I’ve discovered some new things! One of them stops me from going much further, though.

86Box seems bugged

I’ve been using 86Box 4.2.1 to emulate a 486, but it crashes a lot and has mystifying bugs:

  • Changing ISOs crashes the emulator.

  • Networking is entirely borked; the network cards show up with all-0xFF MAC addresses, as well as NETDEV WATCHDOG errors related to timed-out transmissions.

  • Running mkfs.ext4 from within the liveCD causes it to die with a fatal glibc error:

    Fatal glibc error: malloc.c:4376 (_int_malloc): assertion failed: (unsigned long) (size) >= (unsigned long) (nb)
Aborted (core dumped)

Since I don’t have a real 486 on hand, this looks to be the end of the line for me.

The Gentoo install CD boots, but SIGILLs violently during initramfs

As of this writing, the Gentoo install CD has been updated to build the kernel for i486, but it seems early userspace isn’t too happy with 486s.

[  319.424232] Run /init as init process
[  319.427565]   with arguments:
[  319.430899]     /init
[  319.434232]     dokeymap
[  319.437565]     cdroot
[  319.444232]   with environment:
[  319.447565]     HOME=/
[  319.450899]     TERM=linux
[  319.454232]     BOOT_IMAGE=/boot/gentoo
[  319.464232] invalid opcode: 0000 [#7] PREEMPT SMP
[  319.464232] CPU: 0 PID: 1 Comm: init Tainted: G      D            6.6.57-gentoo-dist #1
[  319.464232] EIP: exit_to_user_mode_prepare+0x3e/0x1a0
[  319.464232] Code: f7 c3 0e 30 02 00 0f 85 b0 00 00 00 f6 c7 08 75 53 f7 c3 00 00 40 00 75 58 80 e7 40 75 5e 3e 8d 74 26 00 64 8b 0d 00 c9 37 cc <0f> 31 c1 c9 05 31 c1 64 89 0d 00 c9 37 cc 8b 35 30 00 3b cc 85 f6
[  319.464232] EAX: c11a4600 EBX: 00004000 ECX: 00000000 EDX: 00000000
[  319.464232] ESI: c11a4600 EDI: c11abfb4 EBP: c11abf84 ESP: c11abf78
[  319.464232] DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068 EFLAGS: 00000046
[  319.464232] CR0: 80050033 CR2: bfa83d5b CR3: 01017000 CR4: 00000000
[  319.464232] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
[  319.464232] DR6: 00000000 DR7: 00000400

This leads very quickly to a kernel panic.

[  319.464232] note: init[1] exited with irqs disabled
[  319.470899] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
[  319.470899] Kernel Offset: disabled
[  319.470899] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---

You don’t need the patch if you’re using an x86 toolchain

The CET instructions are in x86_64 libgcc.

$ objdump --disassemble /usr/lib/gcc/x86_64-pc-linux-gnu/14/libgcc.a | grep endbr
   0:   f3 0f 1e fa             endbr64
   0:   f3 0f 1e fa             endbr64
   0:   f3 0f 1e fa             endbr64
   0:   f3 0f 1e fa             endbr64
   0:   f3 0f 1e fa             endbr64
   0:   f3 0f 1e fa             endbr64
(...)

These get unconditionally included even with -m32 -march=i486, but there’s a fix: use an x86 toolchain.

If you just do not use an x86_64 toolchain, you will not encounter CET, and you will not need to rebuild your compiler with --disable-cet or use that patch I talked about last article.

The easiest way to find yourself an x86 toolchain is just to use an x86 Gentoo stage3, which leads into my next point:

You can use the Gentoo tooling to build stuff

This is pretty simple. You can use an x86 stage3 to build the kernel and initramfs using an appropriately-configured installkernel. Note that you must build the kernel from source, as gentoo-kernel-bin is built for i686 and higher.

What I did was to make my USE look like this:

sys-kernel/installkernel dracut
sys-fs/lvm2 lvm

installed the following package list in the chroot:

app-arch/lz4
sys-fs/lvm2
sys-fs/squashfs-tools
sys-kernel/dracut
sys-kernel/gentoo-sources
sys-kernel/installkernel

and configured Dracut like so:

# dmsquash-live allows the initramfs
# to mount squashfs as live systems
dracutmodules+=" dmsquash-live "

# you need iso9660 and squashfs support,
# either built-in or in the initramfs
add_drivers+=" iso9660 squashfs "

# lz4 is quicker to decompress than xz
compress="lz4"

Navigate to /usr/src/linux, configure the kernel as you wish, and make && make modules_install && make install. installkernel will do all the magic, and you should see a kernel and initramfs in /boot:

$ ls boot/
initramfs-6.6.58-gentoo-r1.img  kernel-6.6.58-gentoo-r1

But where do you place these files?

You could do it properly, or you could take apart and reassemble a Gentoo live install ISO.

Taking apart a minimal install CD

xorriso might be able to do this properly, but I’m in a hacky mood. Use bsdtar to extract the ISO, keeping in mind that it’ll splat files into your current directory; mkdir a directory and cd into it, or use the -C option.

$ mkdir iso
$ bsdtar -C iso -xvf install-x86-minimal-20241202T164822Z.iso

You’ll see a layout like this.

.
├── boot
│   ├── gentoo
│   ├── gentoo-config
│   ├── gentoo.igz
│   ├── grub
│   │ (...)
│   └── System-gentoo.map
├── boot.catalog
(...)
├── image.squashfs
├── livecd
(...)

16 directories, 973 files

The important bits are

  • gentoo (the kernel),

  • gentoo.igz (the initramfs, actually xz-compressed),

  • image.squashfs (the live system).

You can replace the kernel and initramfs with the ones you just made.

If you choose to edit the live system, you can use rdsquashfs and gensquashfs (part of squashfs-tools-ng) to do so.

The boot/grub/grub.cfg on this ISO is configured to look for a specific volume label. Edit that out unless you intend on recreating it.

 menuentry 'Boot LiveCD (kernel: gentoo)' --class gnu-linux --class os {
-       search --no-floppy --set=root -l Gentoo-x86-20241202
-       linux /boot/gentoo dokeymap root=live:CDLABEL=Gentoo-x86-20241202 rd.live.dir=/ rd.live.squashimg=image.squashfs cdroot
+       linux /boot/gentoo dokeymap root=live:/dev/sr0 rd.live.dir=/ rd.live.squashimg=image.squashfs cdroot
        initrd /boot/gentoo.igz
 }

 menuentry 'Boot LiveCD (kernel: gentoo) (cached)' --class gnu-linux --class os {
-       search --no-floppy --set=root -l Gentoo-x86-20241202
-       linux /boot/gentoo dokeymap root=live:CDLABEL=Gentoo-x86-20241202 rd.live.dir=/ rd.live.squashimg=image.squashfs cdroot rd.live.ram=1
+       linux /boot/gentoo dokeymap root=live:/dev/sr0 rd.live.dir=/ rd.live.squashimg=image.squashfs cdroot rd.live.ram=1
        initrd /boot/gentoo.igz
 }

Once you’re done, run grub-mkrescue -o gentoo.iso iso to generate your ISO. For me, this yields an ISO that successfully boots to a shell, though as mentioned earlier, the emulator’s bugs render this triumph short-lived.