test kernel and initrd boot with cirros

README.md

Minimal initramfs

If you are playing around with qemu, sometimes it is useful to have a kernel and initrd easily available to play with. Cirros provides kernel and initramfs that can be used for this purpose.

This init script here provides a initramfs /init that will do nothing but write the kernel command line and then power the system off. You can adjust it to your needs.

The get-krd script will:

• download cirros kernel to 'kernel' and initramfs to 'initramfs.dist'
• create 'initramfs' with init as the /init by appending to the cpio archive.

You can use this basically like below, the kernel and initramfs boot, the provided init which executes echo o >/proc/sysrq-trigger and powers off the system.

$ ./get-krd .
using cached ./kernel
using cached ./initramfs.dist
created ./initramfs with 'init' from /tmp/testing

$ ls -l kernel initramfs
-rw-rw-r-- 1 smoser smoser 6581017 Mar 14 15:12 initramfs
-rw-rw-r-- 1 smoser smoser 9144704 Dec  7 02:53 kernel

$ time qemu-system-x86_64 -enable-kvm -m 1024 -nographic \
    -kernel ./kernel -initrd ./initramfs \
    -append "console=ttyS0 this is my cmdline"
Booting from ROM...
[    0.000000] Linux version 5.3.0-26-generic (buildd@lgw01-amd64-039) (gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1)) #28~18.04.1-Ubuntu SMP Wed Dec 18 16:40:14 UTC 2019 (Ubuntu 5.3.0-26.28~18.04.1-generic 5.3.13)
[    0.000000] Command line: console=ttyS0 this is my cmdline
[    0.000000] KERNEL supported cpus:
[    0.000000]   Intel GenuineIntel
[    0.000000]   AMD AuthenticAMD
[    0.000000]   Hygon HygonGenuine
[    0.000000]   Centaur CentaurHauls
[    0.000000]   zhaoxin   Shanghai
[    0.000000] x86/fpu: x87 FPU will use FXSAVE
...
[    0.000000] BIOS-provided physical RAM map:
[    0.987858] x86/mm: Checked W+X mappings: passed, no W+X pages found.
[    0.988673] Run /init as init process
==== MINI INITRAMFS INIT ====
KERNEL COMMAND LINE: console=ttyS0 this is my cmdline
[    0.993539] sysrq: Power Off
[    0.994119] reboot: Power down

real  0m1.339s
user  0m0.685s
sys   0m0.472s

get-krd

#!/bin/bash
CIRROS_VERSION=${CIRROS_VERSION:-0.5.2}
CIRROS_ARCH=${CIRROS_ARCH:-x86_64}
CIRROS_MIRROR="http://download.cirros-cloud.net/"

set -o pipefail

Usage() {
    cat <<EOF
Usage: ${0##*/} [outd]

  downloads cirros kernel at ${CIRROS_VERSION}.

  if not provided, 'outd' defaults to '.'.

  creates:
    outd/kernel           a cirros kernel
    outd/initramfs.dist   an unmodified cirros initramfs
    outd/initramfs        an updated initramfs with 'init' from .
EOF
}

mkcpio() {
    local tmpf="" rc=0
    tmpf=$(mktemp "${TMPDIR:-/tmp}/${0##*/}.XXXXXX") || {
        stderr "failed to create tmp file"
        return 1
    }
    (for f in "$@"; do echo "$f"; done |
        cpio --create --owner=+0:+0 -H newc ) 2>"$tmpf"
    rc=$?
    [ $rc -eq 0 ] || {
        stderr "failed cpio create newc:"
        cat "$tmpf" 1>&2
    }
    rm -f "$tmpf"
    return $rc
}

fail() { stderr "$@"; exit 1; }
stderr() { echo "$@" 1>&2; }

[ "$1" = "-h" -o "$1" = "--help" ] && { Usage; exit 0; }
[ $# -gt 1 ] && { Usage 1>&2; stderr "got $# args."; exit 1; }

outd=${1:-.}
[ -d "$outd" ] || mkdir -p "$outd" || fail "failed to create $outd"
startd="$PWD"

cd "$outd" || fail "could not cd '$outd'"
burl="${CIRROS_MIRROR}/${CIRROS_VERSION}"
for toks in kernel initramfs:initramfs.dist; do
    rname=${toks%:*}
    lname=${toks#*:}
    cname="cirros-${CIRROS_VERSION}-${CIRROS_ARCH}-$rname"
    [ -f "$lname" ] && {
        stderr "using cached $outd/$lname"
        continue;
    }
    wget "$burl/$cname" -O "$lname.tmp" &&
        mv "$lname.tmp" "$lname" || {
            rm -f "$lname.tmp"
            fail "failed to download $lname from $burl/$cname"
        }
done
  
if [ -f "$startd/init" ]; then
    [ -x "$startd/init" ] || chmod 755 "$startd/init" ||
        fail "failed to make $startd/init executable"
    ( zcat initramfs.dist && cd "$startd" && mkcpio init ) |
        gzip -c > initramfs.tmp &&
        mv "initramfs.tmp" "initramfs" || {
            rm -f "initramfs.tmp"
            fail "failed to create initramfs"
        }
    stderr "created $outd/initramfs with 'init' from $startd"
else
    stderr "no 'init' in $startd, not creating initramfs"
fi
exit

init

#!/bin/sh
# vi: ts=4 expandtab
msg() { 
    local d="" written=false
    for d in /dev/ttyS0 /dev/tty1; do
        [ -c "$d" ] || continue
        echo "$@" >"$d" 2>&1 && written=true;
    done
    [ "$written" = "true" ] && return 0
    echo "$@"
}
panic() {
    msg "FATAL:" "$@"
}

msg "==== MINI INITRAMFS INIT ===="
mkdir -p /proc /dev /tmp /sys
mount -t devtmpfs /dev /dev ||
    panic "failed mount devtmpfs"
mount -t proc /proc /proc ||
    panic "failed mount proc"
mount -t sysfs /sys /sys ||
    panic "failed mount /sys"

echo "6 4 1 7" >/proc/sys/kernel/printk

read cmdline < /proc/cmdline ||
    panic "failed to read cmdline"

msg "KERNEL COMMAND LINE:" "$cmdline"

echo o >/proc/sysrq-trigger

read || panic "Read returned $?"
panic "Read returned success; What to do now?"
exit 1