mjhjmtu

I am booting linux mint(version 4.14.13+) without using initrd/initramfs, and I see that /sbin/modprobe is run before mounting the root file system. I was wondering how that is possible. I instrumented the kernel to printk whenever file system is used and that's how I know that /sbin/modprobe was used.

The following link contains the full dmesg output of my boot: dmesg output

[ 3.175001] Used file system /sbin/modprobe!

[ 3.179080] Used file system /dev/console!

[ 3.844276] Used file system /dev/md0!

[ 3.899302] VFS: Mounted root (ext4 filesystem) on device 8:17.
[ 3.951578] devtmpfs: mounted

[ 3.987527] Used file system /bin/sh!

I don't think it is successfully running /sbin/modprobe. But there is something that interested me here. It looks like it tries to open /dev/console before "devtmpfs: mounted" or "Mounted root (ext4 filesystem)", and there is no other attempt to open /dev/console. But I thought the kernel had to open /dev/console from some filesystem for init (/bin/sh in your case)...

It looks like if you do not have an initramfs, the kernel creates a very simple fake one. If I'm following correctly, this is why it can open /dev/console before it has mounted a real root filesystem.

https://github.com/torvalds/linux/blob/v4.14/init/noinitramfs.c

/*
 * Create a simple rootfs that is similar to the default initramfs
 */
static int __init default_rootfs(void)
{
 int err;

 err = sys_mkdir((const char __user __force *) "/dev", 0755);
 if (err < 0)
 goto out;

 err = sys_mknod((const char __user __force *) "/dev/console",
 S_IFCHR | S_IRUSR | S_IWUSR,
 new_encode_dev(MKDEV(5, 1)));
 if (err < 0)
 goto out;

 err = sys_mkdir((const char __user __force *) "/root", 0700);
 if (err < 0)
 goto out;

 return 0;

The /root directory is relied on elsewhere in the kernel code, to mount and transition into the real root= filesystem.

The commit that introduces this function clarifies that this specific function is only used when both initrd and initramfs support are compiled out. So otherwise, the "default initramfs" would provide the /dev/console and /root, if you did not pass a specific initramfs when booting the kernel, and you did not specify an initramfs to be built-in to the kernel.

https://github.com/torvalds/linux/commit/c33df4eaaf41fd3e34837a6ae9a5f9970c393d9f