Add initramfs support about asterinas

[tatetian]

A few crates or tools that are probably useful for initramfs.

mkinitramfs command

• Man page: https://manpages.ubuntu.com/manpages/bionic/en/man8/mkinitramfs.8.html
• Source code: https://github.com/venomlinux/mkinitramfs
• Inner working
  • Uses bsdcpio and gzip commands to generate compressed initramfs images

cpio-rs crate

• Reads from/writes to cpio archives
• https://github.com/jcreekmore/cpio-rs
• Pros
  • No unsafe
  • No 3rd-party dependencies
• Cons
  • Depends on std

miniz_oxide crate

• DEFLATE/zlib encoder/decoder
• https://crates.io/crates/miniz_oxide
• Pros
  • Supports no_std
  • Features no unsafe
• Cons
  • Don't know if this crate supports Lempel-Ziv coding (LZ77), which is used by gzip.
  • The libflate crate is based on miniz_oxide and documented to support LZ77. But libflate crate is not no_std.

[liqinggd]

The Linux kernel will create a tmpfs file system, extract the contents of the archive on it, and then launch the init script located in the root of the tmpfs file system. This script then mounts the real root file system.

Here is a simple initramfs example:

#!/bin/sh

cat > hello.c << EOF
#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
  printf("Hello world!\n");
  sleep(999999999);
}
EOF
gcc -static hello.c -o init
echo init | cpio -o -H newc | gzip > test.cpio.gz
# Testing external initramfs using the initrd loading mechanism.
qemu -kernel /boot/vmlinuz -initrd test.cpio.gz /dev/zero

The mkintramfs script:

#!/bin/sh

# Copyright 2006 Rob Landley <[email protected]> and TimeSys Corporation.
# Licensed under GPL version 2

if [ $# -ne 2 ]
then
echo "usage: mkinitramfs directory imagename.cpio.gz"
exit 1
fi

if [ -d "$1" ]
then
echo "creating $2 from $1"
(cd "$1"; find . | cpio -o -H newc | gzip) > "$2"
else
echo "First argument must be a directory"
exit 1
fi

As we can see, the format of cpio file is in newc format." newc is short for New portable format and CRC format.
Each file has a 110-byte header, a variable length NULL-terminated filename, and variable length file data.
A header for the filename "TRAILER!!!" indicates the end of the archive.

[liqinggd]

1. Implement a no_std crate to decode the newc format cpio archive.
2. Use the decoded information from cpio to prepare fs in ramfs.
3. investigate how to load the initramfs-image file from the -initrd parameter.
4. implement a no_std crate to unzip the gzip(.gz) file.

[liqinggd]

3. investigate how to load the initramfs image file from the -initrd parameter.

Usually, it is the bootloader's capability to load the initramfs image into memory, then pass the address and length to the kernel.

The Rust bootloader implemented the ramdisk support in January(rust-osdev/bootloader#308), it can load the ramdisk image. So we have to upgrade the bootloader to use this feature.

[tatetian]

Done