 # we might have an embedded fs image on squashfs (compressed live)

modules.d/90dmsquash-live/module-setup.sh

@@ -13,7 +13,7 @@ check() {
 depends() {
     # if dmsetup is not installed, then we cannot support fedora/red hat
     # style live images
-    echo dm rootfs-block
+    echo dm rootfs-block img-lib
     return 0
 }