I've just tried Bash on my Windows 10 PC, and it works fine. However, I found that there is no such thing as loop devices by ls /dev/, and modprobe loop gives an error output.
Does it mean this Bash doesn't support loop devices at all or is there a solution for mounting an image as a loop device?
Windows Subsystem for Linux 1 (WSL, formerly known as Bash on Ubuntu on Windows) did not support loop devices. There was a feature request and an issue about it on Microsoft's Git repo.
WSL 2, however, does support loop devices.
$ uname -a
Linux Blade 5.10.102.1-microsoft-standard-WSL2 #1 SMP Wed Mar 2 00:30:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
$ fallocate -l 1G test.img
$ mkfs.ext3 test.img
mke2fs 1.45.5 (07-Jan-2020)
Discarding device blocks: done
Creating filesystem with 262144 4k blocks and 65536 inodes
Filesystem UUID: 549cca4d-a65f-4f4f-8428-e324feaed3d0
Superblock backups stored on blocks:
32768, 98304, 163840, 229376
Allocating group tables: done
Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done
$ sudo mount -o loop test.img /media/
$ ls /media/
lost+found
Do you know that Bash is just a shell (something that reads your commands, executes them, pipes between them and permits you to write scripts) and is not an operating system?
Loop devices are part of the Linux kernel, and they simply don't exist in the Windows kernel.
Related
I want to limit the CPU usage of the process through CGroup.After searching for some information on the Internet, I learned that it needed to be revised "cpu.cfs_period_us" and "cpu.cfs_quota_us",but now I can`t find these two files in cgroup.
My steps are as follows
The path "/sys/fs/cgroup" already exists in my Linux system, but there are no files in it.So I mount the cgroup/cpu/ file according to the reference standard steps:
cd /sys/fs/
mount -t tmpfs cgroup_root ./cgroup
mkdir cgroup/cpu
mount -t cgroup -ocpu cpu ./cgroup/cpu/
What puzzles me is that my ./cgroup/cpu/ directory doesn't have "cpu.cfs_period_us" and "cpu.cfs_quota_us"... What's the reason? Is it a kernel configuration problem?
I have these files in my ./cgroup/cpu path:
/sys/fs # ls ./cgroup/cpu/
cgroup.clone_children cpu.rt_period_us release_agent
cgroup.procs cpu.rt_runtime_us tasks
cgroup.sane_behavior haoel
cpu.notify_on_migrate notify_on_release
Other information
1)my linux system info
/ # cat /proc/version
Linux version 3.18.20 (gcc version 4.9.2 (GCC) ) #1 PREEMPT Tue Mar 9 03:52:45 CST 2021
/ # uname -a
Linux mdm9607-perf 3.18.20 #1 PREEMPT Tue Mar 9 03:52:45 CST 2021 armv7l GNU/Linux
2)Related kernel configuration
CONFIG_CGROUPS=y
CONFIG_CGROUP_DEBUG=y
CONFIG_CGROUP_FREEZER=y
# CONFIG_CGROUP_DEVICE is not set
# CONFIG_CPUSETS is not set
CONFIG_CGROUP_CPUACCT=y
CONFIG_RESOURCE_COUNTERS=y
# CONFIG_MEMCG is not set
# CONFIG_CGROUP_PERF is not set
CONFIG_CGROUP_SCHED=y
# CONFIG_FAIR_GROUP_SCHED is not set
CONFIG_RT_GROUP_SCHED=y
# CONFIG_BLK_CGROUP is not set
Using ls -l normally results in a long listing that includes the file size...
-rw-r--r--# 1 user1 staff 881344 Sep 1 15:35 someFile.png
On macOS 10.13.5, and Ubuntu 20.04, character special (device) file sizes are very different...
crw------- 1 root wheel 31, 0 Aug 30 16:11 autofs
In this case, what does the "31, 0" mean?
what does the "31, 0" mean?
It's the major/minor numbers of character device.
See these:
https://unix.stackexchange.com/questions/97676/how-to-find-the-driver-module-associated-with-a-device-on-linux
https://www.ibm.com/support/knowledgecenter/linuxonibm/com.ibm.linux.z.lgdd/lgdd_c_udev.html
Read carefully the documentation of ls(1) then about inode(7)
31 is a major device number, 0 is a minor device number.
Remember that ls(1) would use stat(2) (you might check using strace(1)...), so read Advanced Linux Programming then syscalls(2)
Sometimes, ls might be some shell alias or function. So read documentation of GNU bash. Try also /bin/ls --help
On GNU Linux, read documentation of coreutils. And it is free software, you could download and study its source code !
On MacOSX, the operating system kernel might have different system calls.
Be however aware of udev (on Linux).
I have an embedded ARM system with processor AT91SAM9G45.
System consists of two components:
Linux kernel (4.14.79)
Busybox 1.29.3 as initramfs image.
I connect to the device using putty and connecting to serial port.
When kernel starts, everything goes fine. Kernel unpacks initramfs image, all files are found and listed (I see it by debug messages). But when it starts /init, log messages are:
Freeing unused kernel memory: 384K
This architecture does not have kernel memory protection.
run_init_process BEFORE /init
run_init_process AFTER /init, result = 0
Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004
/init is symlink to /bin/busybox. I tried to replace /init with /sbin/init, /bin/busybox, /linuxrc, but results are the same.
/etc/inittab file:
# Begin /etc/inittab
id::initdefault:
si::sysinit:/etc/init.d/rc S
#l0::wait:/etc/rc.d/init.d/rc 0
#l1::wait:/etc/rc.d/init.d/rc 1
#l2::wait:/etc/rc.d/init.d/rc 2
#l3::wait:/etc/rc.d/init.d/rc 3
#l4::wait:/etc/rc.d/init.d/rc 4
#l5::wait:/etc/rc.d/init.d/rc 5
#l6::wait:/etc/rc.d/init.d/rc 6
ca::ctrlaltdel:/sbin/shutdown -t1 -a -r now
su::once:/sbin/sulogin
1::respawn:/sbin/getty ttyS1 115200
2::respawn:/sbin/getty ttyS2 115200
3::respawn:/sbin/getty ttyS3 115200
4::respawn:/sbin/getty ttyS4 115200
5::respawn:/sbin/getty ttyS5 115200
6::respawn:/sbin/getty ttyS6 115200
# End /etc/inittab
/etc/init.d/rcS file (this file is allowed to execute):
#!/bin/busybox sh
echo "Hello world!"
I don't know if even /init process starts parsing /etc/inittab or it falls before getting /etc/inittab by some reasons I cannot find out. Maybe there are
some mistakes in my /etc/inittab and /etc/init.d/rcS files. Maybe there are some errors with terminal (/etc/init.d/rcS cannot write to stdout cause it's blocked, suspended, being used by another process and so on).
How to definitely get sured, that /etc/inittab is started?
Welcome to StackOverflow.
I see there is space between rc and S
si::sysinit:/etc/init.d/rc S
change it to
si::sysinit:/etc/init.d/rcS
let me know if it works.
/init is symlink to /bin/busybox.
The typical /init file in an initramfs built by Buildroot that incorporates Busybox is a script of seven lines:
#!/bin/sh
# devtmpfs does not get automounted for initramfs
/bin/mount -t devtmpfs devtmpfs /dev
exec 0</dev/console
exec 1>/dev/console
exec 2>/dev/console
exec /sbin/init $*
Note the comment ("devtmpfs does not get automounted for initramfs") and the mount command for /dev.
It's /sbin/init (rather than /init) that is linked to /bin/busybox.
IOW without the proper setup of the /dev directory, userland has no I/O capabilty.
Only after devtmpfs has been mounted should the init program in Busybox be executed, which will then access /etc/inittab.
See Is there a way to get Linux to treat an initramfs as the final root filesystem?
and
Make CONFIG_DEVTMPFS_MOUNT apply to initramfs/initmpfs
I am building a Linux system from the bottom for a Beagle Bone board. I have compiled the vanilla kernel and built a basic root file system with busybox. The system is booted with U-boot, while the rootfs is located on a Linux PC and exported through NFS:
/path/to/rootfs 10.42.0.17(rw,wdelay,no_root_squash,no_subtree_check,sec=sys,rw,secure,no_root_squash,no_all_squash)
The U-boot bootargs are:
bootargs console=ttyO0,115200n8 root=/dev/nfs rw nfsroot=${serverip}:/path/to/rootfs,v3,tcp ip=dhcp
I've encountered a problem when trying to get su working for non-root users. In order to work around the problem people over internet are suggesting to set the suid bit for the busybox binary.
After doing so:
$ sudo chmod u+s busybox
and verifying:
$ ls -la
...
-rwsr-xr-x 1 myuser myuser 1882976 Jan 13 21:47 busybox
...
$ stat -c "%a %n" busybox
4755 busybox
Something went wrong. The kernel is booting and all of the usual messages are displayed, but it is getting stuck at the end, and no login line is displayed. Here are last few lines of the booting sequence:
[ 3.776185] IP-Config: Complete:
[ 3.779656] device=eth0, hwaddr=c8:a0:30:c5:80:e9, ipaddr=10.42.0.17, mask=255.255.255.0, gw=10.42.0.1
[ 3.789877] host=10.42.0.17, domain=, nis-domain=(none)
[ 3.795822] bootserver=10.42.0.1, rootserver=10.42.0.1, rootpath=
[ 3.802492] nameserver0=10.42.0.1
[ 3.871575] VFS: Mounted root (nfs filesystem) on device 0:15.
[ 3.879903] devtmpfs: mounted
[ 3.883713] Freeing unused kernel memory: 380K (c07ef000 - c084e000)
If removing the flag, the things are returning to normal:
....
[ 3.862291] Freeing unused kernel memory: 380K (c07ef000 - c084e000)
10.42.0.17 login:
If setting the flag from within the running shell on the Beagle Bone board itself, the shell is stopping responding right after the chmod is performed.
I suspect it is something to do with the way the NFS is exporting the rootfs, but it's only a guess, so qualified explanation and possible solution would be helpful.
After some research I will answer my question myself. The answer is very simple. In order the above to work, the busybox binary should be owned by root:root. The simplest solution is just to change the ownership.
I have virtual image of a FreeBSD system and when I mount it I don't see the /etc/ directory and other files, instead is a big loader.gz on the filesystem, that I believe that is extracted during the boot process. I decompressed the loader.gz with gzip and I got it:
$ file loader
loader: ELF 64-bit LSB executable, x86-64, version 1 (FreeBSD), dynamically linked (uses shared libs), not stripped
Using grep I'm able to confirm that the files that I need to edit are inside, however I don't know how to edit it. I tried to mount it without success. How can I modify the contents of loader.gz and use it again?
Can you please give me an example?
I have a Linux system and a Mac to install tools and this FreeBSD image.
Please, help me.
The loader program is generally the last stage of the kernel bootstrapping process.
A recent image should have another signature. e.g. for a memory stick image;
> file tmp/FreeBSD-10.0-RELEASE-amd64-memstick.img
tmp/FreeBSD-10.0-RELEASE-amd64-memstick.img: Unix Fast File system
[v1] (little-endian), last mounted on ,
last written at Fri Jan 17 00:24:02 2014,
clean flag 1, number of blocks 681040, number of data blocks 679047,
number of cylinder groups 13, block size 8192, fragment size 1024,
minimum percentage of free blocks 8, rotational delay 0ms,
disk rotational speed 60rps, TIME optimization
Mounting an image on FreeBSD:
# mdconfig -a -t vnode -f tmp/FreeBSD-10.0-RELEASE-amd64-memstick.img -u 1
# mount /dev/md1a /mnt/root/
(Linux has the same capability, I just don't remember what its called.)
This image contains loader in the boot/ directory:
# ls /mnt/root/
.cshrc ERRATA.TXT README.TXT boot/ lib/ proc/ sys#
.profile HARDWARE.HTM RELNOTES.HTM dev/ libexec/ rescue/ tmp/
COPYRIGHT HARDWARE.TXT RELNOTES.TXT docbook.css media/ root/ usr/
ERRATA.HTM README.HTM bin/ etc/ mnt/ sbin/ var/
# ls /mnt/root/boot/
beastie.4th check-password.4th gptzfsboot menu.4th support.4th
boot color.4th kernel/ menu.rc userboot.so
boot0 defaults/ loader* menusets.4th version.4th
boot0sio delay.4th loader.4th modules/ zfs/
boot1 device.hints loader.help pmbr zfsboot
boot2 firmware/ loader.rc pxeboot zfsloader*
brand.4th frames.4th mbr screen.4th
cdboot gptboot menu-commands.4th shortcuts.4th
On my FreeBSD 10 system, loader has another signature;
/boot/loader: FreeBSD/i386 demand paged executable