source: postlfs/config/bootdisk.xml@ c442d19

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Last change on this file since c442d19 was c442d19, checked in by Larry Lawrence <larry@…>, 19 years ago

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1<sect1 id="postlfs-config-bootdisk">
2<?dbhtml filename="bootdisk.html" dir="postlfs"?>
3<title>Creating a Custom Bootdisk</title>
4
5<sect2>
6<title>Decent Rescue Bootdisk Needs</title>
7<para>This section is really about creating a <emphasis>rescue</emphasis>
8diskette. As the name <emphasis>rescue</emphasis> implies, the host
9system has a problem, often lost partition information or corrupted file
10systems, that prevents it from booting and/or operating normally. For
11this reason, you <emphasis>must not</emphasis> depend on resources from
12the host being "rescued". To presume that any given partition or hard
13drive <emphasis>will</emphasis> be available is a risky presumption.</para>
14
15<para>Heeding the warning, the rescue disk created here has no
16dependency on the host system's resources, other than basic bootability
17and hardware soundness. At a minimum, the most common sorts of failures
18requiring a rescue bootdisk should be addressed by the contents of the
19bootdisk. This would include the common loss of partitioning (master
20boot record is lost or corrupted), file system corruption, and the need
21to allow creation and editing of files that may have been lost or
22corrupted, possibly as an effect of the other two problems.</para>
23
24<para>Additional utilities should be available to search for text or
25files, copy, move and remove files, and many other normal operations
26that might be expected to be needed when reconstructing.</para>
27</sect2>
28
29<sect2>
30<title>This Minimal Decent Rescue Disk</title>
31
32<para>The intent here is to create a "rescue bootdisk" that will support
33the common operations listed above. These functions are provided by
34including selected executables from <application><ulink
35url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox
36</ulink></application> and <application><ulink
37url="http://freshmeat.net/projects/e2fsprogs/">e2fsprogs</ulink></application>.
38A basic editor and rudimentary disk partitioning utility may also be
39optionally included.</para>
40
41<para>This, however, is not the limit. A minimal disk is described
42here, but you can add anything you can fit on the floppy. Furthermore,
43if one floppy is not enough to meet your needs, you can make a
44multi-diskette rescue set that means, essentially, the sky is the limit.
45This is discussed below. The number of other possible variations are
46just too numerous to mention here.</para>
47</sect2>
48
49<sect2>
50<title>Build the Rescue Bootdisk</title>
51<sect3>
52<title>Prerequisites</title>
53
54<para>You should have known-good floppy diskettes available. Some
55people prefer to use the <command>fdformat</command> command to prepare
56these because it also does a verification. See the man page for more
57details. Another good idea is to always prepare duplicates of the
58rescue diskette. Media does deteriorate.</para>
59
60<para>These instructions presume a base <acronym>LFS</acronym> install
61using ext2/ext3 file systems.</para>
62
63<para>You need to have loopback device support enabled in your host's
64kernel to use this procedure.</para>
65
66<para>You should make a custom kernel that includes only those features
67needed to rescue your system, so it will have the least size. No
68sense in building in support for things like <application>xfree86</application>,
69<application>dri</application>, etc, as most rescues are performed from the
70command prompt. Along the same lines, if you have <xref linkend="gcc2"/>, it
71is known to produce smaller kernels. So you might want to use that
72compiler for this kernel. If you do so, don't overlook any loadable
73modules (which are not addressed here) you might need - they need to be
74compiled with same compiler used to make the kernel.</para>
75
76<para>The rescueimage must include support for the file system of your
77choice (we presume ext2/3 here), ramdisk and initial ramdisk (initrd).
78Disable everything that you can in the kernel configuration. You should
79keep support for the proc file system and tempfs file system enabled
80because of their general utility. The proc file system is needed for
81the <command>mount</command> to report properly.</para>
82
83<para>If you install <emphasis>only</emphasis> the minimal set of
84components shown in this document, you will need a kernel that is 643 or
85fewer blocks in size. If you want the optional programs - a very basic
86editor, like <command>ed</command>, and rudimentary disk partitioning, like
87<command>sfdisk</command> - the kernel will need to be 595 or fewer blocks in
88size. This should not be a major problem unless your needs are fairly
89esoteric. On the system used to develop this version of the procedures, using
90only ext2 file systems and not using networking or <acronym>CD</acronym>s for
91recovery, the kernel image is only 481 blocks. And there may be more to gain - it has not been closely examined for additional gains.</para>
92
93<para>This kernel image will be called "rescueimage" hereinafter. You
94can actually name your image anything you like and just use its name
95instead in any commands that include "rescueimage".</para>
96
97<para>If you can not get your kernel down to the size needed to allow
98all you need on the ramdisk image, don't fret. You can always build a
99two diskette set, one boot and one root diskette. The kernel will prompt
100you to insert the root file system diskette. This will allow room for a
101compressed ramdisk image of 1440 blocks and a kernel of the same
102size.</para>
103
104<para>The kernel size limits given above are likely to vary as
105local system-specific configurations change. Use them only as a
106guideline and not as gospel. The size of the kernel image as shown by
107<command>ls -sk</command> is only an approximation because of some
108"overhead". On the system used to develop this version of these
109procedures, that command shows 488 blocks but the actual number of
110blocks written is only 480 and a fraction, which means that 481 blocks
111are actually used.</para>
112</sect3>
113
114<sect3>
115<title>Rescue Disk Build Process</title>
116
117<para>The basic process will be:</para>
118<itemizedlist>
119<listitem><para>make a mount point for a file system</para></listitem>
120<listitem><para>make an empty file to hold the file system</para></listitem>
121<listitem><para>bind the empty file to a loopback device</para></listitem>
122<listitem><para>make a 4MB file system</para></listitem>
123<listitem><para>mount the file system</para></listitem>
124<listitem><para>add components to the file system</para></listitem>
125<listitem><para>make the compressed initrd</para></listitem>
126<listitem><para>join a kernel image and initrd onto a diskette</para></listitem>
127</itemizedlist>
128
129<para>The initial ramdisk will be automatically loaded at boot time if
130setup is done correctly.</para>
131
132<para><emphasis>Make a mount point and an empty file to hold a file
133system</emphasis></para>
134
135<screen><userinput><command>mkdir -p /mnt/loop1
136dd if=/dev/zero of=/tmp/rfloppy bs=1k count=4096</command></userinput></screen>
137
138<para><emphasis>Command explanations</emphasis></para>
139
140<para><command>dd</command>: This is a generalized input-to-output copy
141utility that also has many transformation capabilities.</para>
142
143<para><command>if=/dev/zero</command>: This parameter assigns
144<command>dd</command>'s input file to a device that returns an infinite
145stream of zeroes.</para>
146
147<para><command>of=/tmp/rfloppy</command>: This parameter directs
148<command>dd</command>'s output to <filename>/tmp/rfloppy</filename>.</para>
149
150<para><command>bs=1k count=4096</command>: These parameters tell
151<filename>dd</filename> to read and write in "chunks" of 1024 bytes and
152process 4096 "chunks".</para>
153
154<para><emphasis>Bind the file to a loopback device, make a file system and mount it.</emphasis></para>
155
156<para>The reason these commands are used is that they work regardless of
157the version of <command>mount</command> (older ones don't have the
158<userinput>-o loop</userinput> option) or if <filename>/etc/mtab</filename>
159is symlinked to <filename>/proc</filename> (which causes mount to be
160unable to properly "unbind" a loop device, due to "lost" information).
161An alternate set of commands is provided, after these three commands,
162that you can use if you don't have either of these situations.</para>
163
164<screen><userinput><command>losetup /dev/loop1 /tmp/rfloppy
165mke2fs -m 0 -N 504 /dev/loop1
166mount -t ext2 /dev/loop1 /mnt/loop1</command></userinput></screen>
167
168<para><emphasis>Command explanations</emphasis></para>
169
170<para><command>losetup /dev/loop1 /tmp/rfloppy</command>: This
171command "binds" a loopback device to the empty file.</para>
172
173<para><command>mke2fs -m 0 -N 504 /dev/loop1</command>: This
174command makes an ext2 file system on the loopback device (which really
175means it is created in the file to which the loopback device is bound)
176and reserves no blocks. The <userinput>-N 504</userinput> parameter causes
177only 504 inodes to be allocated, leaving more space for other things needed
178in the file system.</para>
179
180<para><command>mount -t ext2 /dev/loop1 /mnt/loop1</command>: This
181mounts the file system just created, just as if it were a real device,
182like a hard drive or diskette. This allows all the normal system I/O
183commands to operate as if a real device were present.</para>
184
185<para>If your <command>mount</command> supports the
186<userinput>-o loop</userinput> option <emphasis>and</emphasis> your
187<filename>/etc/mtab</filename> is a real file, rather than a symlink to
188<filename>/proc</filename>, the three above commands can be replaced
189by these next two commands.</para>
190
191<screen><userinput><command>mke2fs -F -m 0 -N 504 /tmp/rfloppy
192mount -o loop /tmp/rfloppy /mnt/loop1</command></userinput></screen>
193
194<para><emphasis>Command explanations</emphasis></para>
195
196<para><command>mke2fs -F -m 0 -N 504 /tmp/rfloppy</command>: As before,
197a file system is made, with only 504 inodes and no reserved blocks, that
198will be bound to a loopback device. The <userinput>-F</userinput> parameter
199just suppresses an irritating question issued when
200<command>mke2fs</command> realizes that you are not accessing a
201device.</para>
202
203<para><command>mount -o loop /tmp/rfloppy /mnt/loop1</command>: This
204command tells <command>mount</command> to bind the named file to a
205loopback device it automatically selects (the first available) and mount
206the device on <filename>/mnt/loop1</filename>.</para>
207
208<para><emphasis>Add components to the file system</emphasis></para>
209
210<para><emphasis>A cautionary note:</emphasis> if you are not running in a
211<command>chroot</command> environment, be sure that you do not accidently
212omit the <filename>/mnt/loop1</filename> reference in the commands. If
213you do so, you might replace the equivalent components on your host with
214the components that are installed by these procedures. Even if you are
215in a chroot environment, you may need to be careful if the environment
216is your freshly built <acronym>LFS</acronym> system which you intend to
217use as a host in the future.</para>
218
219<para>First, to have as much free space as possible, remove the
220<filename>lost+found</filename> directory, which is not needed because
221it is only used by <command>fsck</command>. Since <command>fsck</command>
222will never be run on this file system, it is unneeded.</para>
223
224<screen><userinput><command>rmdir /mnt/loop1/lost+found/</command></userinput></screen>
225
226<para>Now make a minimal set of directories.</para>
227
228<screen><userinput><command>mkdir /mnt/loop1/{dev,proc,etc,sbin,bin,lib,mnt,usr,var}</command></userinput></screen>
229
230<para>Add needed device files to the initrd image. If you use devfs,
231the following command works well, as you only have the devices you use
232anyway.</para>
233
234<screen><userinput><command>cp -dpR /dev/* /mnt/loop1/dev</command></userinput></screen>
235
236<para>If you used <command>MAKEDEV</command> to create your devices on
237your host, you'll want to use something similar to this longer command,
238to minimize wasting space with unneeded inodes.</para>
239
240<para><emphasis>You must modify this to suit your kernel configuration and
241other needs.</emphasis> For example, you may need scsi devices and may not need
242frame buffer devices or the pseudo-terminal directory. Also, the number
243of hard drives and partitions that you include should be the minimal
244that you need. Extensive analysis has not been done on the list below,
245so there are more inodes and space to be gained by "fine tuning" this
246set.</para>
247
248<screen><userinput><command>mkdir /mnt/loop1/dev/pts
249cp -a \
250&nbsp;&nbsp;&nbsp;&nbsp;/dev/null /dev/console \
251&nbsp;&nbsp;&nbsp;&nbsp;/dev/fb[0-7] /dev/fd /dev/fd0 /dev/fd0h1440 /dev/full \
252&nbsp;&nbsp;&nbsp;&nbsp;/dev/hda* /dev/hdb* /dev/hdc* /dev/hdd* /dev/initctl /dev/kmem \
253&nbsp;&nbsp;&nbsp;&nbsp;/dev/loop[0-3] /dev/lp0 /dev/mem /dev/port \
254&nbsp;&nbsp;&nbsp;&nbsp;/dev/psaux /dev/ram \
255&nbsp;&nbsp;&nbsp;&nbsp;/dev/ram0 /dev/ram1 /dev/ram2 /dev/ram3 /dev/random /dev/rtc \
256&nbsp;&nbsp;&nbsp;&nbsp;/dev/shm /dev/stderr /dev/stdin /dev/stdout /dev/tty \
257&nbsp;&nbsp;&nbsp;&nbsp;/dev/tty[0-9] /dev/ttyS0 /dev/ttyS1 /dev/urandom /dev/zero \
258&nbsp;&nbsp;/mnt/loop1/dev</command></userinput></screen>
259
260<para><emphasis>What is needed in the <filename>/etc</filename>
261directory</emphasis></para>
262
263<para>If you choose, you can copy all or selected parts of your
264<filename>/etc/passwd</filename> and <filename>/etc/group</filename>
265files. But even if each is less than 1024 bytes, you will lose two
266inodes and two blocks of space on the initial ramdisk. This only really
267matters because of trying to squeeze everything onto a 1.44MB
268diskette. Every little bit helps. The strategy taken here is to create
269these two files as part of the rescue boot and initialization process.
270The commands that make the two files will be imbedded inside the
271<filename>rcS</filename> script that <filename>linuxrc</filename>
272(really <application><ulink
273url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox
274</ulink></application>) invokes after the initrd is
275loaded. This way no more inodes or blocks are used on the
276diskette to carry these files.</para>
277
278<para>Some might like to copy their <filename>/etc/rc*</filename>
279directory into the ramdisk image, but this may have no value, other
280than archival use, in a worst-case recovery scenario. If you want
281automatic initialization of the system after repair, they may have some
282value. But few people need or want this to happen. If the file system
283on the hard drives are corrupted, what good will mount scripts do? Some
284scripts may be useful, like access to a network to copy over backup data
285when the hard drive's file systems are usable again. The point is that
286you should copy only the parts that you can use because space is at a
287premium on the diskette.</para>
288
289<para>Here, only the <filename>fstab</filename> will be included. This
290is handy because it eases mounting of partitions that may be useful and
291also can be examined and used as a guide as to what is available and
292what may need reconstruction. Because it may be larger than needed, you
293should edit it to remove any useless entries and minimize commentary.
294No other editing is needed because the boot scripts are not included and
295no automatic mounting will be done using the <filename>fstab</filename>.
296If you decide to include some bootscripts that might try to mount
297things, change the <filename>fstab</filename>'s entries to
298<command>noauto</command> in the options field so they don't cause an
299attempt to mount a potentially corrupt partition. Copy it to
300<filename>/tmp</filename>, edit it as desired and then:</para>
301
302<screen><userinput><command>cp -a /tmp/fstab /mnt/loop1/etc</command></userinput></screen>
303
304<para>Now the initialization script will be added. As mentioned above,
305<command>linuxrc</command> is symlinked to <application><ulink
306url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox
307</ulink></application>.
308After the kernel and initial ramdisk have been loaded, the kernel gives
309control to <command>linuxrc</command> (<application><ulink
310url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox
311</ulink></application>). It wants to run an <filename>/etc/init.d/rcS</filename>
312script to do any initial setup.</para>
313
314<para>If you use devfsd, you will need to set up the
315<filename>rcS</filename> script to handle the devfsd startup. Put the
316following commands in <filename>/mnt/loop1/etc/init.d/rcS</filename>.
317You may also want to add some of the processes shown in the non-devfs
318version that follows.</para>
319
320<screen><userinput>#!/bin/sh
321mount -t devfs devfs /dev
322/sbin/devfsd /dev</userinput></screen>
323
324<para>If you don't use devfsd, but created a static <filename>/dev</filename>
325directory using <command>MAKEDEV</command>, or any similar process, the
326<filename>rcS</filename> script will do slightly different things.
327Also, don't forget that it is creating the
328<filename>/etc/passwd</filename> and <filename>/etc/group</filename>
329files, thus saving space on the diskette.</para>
330
331<para>The script made next will mount <filename>/proc</filename>, turn
332on swap (no harm is done if it fails), make the
333<filename>/etc/passwd</filename> and <filename>/etc/group</filename>
334files, create a log directory and turn on swapping. Create the script
335with:</para>
336
337<screen><userinput><command>mkdir -p /mnt/loop1/etc/init.d
338cat &gt;/mnt/loop1/etc/init.d/rcS &lt;&lt; EOD</command>
339#!/bin/sh
340mount -t proc proc /proc
341swapon -a
342
343echo "root:x:0:0:root:/root:/bin/bash" &gt; /etc/passwd
344
345<command>cat &gt; /etc/group &lt;&lt;EOF</command>
346root:x:0:
347bin:x:1:
348sys:x:2:
349kmem:x:3:
350tty:x:4:
351tape:x:5:
352daemon:x:6:
353floppy:x:7:
354disk:x:8:
355lp:x:9:
356dialout:x:10:
357audio:x:11:
358<command>EOF
359chmod 644 /etc/passwd /etc/group</command>
360
361mkdir /var/log
362
363<command>EOD
364chmod u+x /mnt/loop1/etc/init.d/rcS</command></userinput></screen>
365
366<para>Unless you add a lot to this script, which <emphasis>is</emphasis>
367encouraged, the above should be reasonably close to what you need.</para>
368
369<para><emphasis>Install packages</emphasis></para>
370
371<para>There are two packages that must be installed. The <application><ulink
372url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox</ulink></application>
373package incorporates the core functions that provide a shell and many
374basic utilities. A file system package, like <application><ulink
375url="http://freshmeat.net/projects/e2fsprogs/">e2fsprogs</ulink></application>, or
376a package for the file system you are using will provide a minimal
377set of utilities for file system checking and reconstruction. The whole
378package will not be installed, but only certain needed components.</para>
379
380<para>If you use devfsd, you will also need to install that software.</para>
381
382<para>Install <application><ulink
383url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox</ulink></application>
384into the initial ramdisk image. Busybox incorporates many UNIX utility
385program functions into a single small executable file.</para>
386
387<screen><userinput><command>make &amp;&amp;
388make PREFIX=/mnt/loop1 install &amp;&amp;
389&gt; /mnt/loop1/var/utmp</command></userinput></screen>
390
391<para>A <filename>var/utmp</filename> is made because <application><ulink
392url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox</ulink></application>
393needs it for the reboot command to work properly. If this file doesn't
394exist when <application><ulink
395url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox</ulink></application>
396is started, the reboot command will not work. This would be a bad thing
397for people that have no reset button available to them.</para>
398
399<para>If you use devfs to create devices on the fly and free up precious
400inodes on the floppy, you'll also install devfsd to facilitate the
401devices that <application><ulink
402url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox</ulink></application>
403expects to find. Use the following commands to do the install.</para>
404
405<screen><userinput><command>mv GNUmakefile Makefile &amp;&amp;
406make &amp;&amp;
407make PREFIX=/mnt/loop1 install &amp;&amp;</command></userinput></screen>
408
409<para><emphasis>Install part of <application>e2fsprogs</application></emphasis></para>
410
411<para>If you use the ext2 or ext3 (journaling) file system, you can use
412the commands below to install the minimal functionality that should
413allow you to get your hard drives usable again. If you use ext3, keep in
414mind that it is a part of the <application>e2fsprogs</application>
415package and you can get the components, which are mostly hard links,
416from the same places shown below. If you use some other file system,
417such as reiserfs, you should apply the <emphasis>principals</emphasis>
418you see here to install parts of that package instead.</para>
419
420<screen><userinput><command>LDFLAGS='-s'
421mkdir build &amp;&amp;
422cd build &amp;&amp;
423../configure --prefix=/mnt/loop1/usr --with-root-prefix="" \
424 --disable-swapfs --disable-debugfs \
425 --enable-dynamic-e2fsck --disable-nls --disable-evms \
426 --disable-rpath &amp;&amp;
427make LDFLAGS="$LDFLAGS" &amp;&amp;
428strip -p --strip-unneeded --remove-section=.comment \
429 -o /mnt/loop1/sbin/mke2fs misc/mke2fs &amp;&amp;
430strip -p --strip-unneeded --remove-section=.comment \
431 -o /mnt/loop1/sbin/e2fsck e2fsck/e2fsck &amp;&amp;
432chmod 555 /mnt/loop1/sbin/{mke2f,e2fsck}
433</command></userinput></screen>
434
435<para><emphasis>Two useful utilities</emphasis></para>
436
437<para>There are two very useful utilities that any rescue disk should
438have to help in faster and more accurate recovery. The first is a
439partitioning utility. The <command>sfdisk</command> program is
440used here because of its small size and great power. Be warned though -
441it is not what is considered to be "user friendly". But the
442<command>fdisk</command> and <command>cfdisk</command> programs are
443substantially larger or require more shared objects, like <application>ncurses
444</application>.</para>
445
446<para>The second utility is an editor. Most graphical editors are
447inherently too large and also require additional shared objects. For
448this reason, <command>ed</command> is used here. It is small, requires
449no additional shared objects and is a regex-based editor that is the
450ancestor to almost all subsequent editors that support regex-based
451editing, whether graphical or not. It is a "context editor" and offers
452powerful, but non-graphical, editing features. There are many other
453editors that may be suitable - feel free to use one of them instead.</para>
454
455<para>Read the <application>busybox</application>
456<filename>INSTALL</filename> and <filename>README</filename> files to
457see how to include a <command>vi</command> editor. It has not been
458investigated here yet, so it may or may not easily fit onto a single
459diskette image such as is made here.</para>
460
461<para>You can install these or not, but it is important for you to have
462some capability such as these offer. Exactly how you would install the
463utilities you choose will have to be determined by you.</para>
464
465<para><command>Sfdisk</command> and <command>ed</command> are installed
466by, essentially, copying them from your host. Strip is used, just to
467assure that they carry no "excess baggage", even though the base
468<acronym>LFS</acronym> install should have stripped them already. Use
469the following commands:</para>
470
471<screen><userinput><command>strip -p --strip-unneeded --remove-section=.comment \
472 -o /mnt/loop1/sbin/sfdisk /sbin/sfdisk
473strip -p --strip-unneeded --remove-section=.comment \
474 -o /mnt/loop1/bin/ed /bin/ed
475chmod 555 /mnt/loop1/sbin/sfdisk /mnt/loop1/bin/ed</command></userinput></screen>
476
477<para>Also, keeping in mind your space limitations, copy any other
478binaries and libraries you need to the image. Use the
479<command>ldd</command> command to see which libraries you will need to
480copy over for any executables. Don't forget to also strip them
481<emphasis>before</emphasis> copying them to the ramdisk image or use the
482<command>strip</command>, as above, to "copy" them.</para>
483
484<para><emphasis>Set up the lib directory</emphasis></para>
485
486<para>Once you have installed all the utilities from above and any
487additional ones you want, use the <command>ldd</command> command, as
488mentioned above, on those that were not listed in this document. If
489any additional libraries are needed, add them into the setup commands
490shown next.</para>
491
492<para>If you installed only those things shown above, the shared objects
493needed will be minimal. You can add them to the ramdisk image with:</para>
494
495<screen><userinput><command>strip -p --strip-unneeded --remove-section=.comment \
496&nbsp;&nbsp;&nbsp;&nbsp;-o /mnt/loop1/lib/libc.so.6 /lib/libc-2.3.2.so &amp;&amp;
497strip -p --strip-unneeded --remove-section=.comment \
498&nbsp;&nbsp;&nbsp;&nbsp;-o /mnt/loop1/lib/ld-linux.so.2 /lib/ld-2.3.2.so &amp;&amp;
499strip -p --strip-unneeded --remove-section=.comment \
500&nbsp;&nbsp;&nbsp;&nbsp;-o /mnt/loop1/lib/libdl.so.2 /lib/libdl-2.3.2.so &amp;&amp;
501chmod 555 /mnt/loop1/lib/{libc.so.6,ld-linux.so.2,libdl.so.2}</command></userinput></screen>
502
503<para>Note that the above commands change the names of the libraries,
504eliminating the need for the usual symlinks. If you add any additional
505shared objects, be alert for similar opportunities and also the pitfalls
506that may be present.</para>
507
508<para><emphasis>Make the compressed initrd</emphasis></para>
509
510<para>Unmount the loopback file. If you used <command>mount</command>'s
511<userinput>-o loop</userinput> option, the "bond" between the loop device
512and the file will be removed when the unmount is done. Just omit the
513<command>losetup -d /dev/loop1</command> from the following
514sequence. The <userinput>-9</userinput> parameter is used with
515<command>gzip</command> to make the smallest possible compressed image. To
516make sure it will fit on the diskette, list the file's size.</para>
517
518<screen><userinput><command>umount /mnt/loop1 &amp;&amp;
519losetup -d /dev/loop1 &amp;&amp; # Omit if mount's -o loop was used
520gzip -9 &lt; /tmp/rfloppy &gt; /tmp/rootfs.gz
521ls -l /tmp/rootfs.gz</command></userinput></screen>
522
523<para><emphasis>Join a kernel image and initrd onto a diskette</emphasis></para>
524
525<para>Now the kernel image and initial ramdisk image will be written to
526the boot diskette. Before doing this, calculate the number of blocks
527needed for the kernel and for the initrd, individually, by dividing each
528size by 1024 and adding one if there is any remainder. Add these two
529results together. They must total 1,440 or fewer blocks. If they total
530more than this, don't worry too much. Changes to make a two-diskette
531set are presented later. Of course, you could reexamine your choices and
532try to shrink either the kernel or the initial ramdisk image.</para>
533
534<para>To make a single-floppy rescue, using devfs, use the following
535commands. If you use the static <filename>/dev</filename> setup, use
536<filename>/dev/fd0</filename> instead of the /dev/floppy/0.</para>
537
538<screen><userinput><command>dd if=rescueimage of=/dev/floppy/0 bs=1k
539rdev /dev/floppy/0 0,0
540rdev -R /dev/floppy/0 0</command></userinput></screen>
541
542<para><emphasis>Command explanations</emphasis></para>
543
544<para><command>rdev /dev/floppy/0 0,0</command>: sets the root file system
545the kernel will use when it boots. Because it loads an initrd, it will
546automatically set that as the root device, initially. So, the
547<userinput>0,0</userinput> gives it "no value", telling the kernel to not
548mount any other device. Some folks give <filename>/dev/fd0</filename> or
549something similar. But this has effect <emphasis>only</emphasis> when
550<command>linuxrc</command> (really <application><ulink
551url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox
552</ulink></application>) exits and the normal <command>init</command>
553processes get invoked. Since this is not being done here, and the floppy
554is <emphasis>not</emphasis> a valid file system, it would be useless
555here. A hard drive would be a better choice if you are looking to
556automatically bring the system up after repair. Since <application><ulink
557url="http://www.busybox.net/downloads/busybox-0.60.4.tar.bz2">busybox
558</ulink></application> provides the <command>reboot</command> command,
559automatic initialization is not needed.</para>
560
561<para>The <command>rdev -R /dev/floppy/0 0</command> will set the
562"root flags" to zero. They have no use in this application.</para>
563
564<para>The <command>dd</command> from above showed some results, like</para>
565
566<screen> 480+1 records in
567 480+1 records out</screen>
568
569<para>In this example the rescueimage (kernel) was 480+1 blocks in size.
570Make sure that this number, which may be different for you, matches your
571calculations from above. You need to calculate a "magic number" now
572that will be inserted into the kernel image. The value consists of three
573significant parts. Two are discussed here. The third is touched upon
574later.</para>
575
576<para>Bits 0 - 10 will contain the size of the kernel image, in blocks,
577that you calculated above, and which should match the results from the
578dd above. Bit 14 (the 15th bit, which is 2 to the 14th power, or 16,384)
579is a flag that, when set to 1, tells the kernel an initial ramdisk is to
580be loaded. So for the single-floppy rescue diskette, the two numbers
58116,384 and 481 (or whatever number is right for your kernel size) are
582added together to produce a decimal value, like 16865. This value is
583inserted into the proper place in the kernel image by the
584<command>rdev</command> command done next.</para>
585
586<para>Insert the "magic number" into the kernel image and then write the
587root file system right after the kernel on the floppy by executing the
588following commands, with the proper numbers inserted. Notice that the
589<command>seek</command> parameter's number must be the size, in blocks,
590of your kernel image. If you use the static <filename>/dev</filename>
591setup, use <filename>/dev/fd0</filename> in the commands below, instead
592of <filename>/dev/floppy/0</filename>.</para>
593
594<screen><userinput><command>rdev -r /dev/floppy/0 16865
595dd if=/tmp/rootfs.gz of=/dev/floppy/0 bs=1k seek=481</command></userinput></screen>
596
597<para>In this command, <command>seek</command> was used to position to
598the block following the end of the kernel (480+1) and begin writing the
599root file system to the floppy.</para>
600</sect3>
601</sect2>
602
603<sect2>
604<title>A Two-diskette Rescue Setup</title>
605
606<para>If you just can't live with a single-diskette rescue system, here
607is what to do to make a simple two-diskette system. Note that the
608endless possibilities presented by the availability of
609<command>linuxrc</command> and other components are not addressed
610here. Here you will just use the kernel's ability to prompt for a second
611diskette that contains the initrd image and load it.</para>
612
613<para>Modify the above instructions as follows. First a different magic
614number is needed. The 15th bit (bit 14) still needs to be set, but the
615size of the kernel image, in blocks, is replaced with a zero. The third
616component, which was not discussed above, is now used. This is the 16th
617bit (bit 15) of the "magic number". When set, it tells the kernel to ask
618the user to insert the "root" floppy. It then loads the initrd image
619from that diskette. Because the size of the kernel image was replaced
620by zero, the kernel starts loading from the "zero'th" block (the first
621one) on the second diskette.</para>
622
623<para>The 16th bit (bit 15) represents 2 raised to the 15th power, or
62432,768. So the new magic number is 32,768 + 16384, which is 49,152. This
625value tells the kernel to prompt for, and then load, an initial ramdisk
626image from the first block on the inserted floppy. So your first
627modification is to the command to write the "magic number" to the kernel
628image on the diskette.</para>
629
630<screen><userinput><command>rdev -r /dev/floppy/0 49152</command></userinput></screen>
631
632<para>Note that the initrd image is <emphasis>not</emphasis> copied to
633the diskette yet. Remove the boot diskette and insert another diskette
634that will hold your root file system. Run this modified command (don't
635forget to use <filename>/dev/fd0</filename> if you don't use devfs).
636Note that no <command>seek</command> parameter is used.</para>
637
638<screen><userinput><command>dd if=/tmp/rootfs.gz of=/dev/floppy/0 bs=1k</command></userinput></screen>
639
640<para>That's all there is to it. The possibilities from here are limited only
641by your imagination and tenacity in pursueing enhancements. And your
642willingness to research available documentation. A good starting point
643is the "Documentation" directory in your kernel source tree. More help
644may be gained at
645<ulink url="http://linuxfromscratch.org/hints/news.html"><acronym>LFS </acronym> Hints</ulink>
646(please use a mirror site that is suitable) and
647<ulink url="http://www.tldp.org">TLDP</ulink>.</para>
648
649</sect2>
650</sect1>
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