mtab hint updated

Oliver Brakmann obrakmann at gmx.net
Thu Jan 2 07:03:32 PST 2003


Hi,

as the discussion about the mtab issue came up again on lfs-dev, I thought
I'd update the patch to glibc for 2.3.1 and, lo and behold, the old patch
still works.

Added a note anyway ;-).

Bye,
Oliver
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-------------- next part --------------
TITLE:		How to get rid of the /etc/mtab -> /proc/mounts symlink
LFS VERSION:	LFS-3.2-RC1 or above
AUTHOR:		Oliver Brakmann <obrakmann at gmx.net>

LAST CHANGES:	2003-01-02: Minor stylistic changes
		2003-01-02: Added a note for glibc 2.3.1 users
		2002-11-14: Minor stylistic changes (thanks to DJ Lucas)
		2002-08-06: Restructured the whole hint
		2002-08-04: Corrected a few technical details
		2002-08-04: Minor stylistic changes
		2002-07-30: Added a note for those who do not want to
			    recompile glibc
		2002-07-30: rephrased some parts to prevent misunderstanding
		2002-07-29: fixed a couple of typos

SYNOPSIS:
	A default LFS install sets up /etc/mtab as a symlink to /proc/mounts,
	which breaks (u)mount's behaviour.  This hints tells you how to
	solve this problem elegantly, even on a read-only root partition.

HINT:

CONTENTS
========

1. INTRODUCTION
2. MOTIVATION
3. WRITEABLE ROOT
4. READ-ONLY ROOT
5. CONCLUSION


1. INTRODUCTION
===============

This hint really covers two issues:
1. getting rid of that annoying symlink
2. doing so on a read-only root partition.

Because the instructions to achieve the second goal (discussed in chapter 4)
include those from chapter 3, albeit in slightly modified form, you will find
that a lot of information is more or less duplicated in this hint.  I did not
know how to solve this any other way without this hint becoming way too
confusing.

OK, as a final note before we begin: keep in mind that I take no responsibility
whatsoever for any damage done to your computer.  Be careful doing this, you
might badly screw your system.


2. MOTIVATION
=============

If /etc/mtab is a link to /proc/mounts, the following issues with the
mount/umount programs show up:

	- the fstab `user' option does not work
	  (you have to specify `users' as a workaround)
	- loop devices are not freed properly when unmounting
	  (you have to use `umount -d' or run `losetup -d' manually)
	- certain kernel versions under certain circumstances list /dev/root
	  instead of the real root device in /proc/mounts.

The reasons one might want to have /etc/mtab as a link to /proc/mounts
are the following:

	- if, for some reason, the system crashes and you have to do
	  a hard reboot, /etc/mtab will not contain inconsistent data upon
	  the next boot.
	- you can mount your root partition read-only.

However, those two points can also be dealt with without the need for
the symlink.


3. WRITEABLE ROOT
=================

Follow these instructions if your / and /var directories are on the same
partition.  If they are not, but your / is writeable nevertheless, go to
chapter 4 anyway.  If you want to have a read-only / one day, you do not
need to start all over again with this.


	1. Switch to single-user mode

		# telinit 1

	2. Remove /etc/mtab

		# rm /etc/mtab

	   Be careful: from this point on, you lose all information about the
	   currently mounted filesystems (that umount can work with, at least)!
	   If you have any filesystems mounted that do not get mounted at boot-
	   time (ie. by the /etc/rc.d/init.d/mountfs script), unmount them
	   _before_ this step! If you are not sure what you have mounted, you
	   can still take a look at /proc/mounts.

	3. Create an empty /etc/mtab

		# touch /etc/mtab
		# chown root:root /etc/mtab
		# chmod 644 /etc/mtab

	4. Now you have to modify your init scripts so that they basically
	   do this:

		1. mount -n /proc	# this is in /etc/rc.d/init.d/mountproc
		2. fsck			# this is in /etc/rc.d/init.d/checkfs
		3. remount -n / rw	# so that we can write to /
		4. > /etc/mtab		# clear /etc/mtab
		5. mount -f /		# update mtab with the already
		6. mount -f /proc	# mounted partitions.
		7. mount -a		# mount all remaining partitions

	   
	   The first step requires a change in /etc/rc.d/init.d/mountproc,
	   whereas the second one does not require any changes.
 	   The remainder is handled in /etc/rc.d/init.d/mountfs:

,---[ /etc/rc.d/init.d/mountfs ]------->8--------------------------------------
#!/bin/bash
# Begin $rc_base/init.d/mountfs - File System Mount Script

# Based on mountfs script from LFS-3.1 and earlier.
# Rewritten by Gerard Beekmans  - gerard at linuxfromscratch.org

source /etc/sysconfig/rc
source $rc_functions

case "$1" in
        start)
                echo "Remounting root file system in read-write mode..."
                mount -n -o remount,rw /
                evaluate_retval

                echo "Cleaning /etc/mtab..."
                > /etc/mtab &&
		mount -f -o remount,rw / &&
		mount -f /proc
		evaluate_retval

                # The following mount command will mount all file systems.  If
                # you have other (network based) file system that should not
                # be or cannot be mounted at this time, add them to the NO_FS
                # variable below.  All file systems that are added to the
		# variable in the form of no<filesystem> will be skipped.

                NO_FS="nonfs,nosmbfs,noproc"
                echo "Mounting remaining file systems..."
                mount -a -t $NO_FS
                evaluate_retval
                ;;

[...remainder of the script omitted...]
`-------------------------------------->8--------------------------------------

	   If you use another set of init scripts, you are surely competent
	   enough to know how to implement these changes in your scripts.

	5. Modify your /etc/fstab so that /proc does not get mounted by
	   mount -a:

		proc			/proc proc noauto	0 0

	   The important part here is the `noauto' keyword.
	   While this has nothing to do with the task at hand and is not
	   really necessary with the lfs-bootscripts, it is good to have
	   though, in case you ever run `mount -a' from outside of any
	   bootscripts (like in the following step :-).

	6. If you have not unmounted any other partitions during the process,
	   all those listed in /etc/fstab should still be mounted.  Check out
	   /proc/mounts to be sure.  The userland tools do not know all this
	   anymore, so we have to tell them:

		# mount -f /
		# mount -f /proc
		# mount -af

	   Be sure that /etc/mtab and /proc/mounts contain consistent data
	   after this step.

	7. You can telinit back to your favourite runlevel now.  Enjoy!



4. READ-ONLY ROOT
=================

Follow these instructions if you have your / and /var directories on
separate partitions.


	1. Switch to single-user mode

		# telinit 1

	2. Remount / read-write

		# mount -o remount,rw /

	3. Rebuild glibc with the following patch applied:
	   (or just do the change by hand, which is probably faster *g*)

	   Please note that this patch works with both glibc 2.2.5 and 2.3.1.

,---[ glibc.mtab-patch ]--------------->8--------------------------------------
diff -Naur glibc-2.2.5/sysdeps/unix/sysv/linux/paths.h.orig glibc-2.2.5/sysdeps/unix/sysv/linux/paths.h
--- glibc-2.2.5/sysdeps/unix/sysv/linux/paths.h.orig	2000-11-16 00:06:47.000000000 +0100
+++ glibc-2.2.5/sysdeps/unix/sysv/linux/paths.h	2003-01-02 13:37:12.000000000 +0100
@@ -51,7 +51,7 @@
 #define	_PATH_MAN	"/usr/share/man"
 #define	_PATH_MEM	"/dev/mem"
 #define	_PATH_MNTTAB	"/etc/fstab"
-#define	_PATH_MOUNTED	"/etc/mtab"
+#define	_PATH_MOUNTED	"/var/lib/misc/mtab"
 #define	_PATH_NOLOGIN	"/etc/nologin"
 #define	_PATH_PRESERVE	"/var/lib"
 #define	_PATH_RWHODIR	"/var/spool/rwho"
`-------------------------------------->8--------------------------------------

	Look up the installation instructions for glibc in the LFS Book.

	This patch makes newly compiled programs look for the mtab file
	in /var/lib/misc/mtab.

	4. Now unmount /var

		# umount /var

	5. Create /var/lib/misc/mtab.  This file should contain only
	   one line that lists your root partition mounted read-only.
	   On my system, using devfs, it looked like this:

		# mkdir -p /var/lib/misc
		# echo "/dev/discs/disc0/part2 / ext3 ro 0 0" > /var/lib/misc/mtab
		# chown root:root /var/lib/misc/mtab
		# chmod 644 /var/lib/misc/mtab

	   Substitute your root device and file system according to your setup.

	6. To prevent scripts and broken code from failing, create
	   /etc/mtab as a link to /var/lib/misc/mtab

		# ln -sf ../var/lib/misc/mtab /etc/mtab

	   Be careful: from this point on, you lose all information about the
	   currently mounted filesystems (that umount can work with, at least)!
	   If you have any filesystems mounted that do not get mounted at boot-
	   time (ie. by the /etc/rc.d/init.d/mountfs script), unmount them
	   _before_ this step! If you are not sure what you have mounted, you
	   can still take a look at /proc/mounts.

	7. At this point you have to mount /var again

		# mount -n /var

	8. Create an empty /var/lib/misc/mtab

		# touch /var/lib/misc/mtab
		# chown root:root /var/lib/misc/mtab
		# chmod 644 /var/lib/misc/mtab

	9. Now you have to modify your init scripts so that they basically
	   do this:

		1. mount -n /proc	# this is in /etc/rc.d/init.d/mountproc
		2. fsck			# this is in /etc/rc.d/init.d/checkfs
		3. mount -n /var	# so that /var/lib/misc/mtab shows up
		4. > /var/lib/misc/mtab # clear /var/lib/misc/mtab
		5. mount -f /		# update mtab with the already
		6. mount -f /proc	# mounted partitions.
		7. mount -f /var	#
		8. mount -a		# mount all remaining partitions

	   The first step requires a change in /etc/rc.d/init.d/mountproc,
	   whereas the second one does not require any changes.
 	   The remainder is handled in /etc/rc.d/init.d/mountfs:

,---[ /etc/rc.d/init.d/mountfs ]------->8--------------------------------------
#!/bin/bash
# Begin $rc_base/init.d/mountfs - File System Mount Script

# Based on mountfs script from LFS-3.1 and earlier.
# Rewritten by Gerard Beekmans  - gerard at linuxfromscratch.org

source /etc/sysconfig/rc
source $rc_functions

case "$1" in
        start)
		# Uncomment the following three lines if you do not have
		# a read only root filesystem.

		# echo "Remounting root file system in read-write mode..."
		# mount -n -o remount,rw /
		# evaluate_retval

                echo "Cleaning /var/lib/misc/mtab..."
                mount -n /var &&
		> /var/lib/misc/mtab &&
		mount -f / &&
		mount -f /proc &&
		mount -f /var
		evaluate_retval

                # The following mount command will mount all file systems.  If
                # you have other (network based) file system that should not
                # be or cannot be mounted at this time, add them to the NO_FS
                # variable below.  All file systems that are added to the
		# variable in the form of no<filesystem> will be skipped.

                NO_FS="nonfs,nosmbfs,noproc"
                echo "Mounting remaining file systems..."
                mount -a -t $NO_FS
                evaluate_retval
                ;;

[...remainder of the script omitted...]
`-------------------------------------->8--------------------------------------

	    If you use another set of init scripts, you are surely competent
	    enough to know how to implement these changes in your scripts.

	10. Modify your /etc/fstab so that /proc and /var do not get mounted by
	    mount -a:

		/dev/discs/disc0/part7  /var  xfs  noauto	0 0
		proc			/proc proc noauto	0 0

	    The important part here is the `noauto' keyword.
	    This is, again, what it looks like on my system.  Change it
	    so that it fits your system.
	    Note that with the lfs-bootscripts, /proc does not really need
	    the `noauto' option.  It is good to have it though, in case you
	    ever run `mount -a' from outside of any bootscripts (like in
	    step 13 :-).  So while you are at it...

	11. Rebuild util-linux so that mount, umount and friends are
	    aware of mtab's new location.  There are no patches required,
	    just look up the instructions in the LFS Book.

	    Other packages you might want to rebuild are the GNU fileutils
	    (df) and other programs that might access /etc/mtab.
	    As long as these programs do not write to that file, it is not
	    absolutely required, though.  That is why we set up that symlink.

	12. If you have a read-only root, remount / read-only now, since 
	    we are not changing any more files.

		# mount -n -o remount,ro /

	13. If you have not unmounted any other partitions during the process,
	    all those listed in /etc/fstab should still be mounted.  Check out
	    /proc/mounts to be sure.  The userland tools do not know all this
	    anymore, so we have to tell them:

		# mount -f /
		# mount -f /proc
		# mount -f /var
		# mount -af

	    Be sure that /etc/mtab and /proc/mounts contain consistent data
	    after this step.

	14. Finally done! You can telinit back to your favourite run-level now.



CONCLUSION
==========

Credits are due to Seth W. Klein for teaching me through the process in the
first place and further corrections and suggestions.  Also thanks to DJ Lucas,
Jesse Tie-Ten-Quee, Richard Lightman and Wouter Vanwalleghem for additional
feedback.

If you have any questions, comments or suggestions regarding this hint, write
to me at <obrakmann at gmx.net> or to the blfs-support mailing list.

I hope this helps some people out there.

Oliver Brakmann


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