Switch_root / pivot_root help needed

[Rob Landley]

On Friday 17 March 2006 4:42 am, Patrick Zwahlen wrote:
> Hi there,
>
> I am new to this mailinglist, and would need some help on solving a
> switch_root problem. I couldn't find examples in the list, but I might
> have looked too fast... :-)
>
> I am working on a project that will ultimately run on a Soekris box, but
> I am doing my current tests using qemu.

svn 14007 applied a fix to switch_root after 1.1.0 came out.  1.1.1 comes out 
monday, might want to grab a current snapshot and test to see if stuff works 
for you...

> Anyway, here is what I am doing:
>
> - I boot a 2.6.15.6 kernel (i586)
> - kernel uncompresses an initramfs (cpio archive passed from the
> bootloader)
> - /init is an ash script
> - I mount /proc /sys
> - I mount /dev as tmpfs (this is what RedHat are doing in their initrd,
> not sure why...)

Mostly to provide an empty directory so you don't have to worry about deleting 
stuff from last time.

> - I populate /dev using udevstart

Might want to look at mdev, but it's your system.

> - I mount my hard drive, which contains my new root filesystems
> (currently ext2 files)
> 	mount /dev/hda2 /mnt/hda2
> - I loopback mount my rootfs
> 	mkdir /mnt/newroot
> 	losetup /dev/loop0 /mnt/hda2/rootfs.ext2
> 	mount /dev/loop0 /mnt/newroot
> - I mount /proc in my new root

You can mount --move the existing /proc and /sys mounts, by the way...

> - And finally:
> 	cd /mnt/newroot
> 	switch_root -c /dev/ttyS0 . /sbin/init
> 	switch_root: not rootfs



> The code generating the error is in switch_root.c
> =================================================
> 	if (stat("/init", &st1) || !S_ISREG(st1.st_mode) || statfs("/",
> &stfs) ||
> 		(stfs.f_type != RAMFS_MAGIC && stfs.f_type !=
> TMPFS_MAGIC) ||
> 		getpid() != 1)
> 	{
> 		bb_error_msg_and_die("not rootfs");
> 	}
> =================================================
> However, I don't understand which test is failing.

You can break 'em up a bit and stick printf() calls between them.

> My /init does exist, 
> but it is a symbolic link to /etc/rc.sysinit (my actual ash script).

Looks like the S_ISREG() is hitting you then.  Yeah, a symlink is legitimate, 
I should tweak that...

> My 
> "/" is not listed when I do a "mount",

cat /proc/mounts

> but it is definitely the kernel's 
> rootfs. And finally, I am pretty sure I am PID 1. I am clearly wrong
> somewhere here :-)
>
> I have also done a test with pivot_root:
> 	cd /mnt/newroot
> 	pivot_root . Initrd
> 	pivot_root: pivot_root: Invalid argument

pivot_root won't work with rootfs.  You can't relocate/umount rootfs.

> At this point, I am stuck ! Are there special kernel requirements ? Does
> busybox's switch_root/pivot_root depends on other applets ?

No, I'm guessing it's the S_ISREG() test.  I suppose we don't really _need_ 
that...

> Thanks a lot for your help and support. - Patrick -

Rob
-- 
Never bet against the cheap plastic solution.