busybox init script. Mount not found - linux

Question

I'm running in to what appears to be a bit of an odd one.
Base machine is Ubuntu 18.04. I'm experimenting with creating a custom initramfs + init script to use with custom compiled kernels that are being used with qemu instances.
From the directory I'm using as the base for the initramfs:
[~/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
Just the basics needed for now. The busybox binary comes from the busybox-static package, and I've confirmed it's statically complied:
[~/initramfs]$ ldd bin/busybox
not a dynamic executable
In the init script, I have:
#!/bin/busybox sh
mount -t proc none /proc
mount -t sysfs none /sys
echo "Hi there"
umount /sys
umount /proc
poweroff
From there, create an initramfs.gz:
find . -print0 | cpio --null --create --verbose --format=newc | pigz --best > ~/initramfs.gz
When I set that as the target initrd for qemu, kernel starts up as expected, then:
[ 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 is part of busybox. So that's strange.
If I modify the init script and put in /bin/busybox sh as the first command to be executed, that gets me to a busybox shell as you'd expect.
[ 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
/ #
And then help shows:
/ # 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
So I go looking for mount, and discover which isn't found either. Oh but it works if I prepend it with /bin/busybox to call it direct...:
/ # type mount
mount is mount
/ # which mount
sh: which: not found
/ # /bin/busybox which mount
/ #
and I can execute the commands successfully if I add /bin/busybox to them:
/ # /bin/busybox mount -t proc none /proc
/ #
It seems really random what will and what won't work from busybox, what does and what doesn't get found, e.g. find is fine:
/ # 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
I can work around this by prepending every command in the init file with /bin/busybox, but I'd really rather not if I don't have to!

Answer 1

You have to symlink all the applets you want, e.g. ln -s /bin/busybox /bin/mount. See 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.
The commands that happen to work without it are the ones implemented as fork-free and therefore can be invoked as builtins.