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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 would normally require root privileges as part of the package
generation process. It is used by the <xref linkend="rootfs_ipkg_class" />
for root filesystem creation and by the <xref linkend="image_class" />
for the creation of filesystem images. Some recipes also use fakeroot to
assist with parts of the package installation (usually) or building where
root privligeses are expected by the package.</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 then
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_class" /> is able to create device nodes and include
them in the final images, all without the need to have root
privileges.</para>
<para>Let's try and create that node again, this time we'll run the
commands from within a 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 successfully 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 exits. 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
this is what we see:<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 library into program which
replaces calls into libc, such as open and stat, and then returns
information to make it look like certain commands succeeded without
actually performing those commands. 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 create an empty file, owned by the user who run
fakeroot;</para>
</listitem>
<listitem>
<para>It remembers the fact that mknod was called by root and it
remembers the properties of the device node;</para>
</listitem>
<listitem>
<para>When a program, such as ls, calls stat on the file fakeroot
remembers 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 received 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, uids or gids will appear on disk.
However if you tar up a directory from within fakeroot (for example),
all of these device, uids and gids will appear correctly 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 confused by the generated root filesystem not
containing any valid device nodes. This is in fact 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
then everything 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 the method for a standard task, such as the
<command>do_install</command> method for the <emphasis>install</emphasis>
task:<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
prefixing the method name 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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