pivot_root off an initramfs broken

[Martin Hicks <mort@wildopensource.com>]

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This is the conclusion that I've come to.  For background please see:

http://www.zytor.com/pipermail/klibc/2003-November/000223.html
which finishes up here:
http://www.zytor.com/pipermail/klibc/2003-December/000254.html


The executive summary is this:

If I boot a 2.6 kernel that first execs /sbin/kinit on the initramfs,
does some setup (mostly to find the real root filesystem), then
pivot_roots over to the real root filesystem and execs /sbin/init the
kernel spins inside check_mnt() while mounting /proc in the initscripts.

The reason is because the vfsmount structures for the root filesystem
looks like this:

mnt->mnt_devname = /dev/root                    (this is good)
mnt->mnt_parent->mnt_devname = rootfs           (this is good, I think)
mnt->mnt_parent->mnt_parent->mnt_devname = /dev/root   (wrong!)

I think it's in pivot_root that something is going wrong.

The attached patch fixes the problem, but is just a hack.
Any comments on how to really fix this?

TIA,
mh

-- 
Martin Hicks                Wild Open Source Inc.
mort@wildopensource.com     613-266-2296


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===== fs/namespace.c 1.50 vs edited =====
--- 1.50/fs/namespace.c	Tue Oct  7 21:52:02 2003
+++ edited/fs/namespace.c	Wed Dec  3 18:48:59 2003
@@ -1064,6 +1064,9 @@
 	detach_mnt(user_nd.mnt, &root_parent);
 	attach_mnt(user_nd.mnt, &old_nd);
 	attach_mnt(new_nd.mnt, &root_parent);
+	if (new_nd.mnt == new_nd.mnt->mnt_parent->mnt_parent &&
+	    new_nd.mnt != new_nd.mnt->mnt_parent)  /* initramfs loop */
+		new_nd.mnt->mnt_parent->mnt_parent = new_nd.mnt->mnt_parent;
 	spin_unlock(&vfsmount_lock);
 	chroot_fs_refs(&user_nd, &new_nd);
 	security_sb_post_pivotroot(&user_nd, &new_nd);