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环境
Qemu：QEMU emulator version 3.1.0
Linux：Linux-4.14.13
工具链：arm-none-linux-gnueabi-gcc （gcc version 4.8.3 20140320）
Android：7.1.2
busybox：BusyBox v1.24.2
概述
Android系统使用的根文件系统是用mkbootfs和minigzip制作的，其中mkbootfs用于将根文件系统打包成cpio格式，也可以用cpio工具来打包，将来Linux内核在启动时会调用init/initramfs.c中的函数unpack_to_rootfs对cpio格式进行解包[调用路径：start_kernel --> rest_init --> kernel_init --> kernel_init_freeable --> do_basic_setup --> do_initcalls --> do_initcall_level --> do_one_initcall --> populate_rootfs --> unpack_to_rootfs]，在内存中构造出根文件系统结构，mkbootfs工具是Android自己实现的，支持的功能也比cpio弱很多。minigzip也是Android实现的一个压缩工具，是对gzip的简化。
minigzip的源码位于：external/zlib/
mkbootfs的源码位于：system/core/cpio/mkbootfs.c
不过需要注意mkbootfs的功能要比cpio弱，从mkbootfs.c的代码注释中可以看出来：
/* NOTES
**
** - see buffer-format.txt from the linux kernel docs for
** an explanation of this file format
** - dotfiles are ignored
** - directories named 'root' are ignored
** - device notes, pipes, etc are not supported (error)
*/
上面的信息说明了如下几点：
1. 对cpio格式的说明，Linux内核文档Documentation/early-userspace/buffer-format.txt中有详细说明说明：
The full format of the initramfs buffer is defined by the following
grammar, where:
* is used to indicate "0 or more occurrences of"
(|) indicates alternatives
+ indicates concatenation
GZIP() indicates the gzip() of the operand
ALGN(n) means padding with null bytes to an n-byte boundary initramfs := ("\0" | cpio_archive | cpio_gzip_archive)* cpio_gzip_archive := GZIP(cpio_archive) cpio_archive := cpio_file* + (<nothing> | cpio_trailer) cpio_file := ALGN() + cpio_header + filename + "\0" + ALGN() + data cpio_trailer := ALGN() + cpio_header + "TRAILER!!!\0" + ALGN()
2. 名为"."的文件会被忽略，不会进行打包。
3. 名为"root"的文件也会被忽略，不会进行打包。
4. 不支持设备节点以及管道文件。所以在使用mkbootfs时，需要确保被打包的路径下没有这两种文件，否者会导致错误，并且Linux内核也无法正常访问指定的文件。
此外，在使用mkbootfs时还需要注意的时，mkbootfs会对打包的文件的权限以及uid和gid进行修改（在函数fix_stat中），有可能会遇到本来具备可执行权限的文件，在用mkbootfs打包成cpio格式后，该文件的可执行权限丢失了，比如/etc/init.d/rcS。
用法
lib_path=`readlink -f ./lib`
bin_path=`readlink -f ./bin/` export LD_LIBRARY_PATH=${lib_path}:$LD_LIBRARY_PATH
export PATH=${bin_path}:$PATH rm -f ramdisk.img ramdisk.cpio
rm -rf ./tmp/ # pack
pushd rootfs2
mkbootfs -f ../config.txt . | minigzip > ../ramdisk.img
mkbootfs -f ../config.txt . > ../ramdisk.cpio
popd # unpack
mkdir -p tmp
pushd tmp
cpio -i < ../ramdisk.cpio
popd
其中config.txt是一个配置文件，内容如下：
etc/init.d/rcS
第1行，把"etc/init.d/rcS"文件的uid设置为0，gid设置为0，权限设置为0755
第2行，所有其他文件的uid都设置0，gid也设置为0，权限设置为0744
需要注意的是，第2行开始有一个空格，并且需要放在最后一行，对具体文件的设置要放到前面。详见mkbootfs.c中函数read_canned_config。
如果没有个指定config.txt，那么会使用Android代码中自带的配置机制[在fix_stat中会调用fs_config]，功能要比config.txt更完善，如果在使用mkbootfs时还设置了-d <path>，那么函数fs_config会优先使用<path>/system/etc/fs_config_dirs和<path>/system/etc/fs_config_files中描述的规则，前者针对目录，后者针对其他文件。如果没有给mkbootfs传递-d <path>参数，那么使用Android在源码中指定的规则，它们存放在两个数组中，一个针对目录，一个针对其他文件：
针对目录的配置规则：
/* Rules for directories.
** These rules are applied based on "first match", so they
** should start with the most specific path and work their
** way up to the root.
*/ static const struct fs_path_config android_dirs[] = {
{ , AID_SYSTEM, AID_CACHE, , "cache" },
{ , AID_ROOT, AID_ROOT, , "config" },
{ , AID_SYSTEM, AID_SYSTEM, , "data/app" },
{ , AID_SYSTEM, AID_SYSTEM, , "data/app-private" },
{ , AID_SYSTEM, AID_SYSTEM, , "data/app-ephemeral" },
{ , AID_ROOT, AID_ROOT, , "data/dalvik-cache" },
{ , AID_SYSTEM, AID_SYSTEM, , "data/data" },
{ , AID_SHELL, AID_SHELL, , "data/local/tmp" },
{ , AID_SHELL, AID_SHELL, , "data/local" },
{ , AID_SYSTEM, AID_MISC, , "data/misc" },
{ , AID_DHCP, AID_DHCP, , "data/misc/dhcp" },
{ , AID_SHARED_RELRO, AID_SHARED_RELRO, , "data/misc/shared_relro" },
{ , AID_MEDIA_RW, AID_MEDIA_RW, , "data/media" },
{ , AID_MEDIA_RW, AID_MEDIA_RW, , "data/media/Music" },
{ , AID_ROOT, AID_SHELL, , "data/nativetest" },
{ , AID_ROOT, AID_SHELL, , "data/nativetest64" },
... ...
针对其他文件的规则：
static const struct fs_path_config android_files[] = {
{ , AID_ROOT, AID_SHELL, , "system/etc/init.goldfish.rc" },
{ , AID_ROOT, AID_SHELL, , "system/etc/init.goldfish.sh" },
{ , AID_ROOT, AID_SHELL, , "system/etc/init.ril" },
{ , AID_ROOT, AID_ROOT, , "system/etc/ppp/*" },
{ , AID_ROOT, AID_ROOT, , "system/etc/rc.*" },
{ , AID_ROOT, AID_ROOT, , "system/etc/recovery.img" },
{ , AID_ROOT, AID_ROOT, , conf_dir + },
{ , AID_ROOT, AID_ROOT, , conf_file + },
{ , AID_SYSTEM, AID_SYSTEM, , "data/app/*" },
{ , AID_MEDIA_RW, AID_MEDIA_RW, , "data/media/*" },
{ , AID_SYSTEM, AID_SYSTEM, , "data/app-private/*" },
{ , AID_SYSTEM, AID_SYSTEM, , "data/app-ephemeral/*" },
{ , AID_APP, AID_APP, , "data/data/*" },
{ , AID_ROOT, AID_SHELL, , "data/nativetest/tests.txt" },
{ , AID_ROOT, AID_SHELL, , "data/nativetest64/tests.txt" },
... ...
验证
1. 使用压缩格式的ramdisk.img (mkbootfs -f ../config.txt . | minigzip > ../ramdisk.img)
kernel_dir=./Linux-4.14.
kernel_image=${kernel_dir}/arch/arm/boot/zImage
dtb_image=${kernel_dir}/arch/arm/boot/dts/vexpress-v2p-ca9.dtb
qemu_path=/home/pengdonglin/disk_ext/Qemu/qemu-3.1./build/install/bin ${qemu_path}/qemu-system-arm \
-M vexpress-a9 \
-m 1024M \
-smp \
-kernel ${kernel_image} \
-nographic \
-append "root=/dev/ram0 rw rootfstype=ramfs console=ttyAMA0 init=/init ignore_loglevel" \
-initrd ./rootfs/ramdisk.img \
-dtb ${dtb_image}
部分启动log：
[ 0.609270] Trying to unpack rootfs image as initramfs...
[ 0.965940] Freeing initrd memory: 3616K
2. 使用非压缩的ramdisk.cpio （mkbootfs -f ../config.txt . > ../ramdisk.cpio）
kernel_dir=./Linux-4.14.
kernel_image=${kernel_dir}/arch/arm/boot/zImage
dtb_image=${kernel_dir}/arch/arm/boot/dts/vexpress-v2p-ca9.dtb
qemu_path=/home/pengdonglin/disk_ext/Qemu/qemu-3.1./build/install/bin ${qemu_path}/qemu-system-arm \
-M vexpress-a9 \
-m 1024M \
-smp \
-kernel ${kernel_image} \
-nographic \
-append "root=/dev/ram0 rw rootfstype=ramfs console=ttyAMA0 init=/init ignore_loglevel" \
-initrd ./rootfs/ramdisk.cpio \
-dtb ${dtb_image}
部分启动log：
[ 0.610055] Trying to unpack rootfs image as initramfs...
[ 0.760468] Freeing initrd memory: 7040K
此外，Linux内核本身也支持在编译时将指定的外部根文件系统编译成cpio.gz格式，然后跟Linux内核链接到一起，在启动时就不需要指定initrd了，Linux内核支持的打包工具usr/gen_init_cpio.c也比mkbootfs强大：
配置内核：
General setup --->
[*] Initial RAM filesystem and RAM disk (initramfs/initrd) support
(/home/pengdonglin/aarch32/rootfs/rootfs) Initramfs source file(s)
Linux内核对这部分的处理请参考usr/Makefile，下面是加入上面的配置之后的部分内核编译log：
GEN usr/initramfs_data.cpio.gz
GZIP kernel/config_data.gz
CHK kernel/config_data.h
UPD kernel/config_data.h
CC kernel/configs.o
AR kernel/built-in.o
AS usr/initramfs_data.o
AR usr/built-in.o
展开：
/bin/bash ./scripts/gen_initramfs_list.sh -o usr/initramfs_data.cpio.gz -u -g /home/pengdonglin/disk_ext/Qemu/aarch32/rootfs/rootfs
下面是测试命令:
kernel_dir=./Linux-4.14.
kernel_image=${kernel_dir}/arch/arm/boot/zImage
dtb_image=${kernel_dir}/arch/arm/boot/dts/vexpress-v2p-ca9.dtb
qemu_path=/home/pengdonglin/disk_ext/Qemu/qemu-3.1./build/install/bin ${qemu_path}/qemu-system-arm \
-M vexpress-a9 \
-m 1024M \
-smp \
-kernel ${kernel_image} \
-nographic \
-append "root=/dev/ram0 rw rootfstype=ramfs console=ttyAMA0 init=/init ignore_loglevel" \
-dtb ${dtb_image}
此外，在Documentation/filesystems/ramfs-rootfs-initramfs.txt提供了一个使用cpio打包的脚本：
#!/bin/sh # Copyright Rob Landley <rob@landley.net> and TimeSys Corporation.
# Licensed under GPL version if [ $# -ne ]
then
echo "usage: mkinitramfs directory imagename.cpio.gz"
exit
fi if [ -d "$1" ]
then
echo "creating $2 from $1"
(cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
else
echo "First argument must be a directory"
exit
fi
完。