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[QEMU] build root filesystem.md

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[QEMU] build root filesystem.md

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You are stuck at "Kernel panic" in the previous note. Since kernel can't load the filesystem. We will get through this error in this tutorial.

Staging directory

You should begin by creating a staging directory on your host computer where you can assemble the files that will eventually be transferred to the target. In the following examples, I have used ~/rootfs . You need to create a skeleton directory structure in that, for example:

    mkdir ~/rootfs
    cd ~/rootfs
    mkdir bin dev etc home lib proc sbin sys tmp usr var
    mkdir usr/bin usr/lib usr/sbin
    mkdir var/log

Then use tree -d to show th results

    .
    ├── bin
    ├── dev
    ├── etc
    ├── home
    ├── lib
    ├── proc
    ├── sbin
    ├── sys
    ├── tmp
    ├── usr
    │   ├── bin
    │   ├── lib
    │   └── sbin
    └── var
        └── log

The first thing to build is busy box

Get busybox source code

    git clone git://busybox.net/busybox.git
    cd busybox

Build busybox

Clean everything first.

    make distclean
    make ARCH=arm CROSS_COMPILE=arm-unknown-linux-gnueabi- defconfig
    ...

Change the install directory which will be installed in your staging directory.

    vi .config
    ...
    /* Note: CONFIG_PREFIX=~/rootfs doesn't work*/
    CONFIG_PREFIX=${HOME}/rootfs

Then build

    make ARCH=arm CROSS_COMPILE=arm-unknown-linux-gnueabi-

Copy the binary to the directory configured in CONFIG_PREFIX and create all the symbolic links to it.

    make ARCH=arm CROSS_COMPILE=arm-unknown-linux-gnueabi- install

Check your staging directory to see if there are executable files of busybox in it. For example:

ls -l bin/
...
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 arch -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 ash -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 base32 -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 base64 -> busybox
-rwxr-xr-x 1 dcthinh dcthinh 1100904 Thg 3   5 16:17 busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 cat -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 chattr -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 chgrp -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 chmod -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 chown -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 conspy -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 cp -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 cpio -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 cttyhack -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 date -> busybox
lrwxrwxrwx 1 dcthinh dcthinh       7 Thg 3   5 16:17 dd -> busybox
...

The busybox need libraries to work properly. So you should copy the need libraries from sysroot directory in your toolchain to the your stagaing directory.

    cd ~/rootfs
    cp -a ${HOME}/x-tools/arm-unknown-linux-gnueabi/arm-unknown-linux-gnueabi/sysroot/lib/* lib

    ls -l lib/
    ...
    -r-xr-xr-x 1 dcthinh dcthinh  1233544 Thg 3   4 20:45 ld-linux.so.3
    -r-xr-xr-x 1 dcthinh dcthinh    11968 Thg 3   4 20:45 libanl.so.1
    -r--r--r-- 1 dcthinh dcthinh   545928 Thg 3   4 21:06 libatomic.a
    ...

Create device node

You need just two nodes to boot with BusyBox: console and null.

  • The console only needs to be accessible to root, the owner of the device node, so the access permissions are 600.
  • The null device should be readable and writable by everyone, so the mode is 666.

You can use the -m option to mknod to set the mode when creating the node. You need to be root to create a device node:

    Note: If you uses docker to build, you need to enter root to make these 2 nodes by `docker exec -u root -it <container name> `
    cd ~/rootfs
    sudo mknod -m 666 dev/null c 1 3
    sudo mknod -m 600 dev/console c 5 1

    ls -l dev
    crw------- 1 root root 5, 1 Thg 3   5 16:40 console
    crw-rw-rw- 1 root root 1, 3 Thg 3   5 16:39 null

The proc and sysfs filesystems

They both represent kernel data as files and provide another way to interact with device drivers and other kernel code.

proc and sysfs should be mounted on the directories /proc and /sys:

    mount -t proc proc /proc
    mount -t sysfs sysfs /sys

Transfering the root filesystem to the target board

Having created a skeleton root filesystem in your staging directory, the next task is to transfer it to the target.

Standalone ramdisk

The following sequence of instructions creates the archive, compresses it and adds a U-Boot header ready for loading to the target:

    cd ~/rootfs
    find . | cpio -H newc -ov --owner root:root > ../initramfs.cpio
    cd ..
    gzip initramfs.cpio
    mkimage -A arm -O linux -T ramdisk -d initramfs.cpio.gz uRamdisk

Boot the device

    cd linux-stable

    QEMU_AUDIO_DRV=none qemu-system-arm -m 256M -nographic \
    -M vexpress-a9 -kernel arch/arm/boot/zImage \
    -dtb arch/arm/boot/dts/vexpress-v2p-ca9.dtb \
    -append "console=ttyAMA0 rdinit=/bin/sh" \
    -initrd ${HOME}/initramfs.cpio.gz

If all goes well, you will get a root shell prompt on the console.

[    0.000000] Booting Linux on physical CPU 0x0
[    0.000000] Linux version 4.10.17 (dcthinh@dcthinh) (gcc version 11.2.0 (crosstool-NG 1.24.0.551_1e47ca1) ) #1 SMP Sun Mar 6 11:03:23 +07 2022
[    0.000000] CPU: ARMv7 Processor [410fc090] revision 0 (ARMv7), cr=10c5387d
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT nonaliasing instruction cache
[    0.000000] OF: fdt:Machine model: V2P-CA9
[    0.000000] efi: Getting EFI parameters from FDT:
[    0.000000] efi: UEFI not found.
[    0.000000] cma: Reserved 64 MiB at 0x6c000000
[    0.000000] Memory policy: Data cache writeback
[    0.000000] CPU: All CPU(s) started in SVC mode.
[    0.000000] percpu: Embedded 14 pages/cpu @cbd94000 s26188 r8192 d22964 u57344
[    0.000000] Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 65024
[    0.000000] Kernel command line: console=ttyAMA0 rdinit=/bin/sh
[    0.000000] PID hash table entries: 1024 (order: 0, 4096 bytes)
[    0.000000] Dentry cache hash table entries: 32768 (order: 5, 131072 bytes)
[    0.000000] Inode-cache hash table entries: 16384 (order: 4, 65536 bytes)
[    0.000000] Memory: 154452K/262144K available (10240K kernel code, 1140K rwdata, 3928K rodata, 2048K init, 534K bss, 42156K reserved, 65536K cma-reserved, 0K highmem)
[    0.000000] Virtual kernel memory layout:
[    0.000000]     vector  : 0xffff0000 - 0xffff1000   (   4 kB)
[    0.000000]     fixmap  : 0xffc00000 - 0xfff00000   (3072 kB)
[    0.000000]     vmalloc : 0xd0800000 - 0xff800000   ( 752 MB)
[    0.000000]     lowmem  : 0xc0000000 - 0xd0000000   ( 256 MB)
[    0.000000]     pkmap   : 0xbfe00000 - 0xc0000000   (   2 MB)
[    0.000000]     modules : 0xbf000000 - 0xbfe00000   (  14 MB)
[    0.000000]       .text : 0xc0208000 - 0xc0d00000   (11232 kB)
[    0.000000]       .init : 0xc1200000 - 0xc1400000   (2048 kB)
[    0.000000]       .data : 0xc1400000 - 0xc151d000   (1140 kB)
[    0.000000]        .bss : 0xc151e000 - 0xc15a3990   ( 535 kB)
...
...
[    3.452270] Key type dns_resolver registered
[    3.452588] ThumbEE CPU extension supported.
[    3.452740] Registering SWP/SWPB emulation handler
[    3.461470] rtc-pl031 10017000.rtc: setting system clock to 2022-03-06 06:02:28 UTC (1646546548)
[    3.465434] uart-pl011 10009000.uart: no DMA platform data
[    3.496453] Freeing unused kernel memory: 2048K
/ # 
/ # 
/ # 
/ # busybox
BusyBox v1.36.0.git (2022-03-06 11:07:42 +07) multi-call binary.
BusyBox is copyrighted by many authors between 1998-2015.
Licensed under GPLv2. See source distribution for detailed
copyright notices.