Writing my own init executable

Question

I wanted to create my own init and some Linux fun for a snowy weekend. I know, kernel boots with rootfs and gives the flow to /sbin/init after some driver loading and disk mounting. I downloaded ubuntu cloud image and tried direct kernel boot with kvm as follows:

kvm -m 1G -nographic -kernel vmlinuz-3.19.0-32-generic -initrd initrd.img-3.19.0-32-generic -append "console=ttyS0 root=/dev/sda1 rw init=/myinit" -hda mydisk.img 

It works somewhat fine with trusty-server-cloudimg-amd64-disk1.img (if you don't mind hanging at cloud-init), then I proceeded to copy it and delete its content.

modprobe nbd
qemu-nbd -c /dev/nbd0 mydisk.img 
fdisk -l /dev/nbd0 # confirm partition
mount /dev/nbd0p1 disk/
# Delete all files with myinit.c and myinit

And here is my magical init:

int main(){
    printf("Welcome to my kernel\n");
    printf("Welcome to my kernel\n");
    printf("Welcome to my kernel\n");
    while(1);
}

I compile it with gcc -static myinit.c -o myinit. However a kernel panic occurs because of my init. I verified it by renaming myinit to myinit2 and kernel could not find it, and it did not crash. I know writing init cannot be as simple as the above, but what are the steps required for it? I am reading upstart source code

Begin: Mounting root file system ... Begin: Running /scripts/local-top ... done.
Begin: Running /scripts/local-premount ... [    1.460164] tsc: Refined TSC clocksource calibration: 2394.558 MHz
[    1.866560] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3
done.
[    6.251763] EXT4-fs (sda1): recovery complete
[    6.253623] EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)
Begin: Running /scripts/local-bottom ... done.
done.
Begin: Running /scripts/init-bottom ... mount: mounting /dev on /root/dev failed: No such file or directory
done.
mount: mounting /sys on /root/sys failed: No such file or directory
mount: mounting /proc on /root/proc failed: No such file or directory
[    6.299404] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000200
[    6.299404] 
[    6.300013] CPU: 0 PID: 1 Comm: init Not tainted 3.19.0-32-generic #37~14.04.1-Ubuntu
[    6.300013] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[    6.300013]  ffff88003c118700 ffff88003dee7e38 ffffffff817af41b 00000000000017d6
[    6.300013]  ffffffff81a90be8 ffff88003dee7eb8 ffffffff817a925b ffff88003dee8000
[    6.300013]  ffffffff00000010 ffff88003dee7ec8 ffff88003dee7e68 ffffffff81c5ee20
[    6.300013] Call Trace:
[    6.300013]  [<ffffffff817af41b>] dump_stack+0x45/0x57
[    6.300013]  [<ffffffff817a925b>] panic+0xc1/0x1f5
[    6.300013]  [<ffffffff81077b01>] do_exit+0xa11/0xb00
[    6.300013]  [<ffffffff811ec53c>] ? vfs_write+0x15c/0x1f0
[    6.300013]  [<ffffffff81077c7f>] do_group_exit+0x3f/0xa0
[    6.300013]  [<ffffffff81077cf4>] SyS_exit_group+0x14/0x20
[    6.300013]  [<ffffffff817b6dcd>] system_call_fastpath+0x16/0x1b
[    6.300013] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[    6.300013] drm_kms_helper: panic occurred, switching back to text console
[    6.300013] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000200
[    6.300013] 

I know myinit is completely static:

# ldd disk/myinit
    not a dynamic executable

So it should not depend anything else, I guess. But what am I doing wrong and why is kernel panicking? (Kernel panicks without printfs too)

I am reading sysvinit source (it should be simpler then upstart & systemd & openrc) but its too long, but the main idea of init is to own the processes and it rests in while(1) loop too.

Answer 1

Your stdin, stdout and stderr might not be connected when your init starts. It's common to see a sequence similar to the following at the start of an init program:

    int onefd = open("/dev/console", O_RDONLY, 0);
    dup2(onefd, 0); // stdin
    int twofd = open("/dev/console", O_RDWR, 0);
    dup2(twofd, 1); // stdout
    dup2(twofd, 2); // stderr

    if (onefd > 2) close(onefd);
    if (twofd > 2) close(twofd);

This ensures that stdin, stdout and stderr are connected to the system console.