[PATCH] swsusup with dm-crypt mini howto

The attached patch contains a mini howto for using dm-crypt together with
swsusp.

Signed-off-by: Andreas Steinmetz <ast@domdv.de>
Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/power/swsusp-dmcrypt.txt

@@ -0,0 +1,138 @@
+Author: Andreas Steinmetz <ast@domdv.de>
+
+
+How to use dm-crypt and swsusp together:
+========================================
+
+Some prerequisites:
+You know how dm-crypt works. If not, visit the following web page:
+http://www.saout.de/misc/dm-crypt/
+You have read Documentation/power/swsusp.txt and understand it.
+You did read Documentation/initrd.txt and know how an initrd works.
+You know how to create or how to modify an initrd.
+
+Now your system is properly set up, your disk is encrypted except for
+the swap device(s) and the boot partition which may contain a mini
+system for crypto setup and/or rescue purposes. You may even have
+an initrd that does your current crypto setup already.
+
+At this point you want to encrypt your swap, too. Still you want to
+be able to suspend using swsusp. This, however, means that you
+have to be able to either enter a passphrase or that you read
+the key(s) from an external device like a pcmcia flash disk
+or an usb stick prior to resume. So you need an initrd, that sets
+up dm-crypt and then asks swsusp to resume from the encrypted
+swap device.
+
+The most important thing is that you set up dm-crypt in such
+a way that the swap device you suspend to/resume from has
+always the same major/minor within the initrd as well as
+within your running system. The easiest way to achieve this is
+to always set up this swap device first with dmsetup, so that
+it will always look like the following:
+
+brw-------  1 root root 254, 0 Jul 28 13:37 /dev/mapper/swap0
+
+Now set up your kernel to use /dev/mapper/swap0 as the default
+resume partition, so your kernel .config contains:
+
+CONFIG_PM_STD_PARTITION="/dev/mapper/swap0"
+
+Prepare your boot loader to use the initrd you will create or
+modify. For lilo the simplest setup looks like the following
+lines:
+
+image=/boot/vmlinuz
+initrd=/boot/initrd.gz
+label=linux
+append="root=/dev/ram0 init=/linuxrc rw"
+
+Finally you need to create or modify your initrd. Lets assume
+you create an initrd that reads the required dm-crypt setup
+from a pcmcia flash disk card. The card is formatted with an ext2
+fs which resides on /dev/hde1 when the card is inserted. The
+card contains at least the encrypted swap setup in a file
+named "swapkey". /etc/fstab of your initrd contains something
+like the following:
+
+/dev/hda1   /mnt    ext3      ro                            0 0
+none        /proc   proc      defaults,noatime,nodiratime   0 0
+none        /sys    sysfs     defaults,noatime,nodiratime   0 0
+
+/dev/hda1 contains an unencrypted mini system that sets up all
+of your crypto devices, again by reading the setup from the
+pcmcia flash disk. What follows now is a /linuxrc for your
+initrd that allows you to resume from encrypted swap and that
+continues boot with your mini system on /dev/hda1 if resume
+does not happen:
+
+#!/bin/sh
+PATH=/sbin:/bin:/usr/sbin:/usr/bin
+mount /proc
+mount /sys
+mapped=0
+noresume=`grep -c noresume /proc/cmdline`
+if [ "$*" != "" ]
+then
+  noresume=1
+fi
+dmesg -n 1
+/sbin/cardmgr -q
+for i in 1 2 3 4 5 6 7 8 9 0
+do
+  if [ -f /proc/ide/hde/media ]
+  then
+    usleep 500000
+    mount -t ext2 -o ro /dev/hde1 /mnt
+    if [ -f /mnt/swapkey ]
+    then
+      dmsetup create swap0 /mnt/swapkey > /dev/null 2>&1 && mapped=1
+    fi
+    umount /mnt
+    break
+  fi
+  usleep 500000
+done
+killproc /sbin/cardmgr
+dmesg -n 6
+if [ $mapped = 1 ]
+then
+  if [ $noresume != 0 ]
+  then
+    mkswap /dev/mapper/swap0 > /dev/null 2>&1
+  fi
+  echo 254:0 > /sys/power/resume
+  dmsetup remove swap0
+fi
+umount /sys
+mount /mnt
+umount /proc
+cd /mnt
+pivot_root . mnt
+mount /proc
+umount -l /mnt
+umount /proc
+exec chroot . /sbin/init $* < dev/console > dev/console 2>&1
+
+Please don't mind the weird loop above, busybox's msh doesn't know
+the let statement. Now, what is happening in the script?
+First we have to decide if we want to try to resume, or not.
+We will not resume if booting with "noresume" or any parameters
+for init like "single" or "emergency" as boot parameters.
+
+Then we need to set up dmcrypt with the setup data from the
+pcmcia flash disk. If this succeeds we need to reset the swap
+device if we don't want to resume. The line "echo 254:0 > /sys/power/resume"
+then attempts to resume from the first device mapper device.
+Note that it is important to set the device in /sys/power/resume,
+regardless if resuming or not, otherwise later suspend will fail.
+If resume starts, script execution terminates here.
+
+Otherwise we just remove the encrypted swap device and leave it to the
+mini system on /dev/hda1 to set the whole crypto up (it is up to
+you to modify this to your taste).
+
+What then follows is the well known process to change the root
+file system and continue booting from there. I prefer to unmount
+the initrd prior to continue booting but it is up to you to modify
+this.