新型的initrd的解压方法 / 开普饭

最近在解决一个问题，需要解压ubuntu的initrd来查看启动脚本。

$ file /boot/initrd.img-4.15.0-32-generic

/boot/initrd.img-4.15.0-32-generic: ASCII cpio archive (SVR4 with no CRC)

$mkdir rootfs

$cd rootfs

$cpio -idvm < /boot/initrd.img-4.15.0-32-generic

$ tree

└── kernel

└── x86

└── microcode

└── AuthenticAMD.bin

3 directories, 1 file

奇怪，没有根文件的目录和文件，只有一个微码的文件。通过Google了解，目前的initrd方式有了变化。通过反复的验证，正确的步骤如下：

步骤一：可以通过lsinitramfs命名来查看initrd含有的文件

$lsinitramfs /boot/initrd.img-4.15.0-32-generic

…

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/amd/amdgpu

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/amd/amdgpu/amdgpu.ko

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/amd/lib

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/amd/lib/chash.ko

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/ast

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/ast/ast.ko

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/i915

lib/modules/4.15.0-32-generic/kernel/drivers/gpu/drm/i915/i915.ko

…..

$ binwalk /boot/initrd.img-4.15.0-32-generic

DECIMAL HEXADECIMAL DESCRIPTION

--------------------------------------------------------------------------------

0 0x0 ASCII cpio archive (SVR4 with no CRC), file name: ".", file name length: "0x00000002", file size: "0x00000000"

112 0x70 ASCII cpio archive (SVR4 with no CRC), file name: "kernel", file name length: "0x00000007", file size: "0x00000000"

232 0xE8 ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86", file name length: "0x0000000B", file size: "0x00000000"

356 0x164 ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode", file name length: "0x00000015", file size: "0x00000000"

488 0x1E8 ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode/AuthenticAMD.bin", file name length: "0x00000026", file size: "0x00006B2A"

28072 0x6DA8 ASCII cpio archive (SVR4 with no CRC), file name: "TRAILER!!!", file name length: "0x0000000B", file size: "0x00000000"

28672 0x7000 ASCII cpio archive (SVR4 with no CRC), file name: "kernel", file name length: "0x00000007", file size: "0x00000000"

28792 0x7078 ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86", file name length: "0x0000000B", file size: "0x00000000"

28916 0x70F4 ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode", file name length: "0x00000015", file size: "0x00000000"

29048 0x7178 ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode/.enuineIntel.align.0123456789abc", file name length: "0x00000036", file size: "0x00000000"

29212 0x721C ASCII cpio archive (SVR4 with no CRC), file name: "kernel/x86/microcode/GenuineIntel.bin", file name length: "0x00000026", file size: "0x00170C00"

1539760 0x177EB0 ASCII cpio archive (SVR4 with no CRC), file name: "TRAILER!!!", file name length: "0x0000000B", file size: "0x00000000"

1540096 0x178000 gzip compressed data, from Unix, last modified: 2018-08-21 22:23:29

通过binwalk能够看到“ gzip compressed data,”的字段，说明从1540096 字节段开始是gzip压缩的格式。从这开始是根文件系统。之前的microcode的文件。说明是microcode文件和根文件是压缩到一起的文件

$ binwalk -y gzip /boot/initrd.img-4.15.0-32-generic

DECIMAL HEXADECIMAL DESCRIPTION

--------------------------------------------------------------------------------

1540096 0x178000 gzip compressed data, from Unix, last modified: 2018-08-21 22:23:29

这里有个数字“1540096 ”,下面开始解压文件

$dd if=/boot/initrd.img-4.15.0-32-generic bs=1540096 skip=1 | zcat | cpio -id --no-absolute-filenames -v

$ls

bin conf etc init kernel lib lib64 run sbin scripts usr var

这里有个注意的地方，如果binwalk显示不都是gzip格式的。比如：

$ binwalk /mnt/casper/initrd

（binwalk工具https://github.com/ReFirmLabs/binwalk）