vmtest: add mkrootfs.sh to build Arch Linux disk image

Generate a disk image for libbpf testing in compressed *.zst format

The mkrootfs.sh has the following stages:
- run pacstrap to install libbpf and selftests dependencies.
- create /etc/fstab w/ bpffs and debugfs filesystems
- create /etc/init.d/rcS to mount in bootime
- create /etc/inittab to invoke /etc/init.d/rcS
- compress an image

In addition ./travis-ci/vmtest/run.sh set up ext4 fs and mounts
it as a loop device:
mkfs.ext4 -q "$tmp"
mount -o loop "$tmp" "$mnt"

Signed-off-by: Julia Kartseva (hex@fb.com)

travis-ci/vmtest/mkrootfs.sh

@@ -0,0 +1,146 @@
+#!/bin/bash
+
+# This script is based on drgn script for generating Arch Linux bootstrap
+# images.
+# https://github.com/osandov/drgn/blob/master/scripts/vmtest/mkrootfs.sh
+
+set -euo pipefail
+
+usage () {
+	USAGE_STRING="usage: $0 [NAME]
+       $0 -h
+
+Build an Arch Linux root filesystem image for testing libbpf in a virtual
+machine.
+
+The image is generated as a zstd-compressed tarball.
+
+This must be run as root, as most of the installation is done in a chroot.
+
+Arguments:
+  NAME   name of generated image file (default:
+         libbpf-vmtest-rootfs-\$DATE.tar.zst)
+
+Options:
+  -h     display this help message and exit"
+
+	case "$1" in
+		out)
+			echo "$USAGE_STRING"
+			exit 0
+			;;
+		err)
+			echo "$USAGE_STRING" >&2
+			exit 1
+			;;
+	esac
+}
+
+while getopts "h" OPT; do
+	case "$OPT" in
+		h)
+			usage out
+			;;
+		*)
+			usage err
+			;;
+	esac
+done
+if [[ $OPTIND -eq $# ]]; then
+	NAME="${!OPTIND}"
+elif [[ $OPTIND -gt $# ]]; then
+	NAME="libbpf-vmtest-rootfs-$(date +%Y.%m.%d).tar.zst"
+else
+	usage err
+fi
+
+pacman_conf=
+root=
+trap 'rm -rf "$pacman_conf" "$root"' EXIT
+pacman_conf="$(mktemp -p "$PWD")"
+cat > "$pacman_conf" << "EOF"
+[options]
+Architecture = x86_64
+CheckSpace
+SigLevel = Required DatabaseOptional
+[core]
+Include = /etc/pacman.d/mirrorlist
+[extra]
+Include = /etc/pacman.d/mirrorlist
+[community]
+Include = /etc/pacman.d/mirrorlist
+EOF
+root="$(mktemp -d -p "$PWD")"
+
+packages=(
+	busybox
+	# libbpf dependencies.
+	libelf
+	zlib
+	# selftests test_progs dependencies.
+	binutils
+	elfutils
+	glibc
+	# selftests test_verifier dependencies.
+	libcap
+)
+
+pacstrap -C "$pacman_conf" -cGM "$root" "${packages[@]}"
+
+# Remove unnecessary files from the chroot.
+
+# We don't need the pacman databases anymore.
+rm -rf "$root/var/lib/pacman/sync/"
+# We don't need D, Fortran, or Go.
+ rm -f "$root/usr/lib/libgdruntime."* \
+	"$root/usr/lib/libgphobos."* \
+	"$root/usr/lib/libgfortran."* \
+	"$root/usr/lib/libgo."*
+# We don't need any documentation.
+rm -rf "$root/usr/share/doc" \
+	"$root/usr/share/help" \
+	"$root/usr/share/man" \
+	"$root/usr/share/texinfo"
+
+chroot "${root}" /bin/busybox --install
+
+cat > "$root/etc/fstab" << "EOF"
+dev /dev devtmpfs rw,nosuid 0 0
+proc /proc proc rw,nosuid,nodev,noexec 0 0
+sys /sys sysfs rw,nosuid,nodev,noexec 0 0
+debugfs /sys/kernel/debug debugfs mode=755,realtime 0 0
+bpffs /sys/fs/bpf bpf realtime 0 0
+EOF
+chmod 644 "$root/etc/fstab"
+
+cat > "$root/etc/inittab" << "EOF"
+::sysinit:/etc/init.d/rcS
+::ctrlaltdel:/sbin/reboot
+::shutdown:/sbin/swapoff -a
+::shutdown:/bin/umount -a -r
+::restart:/sbin/init
+EOF
+chmod 644 "$root/etc/inittab"
+
+mkdir -m 755 "$root/etc/init.d" "$root/etc/rcS.d"
+cat > "$root/etc/rcS.d/S40network" << "EOF"
+#!/bin/sh
+
+ip link set lo up
+EOF
+chmod 755 "$root/etc/rcS.d/S40network"
+
+cat > "$root/etc/init.d/rcS" << "EOF"
+#!/bin/sh
+
+/bin/mount -a
+
+for path in /etc/rcS.d/S*; do
+	[ -x "$path" ] && "$path"
+done
+EOF
+chmod 755 "$root/etc/init.d/rcS"
+
+chmod 755 "$root"
+tar -C "$root" -c . | zstd -T0 -19 -o "$NAME"
+chmod 644 "$NAME"