--- "Pipkins, Jeff" <[EMAIL PROTECTED]> wrote:
> Fascinating. Yes, I'd love to get a copy of your shell script. It would be
> interesting to
> explore this some more, regardless of whether I end up using it for this
> particular application.
> Thanks,
> --Jeff
>
> -----Original Message-----
> From: Christopher Marshall [mailto:[EMAIL PROTECTED]
> Sent: Wed 2007-03-21 9:17 AM
> To: Pipkins, Jeff; [email protected]
> Subject: Re: [uml-user] UML for embedded system development
>
> Jeff:
>
> I don't know if you can use the same code with a host kernel (that manages
> cow files in guest
> kernels), but I can think of at least one way to achieve the same effect.
>
> The unionfs filesystem allows you to stack filesystems on top of each other.
> Say you have two
> directories A, and B, and you mount them on top of each other at directory C.
> C's contents are
> then the union of the files in A and B. When you declare, during the mount,
> that A is read-only
> and B is read-write, the effect is that any modifications to the contents of
> C are stored in B
> without molesting the contents of A. If you delete a file f1 in C (that
> really exists in A/f1),
> the unionfs driver handles that by creating a "whiteout file" in B (B/.wh.f1)
> that indicates
> that
> f1 should no longer exist in the mount at C. If you rewrite a portion of a
> file in C (that
> really
> exists in A), the modified file is written to B.
>
> This has lots of applications. I somtimes use it to see exactly what files a
> program is
> attempting to modify by mounting the root directory as part of a read-only
> stack at /usr/root,
> using /usr/root-rw as the read-write part of the stack. I then chroot into
> /usr/root, run the
> program, and examine the contents of /usr/root-rw to see what files were
> modified / deleted.
>
> In your case, you can alter the boot process by writing a linuxrc file inside
> an initrd.gz to
> setup a union mount of your root filesystem using a ramdisk as a read-write
> stack and the real
> root filesystem as a read-only stack. After every reboot, any changes to the
> root filesystem
> will
> disappear, and new changes will be stored in the ramdisk.
>
> This very technique is how some live-CD or live-DVD distributions of linux
> work.
>
> If you are interested, I have a bash script I use to create the necessary
> files for doing this
> with slackware. There were some nasty details I had to work out that took me
> a while to figure
> out. It may save you some time if you decide to go the same route.
>
> Chris Marshall
>
> --- "Pipkins, Jeff" <[EMAIL PROTECTED]> wrote:
>
> > Is there a way to have an embedded system running just one kernel (i.e.,
> > not UML), but still
> > using a cow file as the root file system?
> >
> > The reason I ask, is that I'm thinking I want to use UML during
> > development, but not ship it
> as
> > part of an embedded system. But the use of the cow file is very
> > attractive, especially since
> > the system can be restored to factory defaults simply by deleting a file.
> >
> > So, does the use of a cow file necessarily require a UML guest kernel? Or
> > can I use the same
> > technique with an ordinary linux kernel?
> >
> > Thanks,
> > --Jeff
> > > -------------------------------------------------------------------------
> > Take Surveys. Earn Cash. Influence the Future of IT
> > Join SourceForge.net's Techsay panel and you'll get the chance to share your
> > opinions on IT & business topics through brief surveys-and earn cash
> > http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV>
> _______________________________________________
> > User-mode-linux-user mailing list
> > [email protected]
> > https://lists.sourceforge.net/lists/listinfo/user-mode-linux-user
> >
>
>
>
Jeff:
Here is the script I used to work out the details of setting up a unionfs mount
of the root
filesystem, where the root filesystem is mounted read-only with a ramdisk
acting as the read-write
area. This is the basic technique several live-CD distributions of linux use.
Let's call is make_union_initrd.sh.
Here's the text:
#!/bin/bash
dev=$1
if [ -z "$1" ] ; then
echo "usage: <hd device>"
exit 1
fi
tree=/boot/initrd-tree
mkinitrd -c -k 2.6.10 -m reiserfs:unionfs
mkdir ${tree}/memory
mkdir ${tree}/union
mkdir ${tree}/union/root
mkdir ${tree}/union/live
mkdir ${tree}/union/live/initrd
cp rc.S ${tree}
(
echo "#!/bin/ash"
echo
echo "mount -n proc /proc -t proc"
echo ". ./load_kernel_modules"
echo
echo "mkdir /memory"
echo "mount -n -t tmpfs none /memory"
echo "mkdir /memory/changes"
echo
echo "mkdir /union"
echo "mkdir /union/root"
echo "mkdir /union/live"
echo
echo "mount -n ${dev} /union/root"
echo "mount -n -t unionfs none /union/live -o
dirs=/memory/changes=rw:/union/root=ro"
echo "mkdir /union/live/initrd"
echo "cat /rc.S > /union/live/etc/rc.d/rc.S"
echo "cd /union/live"
echo
echo "echo \"unmounting proc\""
echo "/bin/umount /proc"
echo "/bin/ash"
echo
echo "pivot_root . initrd"
echo "exec /usr/sbin/chroot . sbin/init -i <dev/console >dev/console 2>&1"
) > /boot/initrd-tree/linuxrc
chmod a+rx /boot/initrd-tree/linuxrc
mkinitrd
mv /boot/initrd.gz /boot/initrd-2.6.10-union.gz
This version of the script assumes you are booting from a root partition on a
hard drive and you
want to use unionfs to mount the root partition as read-only. My reason for
setting it up that
way was to make it easy to change something and see the effects. If I were
working with creating
bootable CDs, I would have to burn another CD every time I did another
experiment, instead of
simply rebooting from a hard drive.
Assuming you are running it from on a slackware workstation the normally boots
from /dev/hda6, you
would invoke the script like this:
make_union_initrd /dev/hda6
Then you would have to add a stanza to your lilo.conf using the resulting
initrz.gz to test it.
In slackware, there is a script for making initrd files called mkinitrd. Close
to the top of this
script, I call it with arguments specifying that I want the resierfs and
unionfs modules pulled
into the initrd.gz for kernel version 2.6.10, which is what I was running back
when I wrote it.
mkinitrd creates the directory tree it uses to create the initrd at
/boot/initrd-tree. If you
call mkinitrd without arguments, it takes whatever is in the tree and sticks it
into
/boot/initrd.gz.
My script, then, calls mkinitrd toward the top to create the directory at
/boot/initrd-tree, makes
some mount points inside /boot/initrd-tree, writes the linuxrc script, and
calls mkinitrd toward
the end to capture the contents of /boot/initrd-tree in /boot/initrd.gz.
Toward the middle of the script you'll notice it writes a line into linuxrc
"/bin/ash". That
causes the booting kernel to give you an ash prompt in the middle of the
process of executing
linuxrc so you can check on things. I used this quite a bit during debugging.
Once you are
booting and hit that point and are done checking things, you need to hit
control-d to exit cleanly
from it. If you type "exit", you'll derail the boot process.
The other tricky parts are the exact mount command that sets up the unionfs
stack, and the
pivot_root + chroot sequence at the end. Notice the "-i" argument to
sbin/init. That forces init
to act as if it were booting the OS and not trying to communicate the an
already running init. I
had to look through the source code for init to discover the -i argument. You
won't find it in
the man page for init.
For some reason I never determined, sbin/init in my linuxrc thinks it is not
booting the OS but
trying to communicate with another init process so I have to force it this way.
Another script detail. The script assumes you have the file rc.S sitting in
the directory you
invoke it from. make_union_initrd.sh copies rc.S into the initrd.gz, where
linuxrc copies it into
the root mount it is about to boot (after the union mount is set up). rc.S is
the slackware boot
script responsible for running fsck on the root filesystem from time to time.
fsck will always
fail when run against a filesystem mounted read-only (which it will be mounted
as, after the union
mount is setup by linuxrc). So I had to modify it by removing the fsck attempt.
You'll have to do something similar for whatever distribution of linux you are
working with.
I hope that is enough to get you started.
Chris Marshall
-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys-and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
_______________________________________________
User-mode-linux-user mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/user-mode-linux-user
