linux - busybox 初始化脚本。未找到挂载点

Question

我遇到了看起来有点奇怪的情况。

基础机器是 Ubuntu 18.04。我正在尝试创建一个自定义 initramfs + init 脚本，以与与 qemu 实例一起使用的自定义编译内核一起使用。

从我用作 initramfs 基础的目录:

[~/initramfs] $ find .
.
./proc
./root
./dev
./dev/console
./dev/sda1
./dev/null
./dev/tty
./sbin
./init
./etc
./lib64
./mnt
./mnt/root
./lib
./bin
./bin/busybox
./sys

只是现在需要的基础知识。 busybox 二进制文件来自 busybox-static 包，我已经确认它是静态编译的:

[~/initramfs]$ ldd bin/busybox
        not a dynamic executable

在初始化脚本中，我有:

#!/bin/busybox sh

mount -t proc none /proc
mount -t sysfs none /sys

echo "Hi there"

umount /sys
umount /proc

poweroff

从那里创建一个 initramfs.gz:

find . -print0 | cpio --null --create --verbose --format=newc | pigz --best > ~/initramfs.gz

当我将其设置为 qemu 的目标 initrd 时，内核按预期启动，然后:

[    0.777443] Run /init as init process
/init: line 3: mount: not found
/init: line 4: mount: not found
Hi there
/init: line 8: umount: not found
/init: line 9: umount: not found
/init: line 11: poweroff: not found

mount 是 busybox 的一部分。所以这很奇怪。

如果我修改 init 脚本并放入 /bin/busybox sh 作为要执行的第一个命令，那么我将如您所期望的那样进入 busybox shell。

[    0.789949] Run /init as init process


BusyBox v1.27.2 (Ubuntu 1:1.27.2-2ubuntu3.2) built-in shell (ash)
Enter 'help' for a list of built-in commands.

sh: can't access tty; job control turned off
/ # [    1.364618] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3
[    1.386482] tsc: Refined TSC clocksource calibration: 3392.105 MHz
[    1.388387] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x30e52cb7a6c, max_idle_ns: 440795310382 ns
[    1.391965] clocksource: Switched to clocksource tsc

/ #

然后帮助显示:

/ # help
Built-in commands:
------------------
        . : [ [[ alias bg break cd chdir command continue echo eval exec[   71.772009] random: fast init done

        exit export false fg getopts hash help history jobs kill let
        local printf pwd read readonly return set shift source test times
        trap true type ulimit umask unalias unset wait [ [[ acpid adjtimex
        ar arp arping ash awk basename blkdiscard blockdev brctl bunzip2
        bzcat bzip2 cal cat chgrp chmod chown chpasswd chroot chvt clear
        cmp cp cpio crond crontab cttyhack cut date dc dd deallocvt depmod
        devmem df diff dirname dmesg dnsdomainname dos2unix dpkg dpkg-deb
        du dumpkmap dumpleases echo ed egrep env expand expr factor fallocate
        false fatattr fdisk fgrep find fold free freeramdisk fsfreeze
        fstrim ftpget ftpput getopt getty grep groups gunzip gzip halt
        head hexdump hostid hostname httpd hwclock i2cdetect i2cdump
        i2cget i2cset id ifconfig ifdown ifup init insmod ionice ip ipcalc
        ipneigh kill killall klogd last less link linux32 linux64 linuxrc
        ln loadfont loadkmap logger login logname logread losetup ls
        lsmod lsscsi lzcat lzma lzop md5sum mdev microcom mkdir mkdosfs
        mke2fs mkfifo mknod mkpasswd mkswap mktemp modinfo modprobe more
        mount mt mv nameif nc netstat nl nproc nsenter nslookup od openvt
        partprobe passwd paste patch pidof ping ping6 pivot_root poweroff
        printf ps pwd rdate readlink realpath reboot renice reset rev
        rm rmdir rmmod route rpm rpm2cpio run-parts sed seq setkeycodes
        setpriv setsid sh sha1sum sha256sum sha512sum shred shuf sleep
        sort ssl_client start-stop-daemon stat static-sh strings stty
        su sulogin svc swapoff swapon switch_root sync sysctl syslogd
        tac tail tar taskset tee telnet telnetd test tftp time timeout
        top touch tr traceroute traceroute6 true truncate tty tunctl
        ubirename udhcpc udhcpd uevent umount uname uncompress unexpand
        uniq unix2dos unlink unlzma unshare unxz unzip uptime usleep
        uudecode uuencode vconfig vi w watch watchdog wc wget which who
        whoami xargs xxd xz xzcat yes zcat

所以我去寻找安装，并发现哪个也没有找到。哦，但是如果我在它前面加上/bin/busybox 来直接调用它，它就可以了...:

/ # type mount
mount is mount
/ # which mount
sh: which: not found
/ # /bin/busybox which mount
/ #

如果我将/bin/busybox 添加到命令中，我就可以成功执行命令:

/ # /bin/busybox mount -t proc none /proc
/ #

它似乎真的是随机的，什么可以从busybox中工作，什么不能工作，什么可以找到，什么不能找到，例如找到就好了:

/ # find
.
./test
./sys
./bin
./bin/busybox
./lib
./mnt
./mnt/root
./lib64
./etc
./init
./sbin
./proc
./root
./dev
./dev/tty
./dev/null
./dev/sda1
./dev/console

我可以通过在 init 文件中的每个命令前面加上 /bin/busybox 来解决这个问题，但如果不需要的话我真的不想这样做!

Answer 1

您必须符号链接(symbolic link)所有您想要的小程序，例如ln -s/bin/busybox/bin/mount。请参阅USAGE in the busybox docs :

USAGE

BusyBox is a multi-call binary. A multi-call binary is an executable program that performs the same job as more than one utility program. That means there is just a single BusyBox binary, but that single binary acts like a large number of utilities. This allows BusyBox to be smaller since all the built-in utility programs (we call them applets) can share code for many common operations.

You can also invoke BusyBox by issuing a command as an argument on the command line. For example, entering

    /bin/busybox ls 

will also cause BusyBox to behave as 'ls'.

Of course, adding '/bin/busybox' into every command would be painful. So most people will invoke BusyBox using links to the BusyBox binary.

For example, entering

    ln -s /bin/busybox ls
    ./ls

will cause BusyBox to behave as 'ls' (if the 'ls' command has been compiled into BusyBox). Generally speaking, you should never need to make all these links yourself, as the BusyBox build system will do this for you when you run the 'make install' command.

If you invoke BusyBox with no arguments, it will provide you with a list of the applets that have been compiled into your BusyBox binary.

在没有它的情况下恰好可以工作的命令是作为无 fork 实现的命令，因此可以作为内置命令调用。