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<?xml version="1.0" encoding="UTF-8"?>
<section id="fakeroot" xreflabel="fakeroot">
<title>fakeroot (device node handling)</title>
<para>The fakeroot program is designed to allow non-root users to perform
actions that normally require root as part of the package generation
process. It is used by <xref linkend="rootfs_ipkg_class" /> the for root
filesystem creation and by <xref linkend="image_ipkg_class" /> the for image
creation. Some recipies also use fakeroot to assist with parts of the
package installation (usually) or building that requires root
priviliges.</para>
<para>In particular fakeroot deals with:</para>
<itemizedlist>
<listitem>
<para>Device nodes; and</para>
</listitem>
<listitem>
<para>Ownership and group (uid & gid) management.</para>
</listitem>
</itemizedlist>
<section>
<title>How fakeroot works</title>
<para>First of all we'll look at an example of how the fakeroot process
works when used manually.</para>
<para>If we attempt to create a device node as a normal (non-root) user
the command will fail, telling is that we do not have permission to create
device nodes:<screen>~%> mknod hdc b 22 0
mknod: `hdc': Operation not permitted</screen>Yet the <xref
linkend="image_ipkg_class" /> is able to create device nodes and include
them in the final images, qll without the need to have root
priviliges.</para>
<para>Let's try and create that node again, this time we'll run the
commands from within the fakeroot process:<screen>~%> ./tmp/staging/x86_64-linux/bin/fakeroot
~#> mknod hdc b 22 0
~#> ls -l hdc
brw------- 1 root root 22, 0 Aug 18 13:20 hdc
~#></screen>So it looks like we have succesfully managed to create a device
node, even though we did not have to give a password for the root user. In
reality this device node still doesn't exist, it just looks like it
exists. Fakeroot is lying to the shell process and telling it that
<emphasis>"yes, this file exists and these are it's
properties"</emphasis>. We'll talk more about how fakeroot actually works
in a minute.</para>
<para>In this case <command>hdc</command> is the cd-rom drive, so let's
try and actually mount the cd-rom:<screen>~#> mkdir disk
~#> mount hdc disk
ERROR: ld.so: object 'libfakeroot.so.0' from LD_PRELOAD cannot be preloaded: ignored.
mount: only root can do that
~#></screen>So even though it appears we have root permissions, and that we
created a device node, you see that the system gives an error about
libfakeroot and about not being able to run mount because we are not
root.</para>
<para>If we exit the fakeroot process and then look at the device node we
see this:<screen>~#> exit
~%> ls -l hdc
brw------- 1 user user 22, 0 Aug 18 13:20 hdc
~#></screen></para>
<para>Note that it isn't a device node at all, just an empty file owned by
the current user!</para>
<para>So what exactly is fakeroot doing? It's using
<command>LD_PRELOAD</command> to load a shared libary into program which
replaces calls into libc, such as open and stat, and then returns
information to make it look like certain commands without actually
performing them. So when creating a device node fakeroot will:</para>
<orderedlist>
<listitem>
<para>Intercept the mknod system call and instead of creating a device
node it'll just created an empty file, owned by the user who run
fakeroot;</para>
</listitem>
<listitem>
<para>It rembers the fact that mknod was called by root and the
properties of the device node;</para>
</listitem>
<listitem>
<para>When a program, such as ls, calls stat on the file fakeroot
remebers that it was device node, owned by root, and modifies that
stat information to return this to ls. So ls sees a device node even
though one doesn't exist.</para>
</listitem>
</orderedlist>
<para>When we tried to run mount we recieved the error <command>"ERROR:
ld.so: object 'libfakeroot.so.0' from LD_PRELOAD cannot be preloaded:
ignored."</command>. This is due to the fact that mount is an suid root
binary, and for security reasons <command>LD_PRELOAD</command> is disabled
on suid binaries.</para>
<para>There are some very important points to remember when dealing with
fakeroot:</para>
<orderedlist>
<listitem>
<para>All information regarding devices nodes, uid and gids will be
lost when fakeroot exists;</para>
</listitem>
<listitem>
<para>None of the device nodes, uid or gid's will appear on disk,
however if you for example tar up a directory from within fakeroot all
of these device, uid's and gid's will appear in the tar
archive;</para>
</listitem>
<listitem>
<para>Any suid binaries will not interact with fakeroot;</para>
</listitem>
<listitem>
<para>Any static binaries will not interact with fakeroot;</para>
</listitem>
</orderedlist>
</section>
<section>
<title>Root filesystem, images and fakeroot</title>
<para>Many people have been been confused by the generated root filesystem
not containing and valid device nodes. But this is the expected
behaviour.</para>
<para>When you look at a generated root filesystem you'll notice that the
device nodes all appear to be incorrectly created:<screen>~%> ls -l tmp/rootfs/dev | grep ttySC
-rw-r--r-- 1 root root 0 Aug 16 13:07 ttySC0
-rw-r--r-- 1 root root 0 Aug 16 13:07 ttySC1
-rw-r--r-- 1 root root 0 Aug 16 13:07 ttySC2
~%></screen>These are empty files and not device nodes at all.</para>
<para>If we look in the image files generated from that root filesystem
everthing is actually ok:<screen>~%> tar -ztvf tmp/deploy/images/titan-titan-20060816030639.rootfs.tar.gz | grep " ./dev/ttySC"
crw-r----- root/root 204,8 2006-08-16 13:07:12 ./dev/ttySC0
crw-r----- root/root 204,9 2006-08-16 13:07:12 ./dev/ttySC1
crw-r----- root/root 204,10 2006-08-16 13:07:12 ./dev/ttySC2
~%></screen>The images are created from within the same fakeroot process as
the creation of the root filesystem and therefore it correctly picks up
all of the special files and permissions from fakeroot.</para>
<para><emphasis role="bold">NOTE: This means that you cannot use the root
filesystem in tmp/rootfs directly on your target device. You need to use
the .tar.gz image and uncompress it as root in order to generate a root
filesystem which is suitable for use directly on the target (or as an NFS
root).</emphasis></para>
</section>
<section>
<title>Recipes and fakeroot</title>
<para>Some applications require that you have root permissions to run
their installation routine, and this is another area where fakeroot can
help. In a recipe a standard task, such as <emphasis>do_install</emphasis>
for example:<screen>do_install() {
install -d ${D}${bindir} ${D}${sbindir} ${D}${mandir}/man8 \
${D}${sysconfdir}/default \
${D}${sysconfdir}/init.d \
${D}${datadir}/arpwatch
oe_runmake install DESTDIR=${D}
oe_runmake install-man DESTDIR=${D}
...</screen>can be modified to run within a fakeroot environment by
prefings the task with fakeroot:<screen><emphasis role="bold">fakeroot</emphasis> do_install() {
install -d ${D}${bindir} ${D}${sbindir} ${D}${mandir}/man8 \
${D}${sysconfdir}/default \
${D}${sysconfdir}/init.d \
${D}${datadir}/arpwatch
oe_runmake install DESTDIR=${D}
oe_runmake install-man DESTDIR=${D}
...</screen></para>
</section>
</section>
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