path: root/etc/etc.i386/INSTALL.linux

$OpenBSD: INSTALL.linux,v 1.4 1998/09/25 01:58:38 espie Exp $

Warning: this document is currently being reviewed. It's not yet complete,
and probably contains loads of errors. As an example, I can't figure out
why Linux doesn't need mkswap as it shares the exact same blocks with
OpenBSD.


   Linux + OpenBSD: it's possible

   by Marc Espie --  Marc.Espie@OpenBSD.org

It is perfectly possible to have Linux and OpenBSD on the same disk.
As of this writing, OpenBSD can read and write Linux' partitions, whereas
Linux cannot access OpenBSD partitions (they differ from NetBSD partitions).

You can even install OpenBSD from an ext2fs partition (choose install from
disk... ext2fs does not appear in the choices, but `default' it is).

First, make a bootable floppy of Linux.  Then, you  have to find some room
for OpenBSD.  Don't worry about swap space: you can share Linux' swap
partition.

If you want to grab space from a Windows/DOS partition, use fips.
Fips20 knows all about FAT32, so windows 95 is no longer a problem.

Other sources of information, especially concerning other BSD systems,
must be taken with a healthy does of scepticism.

First principles
----------------
OpenBSD does not use the DOS partitions for more than booting.  You just
need a small DOS partition to put your OpenBSD root.  Afterwards, OpenBSD
heeds some other information entirely, called the BSD disklabel.  This
disklabel is another completely distinct description of your hard disk. 
It does not even have to be consistent with the usual DOS partitions 
information.

Throughout this document, we will distinguish between DOS partitions and
BSD partitions whenever this is necessary.

For consistency, it's better if all parts of the disks that 
are actually used by OpenBSD are flagged as occupied, type A6, though it 
is not necessary.  The only part of the disk that should appear both as a BSD
partition and as a DOS partition is the root partition: it MUST begin on
the same sector for the boot process to work.

One way to visualize things is to picture OpenBSD embedded inside DOS
partitions: the DOS partitions used by OpenBSD may each contain several
OpenBSD partitions. As long as the DOS partition table has the right
start and length for each partition it will be kept happy.

The OpenBSD disklabel is just another mechanism that yield another 
description of the disk. It is vitally important that the BSD root
partition start precisely where the corresponding DOS partition is supposed
to start, and it is better when all BSD partitions stay inside their
DOS partitions boundaries.  Apart from OpenBSD partitions proper, the
BSD disklabel can contain a BSD description of other DOS partitions, but
this is not mandatory. If you don't have any constraints, having a correct
description of all partitions is better, but with bigger disks, keep in mind
that OpenBSD disklabels can't hold more than 16 partitions.

Contrarily to popular belief, OpenBSD does *NOT* need one contiguous chunk 
of the disk (a `slice' in FreeBSD lingo).  It is probably the simplest setup, 
but other considerations (such as the need to boot several OS, and to have 
several small primary partitions that all within the first  1024 BIOS
cylinders) may lead you to use two chunks for OpenBSD.

If you can, it is MUCH better to devote a full disk to OpenBSD: this limits
the number of mistakes you can do. Admittedly there are some cases where
this isn't a option (my machine is a laptop... I have to cope with the
hard disk I have).

Mapping your disk
-----------------
Starting from Linux, get a grasp of your partitions. Use df to check which
is what, then fdisk to get the actual setup of the disk.
Here is my disk:
--
Disk /dev/hda: 128 heads, 63 sectors, 993 cylinders
Units = cylinders of 8064 * 512 bytes

   Device Boot   Begin    Start      End   Blocks   Id  System
/dev/hda1            1        1      260  1048288+   6  DOS 16-bit >=32M
/dev/hda2          261      261      273    52416   83  Linux native
/dev/hda4          287      287      601  1270080    5  Extended
/dev/hda5          287      287      303    68512   82  Linux swap
/dev/hda6          304      304      456   616864+  83  Linux native
/dev/hda7          457      457      520   258016+  83  Linux native
/dev/hda8          521      521      537    68512+  83  Linux native
/dev/hda9          538      538      601   258016+  83  Linux native

(In case you're wondering, yes this is a big disk. The Linux playground is 
large, the OpenBSD area will be huge. As a developer, I usually have loads
of source & binaries lying around... a simple OpenBSD installation can fit
within 300 Mb with room to spare.)
In my setup hda2 is /, hda6 is /usr, hda7 is /usr/local, hda8 is /var, and
hda9 is /home.

Get the display to sectors with u, and jot down the corresponding
information as well:
--
Disk /dev/hda: 128 heads, 63 sectors, 993 cylinders
Units = sectors of 1 * 512 bytes

   Device Boot   Begin    Start      End   Blocks   Id  System
/dev/hda1           63       63  2096639  1048288+   6  DOS 16-bit >=32M
/dev/hda2      2096640  2096640  2201471    52416   83  Linux native
/dev/hda4      2306304  2306304  4846463  1270080    5  Extended
/dev/hda5      2306368  2306368  2443391    68512   82  Linux swap
/dev/hda6      2443455  2443455  3677183   616864+  83  Linux native
/dev/hda7      3677247  3677247  4193279   258016+  83  Linux native
/dev/hda8      4193343  4193343  4330367    68512+  83  Linux native
/dev/hda9      4330431  4330431  4846463   258016+  83  Linux native

Okay, finally switch to expert mode, and note the corresponding data.
Disk /dev/hda: 128 heads, 63 sectors, 993 cylinders

Nr AF  Hd Sec  Cyl  Hd Sec  Cyl   Start    Size ID
 1 00   1   1    0 127  63  259      63 2096577 06
 2 00   0   1  260 127  63  272 2096640  104832 83
 3 00   0   0    0   0   0    0       0       0 00
 4 00   0   1  286 127  63  600 2306304 2540160 05
 5 00   1   2  286 127  63  302      64  137024 82
 6 00   1   1  303 127  63  455      63 1233729 83
 7 00   1   1  456 127  63  519      63  516033 83
 8 00   1   1  520 127  63  536      63  137025 83
 9 00   1   1  537 127  63  600      63  516033 83

Note that this is STILL the same data. The good point about this last
display is that it is almost what you're going to see in OpenBSD fdisk !

There are some differences though, mostly because Linux fdisk has made
some rather confusing choices:
- in simple mode it starts numbering cylinders at 1... whereas 
everything else starts from 0.
- in simple mode it shows blocks of 1024 bytes, which makes for half-blocks
(marked with a +) and sizes halved from the real block size.
- in expert mode it shows extended partitions offset from the start 
of the extended partition. 
- the hd/sec/cyl is a confusing order, as the sector number is computed
from cyl/hd/sec, in that order.
- it never shows and doesn't care about the real disk geometry.

You will notice that the 3rd primary partition is empty... this is where 
I intend to stick my OpenBSD root partition (both DOS and BSD partitions),
and that I have left cylinders 601--992 empty... this is where I intend to
stick the rest of OpenBSD.
After OpenBSD is installed, partition 3 will show up as:
/dev/hda3          274      274      286    52416   a6  Unknown
/dev/hda3      2201472  2201472  2306303    52416   a6  Unknown

(recent Linux fdisk will display a6 as OpenBSD, but they still don't know
how to deal with the disklabel. Important: NEVER use Linux fdisk to fiddle
with OpenBSD disklabels.)


Before starting to install OpenBSD, now would be a good time to check the
INSTALL.pt document...  Especially note the alignment restriction of 
partitions (first sector of a partition must be at head 0, sector 1 of a
cylinder). This is enforced by Linux' fdisk.

The other point to note is that extended partitions are actually linked
lists. This will show up in OpenBSD' fdisk.

Your clock and OpenBSD
----------------------
OpenBSD expects your hardware clock to be in universal time, and uses 
time zones to give you local time. With Linux, this depends... 
most distributions use a small program called hwclock to set up the 
system time from the hardware clock when booting... there is a --utc 
option if your hardware clock is in universal time, but this is not 
always what happens by default.

For instance, on a redhat 5.1 system, this happens in /etc/rc.d/rc.sysinit
which loads an /etc/sysconfig/clock that defines a variable called UTC, and
then proceeds calling hwclock.
- ensure UTC is set to true,
- adjust your hardware clock from the system time if necessary, e.g.,
hwclock --systohc --utc.

The Linux partition table and OpenBSD
-------------------------------------
There is a problem with many Linux rc that do mount all file systems even
in single-user mode.  After you've installed OpenBSD, if your Linux kernel
knows about BSD disklabels, it may insert lots of BSD partitions in its
list. Then, at the next reboot, you're in trouble. The simplest way around
this problem is probably to make sure you can boot from a Linux kernel 
that doesn't know about disklabels.   Otherwise, you may wish to check 
your inittab and your rc to make deadly sure that single-user boot 
will work.

The OpenBSD installation
------------------------
If you've got the space, you can install from your ext2fs partitions. This
is what I did, as I have a slip connection to the rest of the world, and
the OpenBSD install floppy does not include slip.

REMEMBER TO BACKUP ALL IMPORTANT DATA ON YOUR DISK BEFORE DOING THE 
INSTALLATION !!!

So you cp floppy*.fs /dev/fd0, then reboot...

First, the BSD kernel + ram disk loads, then there is a boot prompt, and
five seconds later, the boot proper starts.

After a while, you will see your disk configuration scroll by.  I got:
  wd0 at wdc0 drive 0: <TOSHIBA MK4006MAV>
  wd0: 3909MB, 7944 cyl, 16 head, 63 sec, 512 bytes/sec, 8007552 sec total
  wd0: using 16-sector 16-bit pio transfers, lba addressing

at which point I got somewhat confused, especially as this is a configuration with
more than 1024 cylinders.  In fact, this is the actual disk geometry, when
you inquire about it, but the geometry that the BIOS does see IS the fdisk
geometry, with 993 cylinders.  As the 1024 cylinders is purely a BIOS
limitation, there is no actual trouble.

If you want to be sure, enquire at the boot prompt:
boot>machine diskconfig
before the automatic boot sequence continues.


Then I got into fdisk, and I proceeded to enter my new OpenBSD partition.
This is what the fdisk dump looked after my changes:

Disk: wd0       geometry: 992/128/63 [7999488 sectors]
Offset: 0       Signatures: 0xAA55,0x0
         Starting        Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
-------------------------------------------------------------------------
 0: 06    0   1   1 -  259 127  63 [        63 -    2096577] DOS > 32MB  
 1: 83  260   0   1 -  272 127  63 [   2096640 -     104832] Linux files*
 2: A6  273   0   1 -  285 127  63 [   2201472 -     104832] OpenBSD     
 3: 05  286   0   1 -  600 127  63 [   2306304 -    2540160] Extended DOS

Selected extended partition 3
New MBR at offset 2306304.
Offset: 2306304 Signatures: 0xAA55,0x0
         Starting        Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
-------------------------------------------------------------------------
 0: 82  286   1   2 -  302 127  63 [   2306368 -     137024] Linux swap  
 1: 05  303   0   1 -  455 127  63 [   2443392 -    1233792] Extended DOS
 2: 00    0   0   0 -    0   0   0 [   2306304 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [   2306304 -          0] unused      

Selected extended partition 1
New MBR at offset 2443392.
Offset: 2443392 Signatures: 0xAA55,0x0
         Starting        Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
-------------------------------------------------------------------------
 0: 83  303   1   1 -  455 127  63 [   2443455 -    1233729] Linux files*
 1: 05  456   0   1 -  519 127  63 [   3677184 -     516096] Extended DOS
 2: 00    0   0   0 -    0   0   0 [   2443392 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [   2443392 -          0] unused      

Selected extended partition 1
New MBR at offset 3677184.
Offset: 3677184 Signatures: 0xAA55,0x0
         Starting        Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
-------------------------------------------------------------------------
 0: 83  456   1   1 -  519 127  63 [   3677247 -     516033] Linux files*
 1: 05  520   0   1 -  536 127  63 [   4193280 -     137088] Extended DOS
 2: 00    0   0   0 -    0   0   0 [   3677184 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [   3677184 -          0] unused      

Selected extended partition 1
New MBR at offset 4193280.
Offset: 4193280 Signatures: 0xAA55,0x0
         Starting        Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
-------------------------------------------------------------------------
 0: 83  520   1   1 -  536 127  63 [   4193343 -     137025] Linux files*
 1: 05  537   0   1 -  600 127  63 [   4330368 -     516096] Extended DOS
 2: 00    0   0   0 -    0   0   0 [   4193280 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [   4193280 -          0] unused      

Selected extended partition 1
New MBR at offset 4330368.
Offset: 4330368 Signatures: 0xAA55,0x0
         Starting        Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
-------------------------------------------------------------------------
 0: 83  537   1   1 -  600 127  63 [   4330431 -     516033] Linux files*
 1: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 2: 00    0   0   0 -    0   0   0 [   4330368 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [   4330368 -          0] unused      

Nothing to it... you just follow the extended partition links using select,
jot down whatever you need, add the OpenBSD partition to look like you want
it to, and save everything.

After you leave fdisk, you get to the interesting part: the disklabel
itself. If all goes well, OpenBSD synthesizes a nice disklabel out of what
it can deduce from the disk, including the ext2fs partitions.

There are only a few subtleties to take care of:
- initially, you can ONLY edit the disklabel part that matches the OpenBSD
DOS partition (a `slice' in FreeBSD lingo). You have to use b 0 *  before
you can edit the whole disk.
- the real disk geometry becomes relevant. The Berkeley fast file system 
can't use partial cylinder groups, hence BSD partitions should start 
on cylinder boundaries, as any remaining sectors  will be lost anyway.
- units for size and offset can be given as sectors (default) or cylinders.

After edition, this is what my disklabel looks like:

# using MBR partition 2: type A6 off 2201472 (0x219780) size 104832 (0x19980)
# /dev/rwd0c:
type: ESDI
disk: 
label: TOSHIBA MK4006M
flags:
bytes/sector: 512
sectors/track: 63
tracks/cylinder: 16
sectors/cylinder: 1008
cylinders: 7944
total sectors: 8007552
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0           # milliseconds
track-to-track seek: 0  # milliseconds
drivedata: 0 

16 partitions:
#        size   offset    fstype   [fsize bsize   cpg]
  a:   104832  2201472    4.2BSD     1024  8192    16   # (Cyl. 2184 - 2287)
  b:   137024  2306368      swap                        # (Cyl. 2288 - 2423*)
  c:  8007552        0    unused        0     0         # (Cyl.    0 - 7943)
  d:   409248  4846464    4.2BSD     1024  8192    16   # (Cyl. 4808 - 5213)
  e:   511056  5255712    4.2BSD     1024  8192    16   # (Cyl. 5214 - 5720)
  f:   204624  5766768    4.2BSD     1024  8192    16   # (Cyl. 5721 - 5923)
  g:  1073520  5971392    4.2BSD     1024  8192    16   # (Cyl. 5924 - 6988)
  h:   962640  7044912    4.2BSD     1024  8192    16   # (Cyl. 6989 - 7943)
  i:  2096577       63     MSDOS                        # (Cyl.    0*- 2079*)
  j:   104832  2096640    ext2fs                        # (Cyl. 2080 - 2183)
  l:  1233729  2443455    ext2fs                        # (Cyl. 2424*- 3647*)
  m:   516033  3677247    ext2fs                        # (Cyl. 3648*- 4159*)
  n:   137025  4193343    ext2fs                        # (Cyl. 4160*- 4295*)
  o:   516033  4330431    ext2fs                        # (Cyl. 4296*- 4807*)

Things to check:
- this disklabel is saved in MBR2 (basic DOS partition 2), as expected.
- all the BSD partitions proper are aligned on a cylinder boundary.
the root partition begins at the precise same offset the corresponding DOS
partition begins, and it extends for the same length.
Other BSD partitions don't show up in the DOS partition setup, hence they
begin precisely on cylinder 601/4808.
- the ext2fs partitions have the exact same layout under the OpenBSD
disklabel.

One point that is somewhat laborious is that the disklabel -E mode
(which you are currently using) tends to move partitions around to ensure
that ALL defined partitions are contiguous. Hence, you may need some fiddling
around and printing to ensure that Linux partitions do show up where they
should.  In my case, disklabel moved the swap and all the ext2fs partitions
slightly, and I add to adjust them manually...

Once the disklabel is written to disk, the installation proceeds as usual.

ext2fs partitions are perfectly usable from OpenBSD. My /home partition
is ext2fs, I have been using it for a week now without any trouble.

Booting
-------
First time I booted my system back, I did not get into OpenBSD as expected... 
I plain forgot I had installed lilo in the master boot block, and lilo 
does not heed the active partition flag. The fix was rather simple: from
the Linux system, I just had to edit lilo.conf to add the obsd entry:

boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
image=/boot/myvmlinuz
   label=linux
   root=/dev/hda2
   vga=4
   read-only
image=/boot/vmlinuz-2.0.34-1
   label=redhat
   root=/dev/hda2
   read-only
other=/dev/hda1
   label=dos
   table=/dev/hda
other=/dev/hda3
   label=obsd
   table=/dev/hda

rerun lilo, and voila, OpenBSD was able to boot !

Linux and OpenBSD partitions
----------------------------
As of 2.0.35/2.1.122, Linux does not support OpenBSD partitions. The way
the Linux kernel works is reasonably straightforward (code in
drivers/block/genhd.c): when detecting a BSD partition (type A5), 
it checks and parses the first block for a disklabel, and use that
information to instantiate some new partitions, 
that will show up in that drive's partition list, exactly like extended
partitions do show up. Unfortunately, that check is hard coded for a type A5
(NetBSD/FreeBSD) partition, and it recognizes 8 partitions disklabels only
anyway.

Just hacking the Linux code to recognize A6 is not quite enough.  You also
need to trim down the resulting list of partitions, as your ext2fs
partitions will now show up twice !
I'm experimenting with the following patch, which should make it to linux-kernel
soon:
--------------------------------------------------------------
diff -ur linux.orig/drivers/block/genhd.c linux/drivers/block/genhd.c
--- linux.orig/drivers/block/genhd.c	Mon Aug  4 20:45:35 1997
+++ linux/drivers/block/genhd.c	Thu Sep 24 18:55:31 1998
@@ -205,11 +206,46 @@
 }
 
 #ifdef CONFIG_BSD_DISKLABEL
+static void check_and_add_bsd_partition(struct gendisk *hd, struct bsd_partition *bsd_p)
+{
+	struct hd_struct *lin_p;
+		/* check relative position of partitions.  */
+	for (lin_p = hd->part + 1; lin_p - hd->part < current_minor; lin_p++) {
+			/* no relationship -> try again */
+		if (lin_p->start_sect + lin_p->nr_sects <= bsd_p->p_offset 
+			|| lin_p->start_sect >= bsd_p->p_offset + bsd_p->p_size)
+			continue;	
+			/* equal -> no need to add */
+		if (lin_p->start_sect == bsd_p->p_offset && 
+			lin_p->nr_sects == bsd_p->p_size) 
+			return;
+			/* bsd living within dos partition */
+		if (lin_p->start_sect <= bsd_p->p_offset && lin_p->start_sect 
+			+ lin_p->nr_sects >= bsd_p->p_offset + bsd_p->p_size) {
+#ifdef DEBUG_BSD_DISKLABEL
+			printk("w: %d %ld+%ld,%d+%d", 
+				lin_p - hd->part, lin_p->start_sect, lin_p->nr_sects,
+				bsd_p->p_offset, bsd_p->p_size);
+#endif
+			break;
+		}
+			/* ouch: bsd and linux overlap. Don't even try for that partition */
+#ifdef DEBUG_BSD_DISKLABEL
+		printk("???: %d %ld+%ld,%d+%d",
+			lin_p - hd->part, lin_p->start_sect, lin_p->nr_sects,
+			bsd_p->p_offset, bsd_p->p_size);
+#endif
+		printk("???");
+		return;
+	} /* if the bsd partition is not described by DOS, we end up there */
+	add_partition(hd, current_minor, bsd_p->p_offset, bsd_p->p_size);
+	current_minor++;
+}
 /* 
  * Create devices for BSD partitions listed in a disklabel, under a
  * dos-like partition. See extended_partition() for more information.
  */
-static void bsd_disklabel_partition(struct gendisk *hd, kdev_t dev)
+static void bsd_disklabel_partition(struct gendisk *hd, kdev_t dev, int max_partitions)
 {
 	struct buffer_head *bh;
 	struct bsd_disklabel *l;
@@ -225,19 +261,15 @@
 		return;
 	}
 
-	p = &l->d_partitions[0];
-	while (p - &l->d_partitions[0] <= BSD_MAXPARTITIONS) {
+	if (l->d_npartitions < max_partitions)
+		max_partitions = l->d_npartitions;
+	for (p = l->d_partitions; p - l->d_partitions <  max_partitions; p++) {
 		if ((current_minor & mask) >= (4 + hd->max_p))
 			break;
-
-		if (p->p_fstype != BSD_FS_UNUSED) {
-			add_partition(hd, current_minor, p->p_offset, p->p_size);
-			current_minor++;
-		}
-		p++;
+		if (p->p_fstype != BSD_FS_UNUSED)
+			check_and_add_bsd_partition(hd, p);
 	}
 	brelse(bh);
-
 }
 #endif
 
@@ -248,6 +280,11 @@
 	struct partition *p;
 	unsigned char *data;
 	int mask = (1 << hd->minor_shift) - 1;
+#ifdef CONFIG_BSD_DISKLABEL
+	/* no bsd disklabel as a default */
+	kdev_t bsd_kdev = 0;
+	int bsd_maxpart;
+#endif
 #ifdef CONFIG_BLK_DEV_IDE
 	int tested_for_xlate = 0;
 
@@ -365,13 +402,29 @@
 				hd->part[minor].nr_sects = 2;
 		}
 #ifdef CONFIG_BSD_DISKLABEL
+			/* tag first disklabel for late recognition */
 		if (SYS_IND(p) == BSD_PARTITION) {
-			printk(" <");
-			bsd_disklabel_partition(hd, MKDEV(hd->major, minor));
-			printk(" >");
+			printk("!");
+			if (!bsd_kdev) {
+				bsd_kdev = MKDEV(hd->major, minor);
+				bsd_maxpart = BSD_MAXPARTITIONS;
+			}
+		} else if (SYS_IND(p) == OPENBSD_PARTITION) {
+			printk("!");
+			if (!bsd_kdev) {
+				bsd_kdev = MKDEV(hd->major, minor);
+				bsd_maxpart = OPENBSD_MAXPARTITIONS;
+			}
 		}
 #endif
 	}
+#ifdef CONFIG_BSD_DISKLABEL
+	if (bsd_kdev) {
+		printk(" <");
+		bsd_disklabel_partition(hd, bsd_kdev, bsd_maxpart);
+		printk(" >");
+	}
+#endif
 	/*
 	 *  Check for old-style Disk Manager partition table
 	 */
diff -ur linux.orig/include/linux/genhd.h linux/include/linux/genhd.h
--- linux.orig/include/linux/genhd.h	Wed Mar 18 20:25:30 1998
+++ linux/include/linux/genhd.h	Thu Sep 24 18:55:43 1998
@@ -69,12 +70,19 @@
 #ifdef CONFIG_BSD_DISKLABEL
 /*
  * BSD disklabel support by Yossi Gottlieb <yogo@math.tau.ac.il>
+ *
+ * updated by Marc Espie <Marc.Espie@openbsd.org>
  */
 
+/* check against BSD src/sys/sys/disklabel.h for consistency */
+
 #define BSD_PARTITION		0xa5	/* Partition ID */
+#define OPENBSD_PARTITION	0xa6
 
 #define BSD_DISKMAGIC	(0x82564557UL)	/* The disk magic number */
 #define BSD_MAXPARTITIONS	8
+#define OPENBSD_MAXPARTITIONS 16
+
 #define BSD_FS_UNUSED		0	/* disklabel unused partition entry ID */
 struct bsd_disklabel {
 	__u32	d_magic;		/* the magic number */
----------------------------------------------------------------------
It does:
- trigger the recognition of OpenBSD disklabels,
- force bsd partitions to appear at the end of the partition list,
- check the bsd disklabel for redundancy and consistency with the dos
partition table, so that the same partition does not show up twice.

You also need a working ufs, such as
<ftp://sunsite.unc.edu/pub/Linux/ALPHA/ufs/u2fs-0.4.3.tar.gz>
This package yields a new module, plus a small patch to tie the module
with the rest of the Linux kernel. I would recommend checking that patch
manually, as Linux module information tends to vary widely, and it is
pretty trivial to add by hand anyway.

Running Linux binaries under OpenBSD
------------------------------------
You just have to recompile your BSD kernel with COMPAT_LINUX, and set up
/emul/linux as explained in compat_linux(1). 

It's a good idea to mount your Linux file system under another point, then
make symbolic links so that you can control what gets used precisely.
Don't bother with the ports emul/linux_lib entry: it's only a set of Linux
libraries for people who don't have a Linux system running.

As of this writing, most applications work, apart from sound.

The Linux sound devices use differing ioctl from OpenBSD, hence anything that
needs to change the audio mode won't work, and produce audio garbage at best.

A small detail that may cause problems: uname still says `OpenBSD', even
under Linux compatibility.  The reason behind that is that we don't want
netscape to tell it was run from a Linux box, when it is used under
OpenBSD.

Some programs, for instance maple, do depend on uname answering `Linux'.
For maple, this is straigthforward: you just have to fudge
/usr/local/maple/bin/maple.system.type to check OpenBSD in the same
class with Linux.

Similar shell scripts are easy to fix.  Binary programs that don't run
suid can be coerced by using LD_PRELOAD.

As a rule, this should be achieved on a program-by-program basis. 
The more networking programs that do tell they're running under OpenBSD,
the merrier !