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authorMichael 'Mickey' Lauer <mickey@vanille-media.de>2009-02-25 01:47:30 +0100
committerMichael 'Mickey' Lauer <mickey@vanille-media.de>2009-02-25 01:47:30 +0100
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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 &amp; 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>~%&gt; 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>~%&gt; ./tmp/staging/x86_64-linux/bin/fakeroot
+~#&gt; mknod hdc b 22 0
+~#&gt; ls -l hdc
+brw------- 1 root root 22, 0 Aug 18 13:20 hdc
+~#&gt;</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>~#&gt; mkdir disk
+~#&gt; mount hdc disk
+ERROR: ld.so: object 'libfakeroot.so.0' from LD_PRELOAD cannot be preloaded: ignored.
+mount: only root can do that
+~#&gt;</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>~#&gt; exit
+~%&gt; ls -l hdc
+brw------- 1 user user 22, 0 Aug 18 13:20 hdc
+~#&gt;</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>~%&gt; 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
+~%&gt;</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>~%&gt; 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
+~%&gt;</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> \ No newline at end of file