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创建initramfs

下一篇将介绍linux系统开机启动流程,这里先引入创建initramfs。对于定制的系统一般不需要专门创建initramfs系统,当然在一些特殊环境下无法直接引导挂载根目录

  • 需要加载额外的内核模块
  • 从网络加载rootfs
  • 从LVM逻辑卷加载rootfs
  • 从加密的rootfs启动
  • 其他一些特殊环境

对于一般发行版,都会提供initramfs以适应不同的用户环境。在linux开机引导过程中,引导加载程序将内核和initramfs映像加载到内存中,并启动内核。内核检查initramfs的存在,如果找到,则将其挂载为/和运行/init。 init程序通常是一个shell脚本。当然,如果使用initramfs,引导过程需要更长时间,可能会显着更长。

mkinitramfs 脚本也可以作为创建一个微型linux系统来用,复制一些必要的程序(用busybox做根目录即已经包含很多功能了),这个系统就可以跑起来的

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cat > /sbin/mkinitramfs << "EOF"
#!/bin/bash
# This file based in part on the mkinitramfs script for the LFS LiveCD
# written by Alexander E. Patrakov and Jeremy Huntwork.

copy()
{
local file

if [ "$2" == "lib" ]; then
file=$(PATH=/lib:/usr/lib type -p $1)
else
file=$(type -p $1)
fi

if [ -n $file ] ; then
cp $file $WDIR/$2
else
echo "Missing required file: $1 for directory $2"
rm -rf $WDIR
exit 1
fi
}

if [ -z $1 ] ; then # 传递内核版本
INITRAMFS_FILE=initrd.img-no-kmods
else
KERNEL_VERSION=$1
INITRAMFS_FILE=initrd.img-$KERNEL_VERSION
fi

if [ -n "$KERNEL_VERSION" ] && [ ! -d "/lib/modules/$1" ] ; then
echo "No modules directory named $1"
exit 1
fi

printf "Creating $INITRAMFS_FILE... "

binfiles="sh cat cp dd killall ls mkdir mknod mount " #需要复制的二进制文件
binfiles="$binfiles umount sed sleep ln rm uname"

# Systemd installs udevadm in /bin. Other udev implementations have it in /sbin
if [ -x /bin/udevadm ] ; then binfiles="$binfiles udevadm"; fi

sbinfiles="modprobe blkid switch_root"

#Optional files and locations
for f in mdadm mdmon udevd udevadm; do
if [ -x /sbin/$f ] ; then sbinfiles="$sbinfiles $f"; fi
done

unsorted=$(mktemp /tmp/unsorted.XXXXXXXXXX) #创建临时目录

DATADIR=/usr/share/mkinitramfs
INITIN=init.in

# Create a temporary working directory
WDIR=$(mktemp -d /tmp/initrd-work.XXXXXXXXXX)

# Create base directory structure
mkdir -p $WDIR/{bin,dev,lib/firmware,run,sbin,sys,proc}
mkdir -p $WDIR/etc/{modprobe.d,udev/rules.d}
touch $WDIR/etc/modprobe.d/modprobe.conf
ln -s lib $WDIR/lib64

# Create necessary device nodes #这两个设备是必须的
mknod -m 640 $WDIR/dev/console c 5 1
mknod -m 664 $WDIR/dev/null c 1 3

# Install the udev configuration files # 复制一些特殊的/自定义的设备配置文件
if [ -f /etc/udev/udev.conf ]; then
cp /etc/udev/udev.conf $WDIR/etc/udev/udev.conf
fi

for file in $(find /etc/udev/rules.d/ -type f) ; do
cp $file $WDIR/etc/udev/rules.d
done

# Install any firmware present
cp -a /lib/firmware $WDIR/lib

# Copy the RAID configuration file if present
if [ -f /etc/mdadm.conf ] ; then
cp /etc/mdadm.conf $WDIR/etc
fi

# Install the init file #initramfs 主程序
install -m0755 $DATADIR/$INITIN $WDIR/init

if [ -n "$KERNEL_VERSION" ] ; then
if [ -x /bin/kmod ] ; then
binfiles="$binfiles kmod"
else
binfiles="$binfiles lsmod"
sbinfiles="$sbinfiles insmod"
fi
fi

# Install basic binaries
for f in $binfiles ; do
ldd /bin/$f | sed "s/\t//" | cut -d " " -f1 >> $unsorted
copy $f bin
done

# Add lvm if present
if [ -x /sbin/lvm ] ; then sbinfiles="$sbinfiles lvm dmsetup"; fi

for f in $sbinfiles ; do
ldd /sbin/$f | sed "s/\t//" | cut -d " " -f1 >> $unsorted
copy $f sbin
done

# Add udevd libraries if not in /sbin
if [ -x /lib/udev/udevd ] ; then
ldd /lib/udev/udevd | sed "s/\t//" | cut -d " " -f1 >> $unsorted
elif [ -x /lib/systemd/systemd-udevd ] ; then
ldd /lib/systemd/systemd-udevd | sed "s/\t//" | cut -d " " -f1 >> $unsorted
fi

# Add module symlinks if appropriate
if [ -n "$KERNEL_VERSION" ] && [ -x /bin/kmod ] ; then
ln -s kmod $WDIR/bin/lsmod
ln -s kmod $WDIR/bin/insmod
fi

# Add lvm symlinks if appropriate
# Also copy the lvm.conf file
if [ -x /sbin/lvm ] ; then
ln -s lvm $WDIR/sbin/lvchange
ln -s lvm $WDIR/sbin/lvrename
ln -s lvm $WDIR/sbin/lvextend
ln -s lvm $WDIR/sbin/lvcreate
ln -s lvm $WDIR/sbin/lvdisplay
ln -s lvm $WDIR/sbin/lvscan

ln -s lvm $WDIR/sbin/pvchange
ln -s lvm $WDIR/sbin/pvck
ln -s lvm $WDIR/sbin/pvcreate
ln -s lvm $WDIR/sbin/pvdisplay
ln -s lvm $WDIR/sbin/pvscan

ln -s lvm $WDIR/sbin/vgchange
ln -s lvm $WDIR/sbin/vgcreate
ln -s lvm $WDIR/sbin/vgscan
ln -s lvm $WDIR/sbin/vgrename
ln -s lvm $WDIR/sbin/vgck
# Conf file(s)
cp -a /etc/lvm $WDIR/etc
fi

# Install libraries
sort $unsorted | uniq | while read library ; do
if [ "$library" == "linux-vdso.so.1" ] ||
[ "$library" == "linux-gate.so.1" ]; then
continue
fi

copy $library lib
done

if [ -d /lib/udev ]; then
cp -a /lib/udev $WDIR/lib
fi
if [ -d /lib/systemd ]; then
cp -a /lib/systemd $WDIR/lib
fi

# Install the kernel modules if requested
if [ -n "$KERNEL_VERSION" ]; then
find \
/lib/modules/$KERNEL_VERSION/kernel/{crypto,fs,lib} \
/lib/modules/$KERNEL_VERSION/kernel/drivers/{block,ata,md,firewire} \
/lib/modules/$KERNEL_VERSION/kernel/drivers/{scsi,message,pcmcia,virtio} \
/lib/modules/$KERNEL_VERSION/kernel/drivers/usb/{host,storage} \
-type f 2> /dev/null | cpio --make-directories -p --quiet $WDIR

cp /lib/modules/$KERNEL_VERSION/modules.{builtin,order} \
$WDIR/lib/modules/$KERNEL_VERSION

depmod -b $WDIR $KERNEL_VERSION
fi

( cd $WDIR ; find . | cpio -o -H newc --quiet | gzip -9 ) > $INITRAMFS_FILE

# Remove the temporary directory and file
rm -rf $WDIR $unsorted
printf "done.\n"

EOF

chmod 0755 /sbin/mkinitramfs
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mkdir -p /usr/share/mkinitramfs &&
cat > /usr/share/mkinitramfs/init.in << "EOF"
#!/bin/sh

PATH=/bin:/usr/bin:/sbin:/usr/sbin
export PATH

problem()
{
printf "Encountered a problem!\n\nDropping you to a shell.\n\n"
sh
}

no_device()
{
printf "The device %s, which is supposed to contain the\n" $1
printf "root file system, does not exist.\n"
printf "Please fix this problem and exit this shell.\n\n"
}

no_mount()
{
printf "Could not mount device %s\n" $1
printf "Sleeping forever. Please reboot and fix the kernel command line.\n\n"
printf "Maybe the device is formatted with an unsupported file system?\n\n"
printf "Or maybe filesystem type autodetection went wrong, in which case\n"
printf "you should add the rootfstype=... parameter to the kernel command line.\n\n"
printf "Available partitions:\n"
}

do_mount_root()
{
mkdir /.root
[ -n "$rootflags" ] && rootflags="$rootflags,"
rootflags="$rootflags$ro"

case "$root" in
/dev/* ) device=$root ;;
UUID=* ) eval $root; device="/dev/disk/by-uuid/$UUID" ;;
LABEL=*) eval $root; device="/dev/disk/by-label/$LABEL" ;;
"" ) echo "No root device specified." ; problem ;;
esac

while [ ! -b "$device" ] ; do
no_device $device
problem
done

if ! mount -n -t "$rootfstype" -o "$rootflags" "$device" /.root ; then
no_mount $device
cat /proc/partitions
while true ; do sleep 10000 ; done
else
echo "Successfully mounted device $root"
fi
}

init=/sbin/init
root=
rootdelay=
rootfstype=auto
ro="ro"
rootflags=
device=

mount -n -t devtmpfs devtmpfs /dev
mount -n -t proc proc /proc
mount -n -t sysfs sysfs /sys
mount -n -t tmpfs tmpfs /run

read -r cmdline < /proc/cmdline

for param in $cmdline ; do
case $param in
init=* ) init=${param#init=} ;;
root=* ) root=${param#root=} ;;
rootdelay=* ) rootdelay=${param#rootdelay=} ;;
rootfstype=*) rootfstype=${param#rootfstype=} ;;
rootflags=* ) rootflags=${param#rootflags=} ;;
ro ) ro="ro" ;;
rw ) ro="rw" ;;
esac
done

# udevd location depends on version
if [ -x /sbin/udevd ]; then
UDEVD=/sbin/udevd
elif [ -x /lib/udev/udevd ]; then
UDEVD=/lib/udev/udevd
elif [ -x /lib/systemd/systemd-udevd ]; then
UDEVD=/lib/systemd/systemd-udevd
else
echo "Cannot find udevd nor systemd-udevd"
problem
fi

${UDEVD} --daemon --resolve-names=never
udevadm trigger
udevadm settle

if [ -f /etc/mdadm.conf ] ; then mdadm -As ; fi
if [ -x /sbin/vgchange ] ; then /sbin/vgchange -a y > /dev/null ; fi
if [ -n "$rootdelay" ] ; then sleep "$rootdelay" ; fi

do_mount_root

killall -w ${UDEVD##*/}

exec switch_root /.root "$init" "$@"

EOF

最后,创建initramfs,并编辑引导程序,使用它启动系统

1
mkinitramfs [KERNEL VERSION]

可选参数是相应内核模块所在的目录。 它必须是/ lib / modules的子目录。 如果没有指定模块,则initramfs命名为initrd.img-no-kmods。 如果指定了内核版本,initrd将命名为initrd.img- $ KERNEL_VERSION,并且仅适用于指定的特定内核。 输出文件将放在当前目录中。