Based on portage-3.0.8 and gentoo-20201029

The specifics of this document (software versions and whatnot) are subject to
change as gentoo evolves, but I hope the big lines of it won't change too
much.

This guide starts off with a system built from build_cross.sh, built for
x86_64 with i686 multilib.
It might be possible to adapt both that script, and this guide for other
architectures, but this guide won't go into detail.

You should be running a system with a x86_64 kernel (with i686 support).
Install the required sources (outlined in gentoo.wget-list) into /sources.

To enter the bootstrap system as a chroot, use the following commands to set
up the filesystems and entering:
mkdir -p proc sys dev tmp
mount -t proc proc proc
mount -t sysfs sysfs sys
mount -t devtmpfs devtmpfs dev
mount -t devpts devpts dev/pts
mount -t tmpfs tmpfs dev/shm
mkdir -p etc
touch etc/resolv.conf
mount --bind /etc/resolv.conf etc/resolv.conf
./bootstrap/bin/chroot . /bootstrap/bin/env -i PATH=/bin:/sbin:/usr/bin:/usr/sbin:/bootstrap/bin:/bootstrap/sbin HOME=/root /bootstrap/bin/sh -l

To install all the prerequisite tools, place the sources (listed in
gentoo.wget-list) under /sources, then run the gentoo_tools.sh script.

Install ebuild repository:
tar xf gentoo-*.tar.xz
mkdir -p /var/db/repos/
mv gentoo-*/ /var/db/repos/gentoo

Configure portage:
echo "root:x:0:0:root:/root:/bin/bash" > /etc/passwd
echo "root:x:0:root" > /etc/group
echo "portage:x:250:250:portage:/var/tmp/portage:/bin/false" >> /etc/passwd
echo "portage::250:portage" >> /etc/group
mkdir -p /tmp /root
chown root.root /tmp /root
chmod 1777 /tmp
chmod 0700 /root
ln -sf ../../var/db/repos/gentoo/profiles/default/linux/amd64/17.1/ /etc/portage/make.profile
cat > /etc/portage/make.conf << 'EOF'
FEATURES=-pid-sandbox
ROOTPATH=/bootstrap/bin:/bootstrap/sbin
EOF

Install some prerequisites:
emerge -O1 sys-apps/gentoo-functions  # Used by elt-patches and a ton of other things
emerge -O1 app-portage/elt-patches  # required by elibtoolize(libtool.eclass), used to install any library

TODO: Doing this is *BAAAAD*, since we're linking the compiler and libraries
with a potentially incompatible version of glibc in /tools, and upgrading it
afterwards. Glibc should always be installed first into a new prefix!
I just can't be assed to discuss the manual upgrade of binutils+gcc and their
dependency tools.

TODO: Consider installing sys-devel/gnuconfig for the config.sub and
config.guess scripts.

Bootstrap GCC:
# Install linker at final location,
#  so programs built with the new compiler can run...
mkdir -p /lib64 /lib
ln -sf /bootstrap/lib/ld-linux-x86-64.so.2 /lib64/ld-linux-x86-64.so.2
ln -sf /bootstrap/lib32/ld-linux.so.2 /lib/ld-linux.so.2
# Target compiler expects target limits.h to be available in target prefix...
mkdir -p /usr/include
ln -sf /bootstrap/include/limits.h /usr/include/limits.h
# GCC errors out at the first incompatible startfile in LIBRARY_PATH,
#   so multilib paths need to be fixed to match the final compiler...
mkdir -p /usr/lib64 /usr/lib
ln -sf /bootstrap/lib/crti.o /bootstrap/lib/crtn.o /bootstrap/lib/Scrt1.o /usr/lib64/
ln -sf /bootstrap/lib32/crti.o /bootstrap/lib32/crtn.o /bootstrap/lib32/Scrt1.o /usr/lib/
# Reconfigure GCC to match the config of the final compiler as much as possible.
cat > /etc/portage/gcc.specs << 'EOF'
*multilib:
. !m64 !m32;64:../lib64 m64 !m32;32:../lib !m64 m32;

*startfile_prefix_spec:
/usr/lib/

*link:
+ %{!shared:%{!static:-dynamic-linker %{m32:/lib/ld-linux.so.2;:/lib64/ld-linux-x86-64.so.2}}}

*cpp:
+ -isystem /usr/include
EOF
cat > /etc/portage/bashrc << 'EOF'
export C_INCLUDE_PATH=/usr/include:/bootstrap/include
export CPLUS_INCLUDE_PATH=/usr/include:/bootstrap/include
export LIBRARY_PATH=/usr/lib64:/usr/lib:/bootstrap/lib:/bootstrap/lib32
export LD_LIBRARY_PATH=/usr/lib64:/usr/lib:/bootstrap/lib:/bootstrap/lib32
export CC='gcc -specs=/etc/portage/gcc.specs'
export CXX='g++ -specs=/etc/portage/gcc.specs'
EOF
emerge -O1 sys-libs/zlib dev-libs/gmp dev-libs/mpfr dev-libs/mpc
emerge -O1 sys-devel/binutils-config sys-devel/binutils
emerge -O1 sys-devel/gcc-config sys-devel/gcc
/usr/bin/gcc-config 1  # Not automatic unlike binutils-config

Make portage forget about everything it's just installed, so it's all rebuilt
first thing when running bootstrap.sh/emerge later:
rm -rf /var/db/pkg

Install initial glibc:
CC=gcc emerge -O1 sys-kernel/linux-headers
mkdir -p /usr/bin/
ln -sf /bootstrap/bin/python3 /usr/bin/python3tmp
PYTHON_COMPAT_OVERRIDE=python3tmp emerge -O1 sys-libs/glibc
rm -f /usr/bin/python3tmp

This compiler and dynamic linker will now pick up everything correctly under
/usr instead of /bootstrap:
rm -f /etc/portage/bashrc /etc/portage/gcc.specs

Build the remainder of bootstrap utilities:
cd /var/db/repos/gentoo/scripts
rm -f /etc/profile.env
mkdir -p /var/run
USE=-nls ./bootstrap.sh

You'll get tons of LD_PRELOAD libsandbox.so errors from programs still linked
against /bootstrap. This is fine to ignore, and will gradually disappear as
programs are rebuilt.

It'd be nice to remove the bootstrap tools now, but among wget/openssl, gawk,
gzip, grep, and other tools, they're still required for some reason.

From this point onward, you're pretty much on your own to beat portage into
performing an `emerge -e @system` and fixing any unspecified dependencies you
encounter along the way. This can be done in a whole number of ways, and will
differ on any given day, since this is painfully unsupported by gentoo devs.
Because the order in which you install everything can differ wildly, and
consequently affect the output of your build, it is advised to do a second
`emerge -e @system` afterwards, for sanity. Below I detail my most recent
procedure, which, I can't stress enough, may or may not work at the time
you're reading this.

Fix circular dependencies:
USE=-filecaps emerge -O1 sys-libs/pam
USE='-acl -xattr' emerge -O1 sys-apps/shadow  # required by acct-group/acct-user

Verify there's no circular deps left, for sanity:
emerge -fe @system

Build everything:
emerge -be @system

Clean up bootstrap tools/configs:
rm -rf /bootstrap /sources
rm -f /etc/portage/make.conf

Reinitialize environment:
env-update
exec bash -l

Consider rebuilding everything again without /bootstrap, as surely a lot
errors popped up during the first emerge -be @system, can't be sure enough.

To install everything into a clean root:
ROOT=/final emerge -K @system

Prepare files for catalyst:
mkdir -p /var/tmp/catalyst/snapshots
wget -O gentoo-latest.tar.xz http://distfiles.gentoo.org/snapshots/gentoo-latest.tar.xz
mkdir -p /var/tmp/catalyst/builds/default
cd /var/tmp/catalyst/builds/default
tar -C /final -cf stage3-amd64-latest.tar .
xz -9v stage3-amd64-latest.tar
rm -rf /final
wget -O stage1.spec "https://gitweb.gentoo.org/proj/releng.git/plain/releases/specs/amd64/stage1.spec"
wget -O stage2.spec "https://gitweb.gentoo.org/proj/releng.git/plain/releases/specs/amd64/stage2.spec"
wget -O stage3.spec "https://gitweb.gentoo.org/proj/releng.git/plain/releases/specs/amd64/stage3.spec"
sed -i -e 's/@TIMESTAMP@/latest/' -e '/^portage_confdir: /d' *.spec

Build the stage3:
emerge catalyst
catalyst -f stage1.spec