[SOLVED] Problems making a small linux image with initrd run in qemu

knarf · 2014-02-17 23:20:40

Hi,

I want to build a image with a kernel and an initramfs (that I can boot from qemu) for learning purpose but I'm stuck, here's what I've done so far:

build the kernel, end up with a bzImage file (compressed kernel image) for x86_64

make a 50MB ext2 disk:

$ dd if=/dev/zero of=disk.raw bs=1M count=50
$ mkfs.ext2 -F disk.raw

make a tiny initrd with a "hello world" init program:

# hello world in asm with embed ELF header & using system calls
$ cat > hello.asm
  BITS 32
  
                org     0x08048000
  
  ehdr:                                                 ; Elf32_Ehdr
                db      0x7F, "ELF", 1, 1, 1, 0         ;   e_ident
        times 8 db      0
                dw      2                               ;   e_type
                dw      3                               ;   e_machine
                dd      1                               ;   e_version
                dd      _start                          ;   e_entry
                dd      phdr - $$                       ;   e_phoff
                dd      0                               ;   e_shoff
                dd      0                               ;   e_flags
                dw      ehdrsize                        ;   e_ehsize
                dw      phdrsize                        ;   e_phentsize
                dw      1                               ;   e_phnum
                dw      0                               ;   e_shentsize
                dw      0                               ;   e_shnum
                dw      0                               ;   e_shstrndx
  
  ehdrsize      equ     $ - ehdr
  
  phdr:                                                 ; Elf32_Phdr
                dd      1                               ;   p_type
                dd      0                               ;   p_offset
                dd      $$                              ;   p_vaddr
                dd      $$                              ;   p_paddr
                dd      filesize                        ;   p_filesz
                dd      filesize                        ;   p_memsz
                dd      5                               ;   p_flags
                dd      0x1000                          ;   p_align
  
  phdrsize      equ     $ - phdr
  
  _start:
  

  ; write(1, "hello\n", 6)
  mov eax, 4
  mov ebx, 1
  mov ecx, msg
  mov edx, msglen
  int 80h
  
  ; exit(0)
  mov eax, 1
  mov ebx, 0
  int 80h

  msg     db  "hello", 10
  msglen  equ $ - msg
  filesize      equ     $ - $$

^D

$ nasm -f bin -o initrd/init hello.asm

# test it out
$ ./initrd/init
hello

# make the img
$ cd initrd && find . | cpio --quiet -H newc -o | gzip -9 -n > ../initrd.img && cd ..

copy everything on the disk

$ mkdir disk
$ mount disk.raw disk
$ mkdir disk/boot
$ cp initrd.img bzImage disk/boot

install syslinux

$ cat > disk/boot/syslinux.cfg
DEFAULT linux
LABEL   linux
SAY     Booting linux
LINUX   /boot/bzImage
APPEND  root=/dev/sda
INITRD  /boot/initrd.img
^D

$ extlinux -i disk/boot
$ umount disk

install mbr

$ dd bs=440 count=1 conv=notrunc if=/usr/lib/syslinux/bios/mbr.bin of=disk.raw

But when I run the image in qemu

$ qemu-system-x86_64 disk.raw

I get the syslinux error: "Operating system load error" and it then tries to boot other devices (floppy, cd, net, ...).

You can get a script of the above command to test it out, along with the kernel here:

http://diobla.info/stuff/boot/mklinux
http://diobla.info/stuff/boot/bzImage (5MB)

Make a directory, cd in it, download the script and the kernel image then run script.
You need cpio, nasm, e2fsprogs, syslinux and finally qemu

The script must be run as root, be careful!

there's only a single filesystem without partition, is the root arg in the syslinux.cfg correct?
is syslinux.cfg the correct file? Could be syslinux.conf, extlinux.conf or extlinux.cfg but none of them seem work either.

Last edited by knarf (2014-02-20 17:42:27)

knarf · 2014-02-19 11:08:25

Ok so

I've switched to partitions instead of whole disk
fixed some bugs in the script: heredoc string (<<EOF style) are expanded if you don't quote the EOF token, leading to the asm source being wrong.

Mounting partitions is not as simple as mounting the whole disk fs. See https://wiki.archlinux.org/index.php/Qe … disk_image

The updated script is at the same url.

...and it works! The kernet boots, runs init which prints hello and exit, and the kernel crashes (which is normal since init is not supposed to exit).

I'm now trying to enable the framebuffer in order to get the /dev/fb0 device. I've enabled a bunch of CONFIG_FB_* stuff in the kernel config which lets me change the resolution and add a logo with the vga=<code> kernel parameter but no /dev/fb0 device!

knarf ~/prog/c/linux-3.7.2 > grep '^C.*_FB' .config  
CONFIG_FB=y
CONFIG_FB_BOOT_VESA_SUPPORT=y
CONFIG_FB_CFB_FILLRECT=y
CONFIG_FB_CFB_COPYAREA=y
CONFIG_FB_CFB_IMAGEBLIT=y
CONFIG_FB_SYS_FILLRECT=y
CONFIG_FB_SYS_COPYAREA=y
CONFIG_FB_SYS_IMAGEBLIT=y
CONFIG_FB_SYS_FOPS=y
CONFIG_FB_MODE_HELPERS=y
CONFIG_FB_TILEBLITTING=y
CONFIG_FB_VGA16=y
CONFIG_FB_UVESA=y
CONFIG_FB_VESA=y
CONFIG_FB_EFI=y
CONFIG_FB_VIRTUAL=y

Last edited by knarf (2014-02-19 23:26:19)

knarf · 2014-02-19 12:10:16

To create the fb0 device you need to use the mknod system call. The framebuffer major device number is 29 and the minor one for fb0 is well 0.
Thus the call is:

mknod("/dev/fb0", S_IFCHR, makedev(29, 0))

I've compiled statically my init C code that draws a dumb xor-texture with musl-c and it works, here's the result

http://diobla.info/stuff/boot/draw.c

xor texture on framebuffer

Last edited by knarf (2014-02-19 12:15:16)

kenn · 2015-06-03 11:28:16

@knarf thank you sharing your experience. I followed your instructions, built a kernel-2.6 bzImage and made an initrd with above hello.asm in /sbin/init. When I run it like qemu-system-x86_64 -kernel bzImage -initrd rootfs.cpio.gz -append "root=/dev/ram0" it prints hello then crashes which is ok. But when I replace hello.asm example with hello.c example it fails to load init. It simply hangs after freeing memory line.
I also tried another advanced initrd images which is built from busybox but it also fails to start init process. Why does it fail to start init? What am I missing? It is most probably related to kernel config. I also followed other kernel building for embedded systems to fit in a floppy but it didn't work. Perhaps I am missing some flags to run qemu-system-x86_64 ?

WorMzy · 2015-06-03 11:30:12

You have been warned about necrobumping before, kenn, please don't do it again.

https://wiki.archlinux.org/index.php/Fo … bumping.22

Closing.

Arch Linux

#1 2014-02-17 23:20:40

[SOLVED] Problems making a small linux image with initrd run in qemu

#2 2014-02-19 11:08:25

Re: [SOLVED] Problems making a small linux image with initrd run in qemu

#3 2014-02-19 12:10:16

Re: [SOLVED] Problems making a small linux image with initrd run in qemu

#4 2015-06-03 11:28:16

Re: [SOLVED] Problems making a small linux image with initrd run in qemu

#5 2015-06-03 11:30:12

Re: [SOLVED] Problems making a small linux image with initrd run in qemu

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