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Subject: Re: MS_NOUSER and rootfs
From: Thayne Harbaugh <tharbaugh@lnxi.com>
Reply-To: tharbaugh@lnxi.com
To: Andries Brouwer <aebr@win.tue.nl>
Cc: Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>,
       lkml <linux-kernel@vger.kernel.org>,
       Al Viro <viro@parcelfarce.linux.theplanet.co.uk>,
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Organization: Linux Networx
Date: Fri, 07 Jan 2005 15:54:37 -0700
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On Fri, 2005-01-07 at 22:45 +0100, Andries Brouwer wrote:
> On Fri, Jan 07, 2005 at 01:53:32PM -0700, Thayne Harbaugh wrote:
> > On Fri, 2005-01-07 at 19:32 +0200, Denis Vlasenko wrote:
> > > On Thursday 06 January 2005 17:08, Thayne Harbaugh wrote:
> > > > What is the purpose of the MS_NOUSER flag serve and why is it set on
> > > > rootfs?
> 
> Some random docs say

Much thanks for the randomness.

> The <tt>FS_NOMOUNT</tt> flag says that this filesystem must never
> be mounted from userland, but is used only kernel-internally.
> This flag was introduced in 2.3.99-pre7 and disappeared in Linux 2.5.22.
> This was used, for example, for pipefs, the implementation of Unix pipes
> using a kernel-internal filesystem (see <tt>fs/pipe.c</tt>).
> Even though the flag has disappeared, the concept remains,
> and is now represented by the MS_NOUSER flag.

Okay, I can buy that there are filesystems, such as pipefs, that
shouldn't be mounted from userland.  I'm wondering, however, why that
applies to rootfs.  Is it a security reason (if it is, why - I'd like to
understand), is it a technical reason to simplify some VFS
requirement/limitation, or was it simply picked because it seemed to
make sense but it could be changed if something else made more sense?

Here's my application:

I have an early-userspace with early init programs and kernel modules.
Hotplug events load drivers and then the early init mounts the real
root.  I want to take the modules from the early-userspace to the real
root by a mount bind of /lib/modules to /newroot/lib/modules.  The mount
bind is the slickest way to do it rather than copying everything from /
to /newroot.  That's where the MS_NOUSER is getting in the way.

I've removed MS_NOUSER from rootfs_get_sb() in my tree and things work
great.  I'm just wondering if this might cause some real, unforeseen
problem or if it's something that might be changed in the main tree.

-- 
Thayne Harbaugh
Linux Networx

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