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trumee
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PostPosted: Mon Aug 20, 2012 8:36 pm    Post subject: Readonly root file system (OpenRC) Reply with quote

Hi all,

I am trying to setup a read only gentoo system following the http://www.gentoo-wiki.info/HOWTO_Read-only_root_filesystem.The wiki recommends to modify /sbin/rc in Step 3. Unfortunately, /sbin/rc is no longer a text file in current gentoo OpenRC.

Is there any alternate way i can mount /etc using unionfs/aufs?

Thanks
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SlashBeast
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PostPosted: Mon Aug 20, 2012 9:32 pm    Post subject: Reply with quote

You can mount tmpfs and aufs from rootfs on the initramfs level. Should be fairly simple but you will need a aufs-enabled kernel and userspace utils in initramfs as well.
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trumee
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PostPosted: Tue Aug 21, 2012 8:06 pm    Post subject: Reply with quote

Thanks for the suggestion. I got a partially working file together. Current problem is that as soon initramfs ends, the / is mounted and my aufs/tmpfs mounts go haywire.

Here is the init script
Code:

#!/bin/busybox sh

export PATH=/usr/local/bin:/usr/bin:/bin

# Mount the /proc and /sys filesystems.
mount -t proc none /proc
mount -t sysfs none /sys

echo /sbin/mdev > /proc/sys/kernel/hotplug
mdev -s


# Mount the root filesystem.
echo "Mount root fs"
mount -o ro /dev/mmcblk0p2 /mnt/root


# test for mount points on aufs file system
[ -d /aufs/etc ] || mkdir -p /aufs/etc

echo "create tmpfs"
# Create tmpfs /etc filesystem in ram.
mount -n -t tmpfs tmpfs /aufs/etc -o size=1M


#combine /mnt/root/etc and /etc with aufs
echo "Mount aufs"
mount -n -t aufs aufs /etc -o dirs=/aufs/etc=rw:/mnt/root/etc=ro

#set in mtab
echo "tmpfs /aufs/etc tmpfs rw,size=1M 0 0" >> /etc/mtab
echo "aufs /etc aufs rw,dirs=/aufs/etc=rw:/etc=ro 0 0" >> /etc/mtab

#cleanup
umount /proc
umount /sys

# Boot the real thing.
exec switch_root /mnt/root /sbin/init


And here is the output of df -h
Code:

 Filesystem      Size  Used Avail Use% Mounted on
rootfs          5.5G  3.8G  1.5G  72% /
/dev/mmcblk0p2  5.5G  3.8G  1.5G  72% /
tmpfs           5.5G  3.8G  1.5G  72% /aufs/etc
aufs            5.5G  3.8G  1.5G  72% /etc
tmpfs           117M  148K  117M   1% /run
rc-svcdir       1.0M   56K  968K   6% /lib/rc/init.d
cgroup_root      10M     0   10M   0% /sys/fs/cgroup
udev             10M     0   10M   0% /dev
shm             117M     0  117M   0% /dev/shm
/dev/mmcblk0p1   70M   14M   56M  20% /boot


The tmpfs partition is the whole of rootfs instead of 1MB of size. Any idea what is wrong?
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SlashBeast
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PostPosted: Thu Aug 23, 2012 6:33 pm    Post subject: Reply with quote

You messed a bit the mountpoints, you do switchroot to /mnt/root so you want to mount aufs there, something like:

Code:

mount -o ro /dev/mmcblk0p2 /mnt/root
mount -n -t tmpfs etc-tmpfs /aufs/etc -o size=1M
mount -n -t aufs etc-aufs /mnt/root/etc -o dirs=/aufs/etc=rw:/mnt/root/etc=ro


first it mounts as read-only the /mnt/root
then it mounts tmpfs with name 'etc-tmpfs' inside /aufs/etc
on the end it mounts a aufs filesystem under /mnt/root/etc, with /aufs/etc as read-write and /mnt/root/etc as readonly branch.

and yes you can use mountpoint and readonly branch as the same path, then you switch_root to /mnt/root where /etc is read-write.

Also instead of writing mtab I would suggest leave it empty OR link it to /proc/mounts.
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trumee
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PostPosted: Thu Aug 23, 2012 9:35 pm    Post subject: Reply with quote

Thanks i have modified the init file to:

Code:

#!/bin/busybox sh

export PATH=/usr/local/bin:/usr/bin:/bin

# Mount the /proc and /sys filesystems.
mount -t proc none /proc
mount -t sysfs none /sys

echo /sbin/mdev > /proc/sys/kernel/hotplug
mdev -s


# Mount the root filesystem.
echo "Mount root fs"
mount -o ro /dev/mmcblk0p2 /mnt/root


# test for mount points on aufs file system
[ -d /aufs/etc ] || mkdir -p /aufs/etc

echo "create tmpfs"
# Create tmpfs /etc filesystem in ram.
mount -n -t tmpfs etc-tmpfs /aufs/etc -o size=1M


#combine /mnt/root/etc and /etc with aufs
echo "Mount aufs"
mount -n -t aufs etc-aufs /mnt/root/etc -o dirs=/aufs/etc=rw:/mnt/root/etc=ro

#set in mtab
#echo "tmpfs /aufs tmpfs rw,size=1M 0 0" >> /etc/mtab
#echo "aufs /etc aufs rw,dirs=/aufs=rw:/etc=ro 0 0" >> /etc/mtab

#cleanup
umount /proc
umount /sys

# Boot the real thing.
exec switch_root /mnt/root /sbin/init


The mounts after boot (/etc/mtab symlinked to /proc/mounts) looks like the following.
Code:

#df -h
Filesystem      Size  Used Avail Use% Mounted on
rootfs          5.5G  3.9G  1.4G  74% /
/dev/mmcblk0p2  5.5G  3.9G  1.4G  74% /
etc-tmpfs       5.5G  3.9G  1.4G  74% /aufs/etc
etc-aufs        1.0M   92K  932K   9% /etc
tmpfs           117M  156K  117M   1% /run
rc-svcdir       1.0M   56K  968K   6% /lib/rc/init.d
cgroup_root      10M     0   10M   0% /sys/fs/cgroup
udev             10M     0   10M   0% /dev
shm             117M     0  117M   0% /dev/shm
/dev/mmcblk0p1   70M   14M   56M  20% /boot
tmpfs           5.0M  4.0K  5.0M   1% /tmp



My /etc/fstab is as follows
Code:

/dev/mmcblk0p1  /boot   vfat    defaults,ro             0 0
/dev/mmcblk0p2  /       ext4    defaults,ro,noatime     0 0
/dev/mmcblk0p3   none    swap    sw                   0 0
tmpfs      /tmp   tmpfs  size=5M         0 0
tmpfs      /var/run   tmpfs  size=10M         0 0


If i write anything to /etc, i dont see anything getting written to /aufs/etc?

Code:

# mkdir /etc/abc
# ls -la /etc/abc
total 0
drwxr-xr-x  2 root root  40 Aug 23 22:31 .
drwxrwxrwt 47 root root 160 Aug 23 22:31 ..

# ls -la /aufs/etc/
total 8
drwxr-xr-x 2 root root 4096 Aug 23 22:24 .
drwxr-xr-x 3 root root 4096 Aug 23 22:24 ..



Also, how do i umount aufs so that i can make permanent changes to etc?

Code:

# mount / -o remount,rw
# umount -n /aufs/etc/
umount: /aufs/etc: not mounted

# umount -n /etc/
umount: /etc: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
umount: /etc: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

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SlashBeast
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PostPosted: Fri Aug 24, 2012 10:46 am    Post subject: Reply with quote

the /aufs/etc is mounted outside /mnt/root, after switch_root you dont have access there as you are on different layer.

you mount your rootfs which container real /etc as read-only, if you want make permament maybe implament something to preserve /etc content, like on shutdown tar /etc and store it on /boot. then after mouting new aufs initramfs would unpack it to /mnt/root/etc or something. Umounting filesystem where something have open descriptior like /etc is impossible without shutting it down which pretty much means reboot.
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