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<tr align="left"><td valign="top" bgcolor="#e0e0e0">Fun and Games<br>
<font size="-1"><a href="http://user-mode-linux.sourceforge.net/cgi-bin/hangman">Kernel Hangman</a></font>
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<font size="-1"><a href="sdotm.html">Disaster of the Month</a></font>
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<td valign="top" align="left">
<center>
<h3>Host file access</h3>
</center>
If you want to access files on the host machine from inside UML, you
can treat it as a separate machine and either nfs mount directories
from the host or copy files into the virtual machine with scp or rcp.
However, since UML is running on the the host, it can access those
files just like any other process and make them available inside the
virtual machine without needing to use the network.
<p>
This is now possible with the hostfs virtual filesystem. With it, you
can mount a host directory into the UML filesystem and access the
files contained in it just as you would on the host.
<p>
Note that hostfs is currently not available on 2.5. The reason is
that there was an fs.h rework early in 2.5 which required filesystem
changes, and I haven't got around to updating hostfs to those changes.
<a name="Using hostfs"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Using hostfs</font>
</b>
</td>
</tr>
</table>
<blockquote head="Using hostfs">
To begin with, make sure that hostfs is available inside the virtual
machine with <blockquote>
<tt>
<font color="#000000" where="UML">UML# cat /proc/filesystems</font>
</tt>
</blockquote>
hostfs should be listed. If it's not, either rebuild the kernel with
hostfs configured into it or make sure that hostfs is built as a
module and available inside the virtual machine, and insmod it.
<p>
Now all you need to do is run mount:
<blockquote>
<tt>
<font color="#000000" where="UML">UML# mount none /mnt/host -t hostfs</font>
</tt>
</blockquote>
will mount the host's / on the virtual machine's /mnt/host.
<p>
If you don't want to mount the host root directory, then you can
specify a subdirectory to mount with the -o switch to mount:
<blockquote>
<tt>
<font color="#000000" where="UML">UML# mount none /mnt/home -t hostfs -o /home</font>
</tt>
</blockquote>
will mount the hosts's /home on the virtual machine's /mnt/home.
</blockquote>
<a name="hostfs command line options"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">hostfs command line options</font>
</b>
</td>
</tr>
</table>
<blockquote head="hostfs command line options">
There is a hostfs option available on the UML command line which can
be used confine all hostfs mounts to a host directory hierarchy or to
prevent a hostfs user from destroying data on the host. The format is
<blockquote>
<tt>
<font color="#000000"> hostfs=<i>directory</i>,<i>options</i>
</font>
</tt>
</blockquote>
The only option available at present is 'append', which forces all
files to be opened in append mode and disallows any deletion of files.
<p>
To specify append mode without confining hostfs to a host directory,
just leave out the directory name so that the argument begins with a
comma:
<blockquote>
<tt>
<font color="#000000"> hostfs=,append</font>
</tt>
</blockquote>
</blockquote>
<a name="hostfs as the root filesystem"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">hostfs as the root filesystem</font>
</b>
</td>
</tr>
</table>
<blockquote head="hostfs as the root filesystem">
It's possible to boot from a directory hierarchy on the host using hostfs
rather than using the standard filesystem in a file.
<p>To start, you need that hierarchy. The easiest way is to loop mount an
existing root_fs file:
<blockquote>
<tt>
<font color="#000000" where="host root">host# mount root_fs uml_root_dir -o loop</font>
</tt>
</blockquote>
You need to change the filesystem type of / in etc/fstab to be 'hostfs', so
that line looks like this:
<tt>
<pre>
<font size="-1">none / hostfs defaults 1 1
</font>
</pre>
</tt>
Then you need to chown to yourself all the files in that directory that are
owned by root. This worked for me:
<blockquote>
<tt>
<font color="#000000" where="host root">host# find . -uid 0 -exec chown jdike {} \;</font>
</tt>
</blockquote>
If you don't want to do that because that's a filesystem image that
you boot as a disk, then run UML as root instead.
<p>
Next, make sure that your UML kernel has hostfs compiled in, not as a
module. Then run UML with the following arguments added to the command line:
<blockquote>
<tt>
<font color="#000000"> root=/dev/root rootflags=/path/to/uml/root rootfstype=hostfs</font>
</tt>
</blockquote>
UML should then boot as it does normally.
</blockquote>
<a name="Building hostfs"/><table width="100%" bgcolor="#e0e0e0">
<tr>
<td>
<b>
<font color="black">Building hostfs</font>
</b>
</td>
</tr>
</table>
<blockquote head="Building hostfs">
If you need to build hostfs because it's not in your kernel, you have
two choices:
<p>
<ul>
<li>Compiling hostfs into the kernel:<p>
Reconfigure the kernel and set the 'Host filesystem' option under
'Processor features' to 'Y'. Recompile the kernel and reboot it.
</li>
<li>Compiling hostfs as a module:<p>
Reconfigure the kernel and set the 'Host filesystem' option under
'Processor features' to 'M'. Rebuild the kernel modules. hostfs will
be in arch/um/fs/hostfs/hostfs.o. Install that in /lib/modules/`uname -r`/fs
in the virtual machine, boot it up, and
<blockquote>
<tt>
<font color="#000000" where="UML">UML# insmod hostfs</font>
</tt>
</blockquote>
</li>
</ul>
</blockquote>
</td>
</tr>
</table>
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