UML
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Installation
We'll only need a UML kernel:
make ARCH=um defconfig make ARCH=um
Usage
We'll need to create a (very basic) root file system first:
dd if=/dev/zero of=disk0.img bs=1M count=128 mkfs.ext2 disk0.img sudo mount -o loop disk0.img /mnt/disk/
As root:
cd /mnt/disk/
mkdir {bin,dev,mnt,proc,sys}
cat > init << EOF
#!/bin/sh
mount -t devtmpfs none /dev
mount -t proc none /proc
mount -t sysfs none /sys
echo
exec /bin/sh
EOF
chmod +x init
cp /sbin/busybox bin/
bin/busybox --install bin
With that in place, we should be able to start UML as a user:
./linux mem=1024M ubd0=disk0.img # uname -nrv (none) 5.3.0-rc6-00115-g9e8312f5e160 #1 Thu Aug 29 00:28:07 PDT 2019
The Gentoo Wiki has more examples on UML usage.