How to Boot the Linux Kernel Without Creating an Initrd Image

Is it possible to boot the Linux kernel without creating an initrd image?

initrd/initramfs is optional and not a requirement. bzImage is the pure kernel image and can be booted directly by the bootloader. However it might be neccesary to execute some tasks (loading filesystem modules, drivers for disk access, mounting the root file system from some exchangeable media without fixed name/path, etc.) that would usually require access to a filesystem and userspace tools.

That's what initramfs is for: It is a CPIO archive that gets attached to the kernel image (the kernel image is the container for the initramfs not other way round) either in the kernel image itself, or by the bootloader at boot time.

That CPIO archive contains an initial rootfs with the modules required to setup all devices to access the proper root filesystem and some programs to identify those devices, load the modules, do some other startup tasks remount the proper root file system to / and start /sbin/init

initrd is similar, with the main difference that it is an filesystem image, that may be and usually is compressed. The kernel must have support for the filesystem used built in and will mount this image as the initial /.

Since CPIO is simpler by several orders of magnitudes, initramfs is prefered over initrd, as this saves both the requirement for any filesystem modules being built in and also makes initramfs creation easier. Instead of having to create an ext2 image, loopdevice mount and populate it, it boils down to a simple archive creation, not unlike using tar.

However if you compile your kernel with all required drivers and modules built into the kernel image, and your root file system device has a fixed name in the system you don't need a initramfs as the kernel can do things by itself then.

If I build linux kernel from source, does it contain initramfs inside by default?

If I build linux kernel from source, does it contain initramfs inside by default?

Yes, but it will be empty (by default).

As stated in Documentation/filesystems/ramfs-rootfs-initramfs.txt:

The 2.6 kernel build process always creates a gzipped cpio format initramfs
archive and links it into the resulting kernel binary.  By default, this
archive is empty (consuming 134 bytes on x86).

You can populate the initramfs with either an internal or external cpio archive.

Use CONFIG_INITRAMFS_SOURCE to point to a cpio archive file, which will be embedded into the kernel image.

Otherwise an (external) cpio archive file can be specified during boot by using the old initrd mechanism.

If an initrd/initramfs is passed to the arm64 kernel at boot, it must reside entirely within a 1 GB aligned physical memory window of up to 32 GB in size that fully covers the kernel Image as well.

---

The cpio archive needs to contain an init file in order to inhibit the further processing for mounting a root filesystem. As documented:

After extracting, the kernel checks to see if rootfs contains a file "init",
and if so it executes it as PID 1.  
If found, this init process is responsible for bringing the system the
rest of the way up, including locating and mounting the real root device (if
any).  
If rootfs does not contain an init program after the embedded cpio
archive is extracted into it, the kernel will fall through to the older code
to locate and mount a root partition, then exec some variant of /sbin/init
out of that.

Busybox can be built to provide that init file.

---

ADDENDUM

I had found CONFIG_INITRAMFS_SOURCE value in my old .config file (year 2013~2014) was set to just empty (set to " ").

A string of a single space does not make sense.

Did you mean an empty string, as in "" which is the default value?
The default value means that no archive for the initramfs is to be built with the kernel image.

Now I just now found CONFIG_RD_GZIP=y too in the file.

That simply enables support for loading of a gzip-encoded initial ramdisk or cpio buffer. It is just one of several compression methods available that can be configured. There could also be support for bzip2, LZMA, XZ, LZO, and LZ4.

The salient configuration symbol would be CONFIG_BLK_DEV_RAM, which would enable RAM disk support.

At that time we put busybox120.gz file under arch/sparc/boot before doing linux 'make sImage'. so I guess when there is a .gz file under arch/sparc/boot, the kernel build system uses that .gz file as the initramfs.

No, simply storing a file in a directory is not going to cause inclusion in the kernel build.

Since no initramfs file is specified, the likeliest use of that busybox120.gz file would be for a ramdisk.

Archives are typically named with .cpio or .tar filename extensions. Your file has neither, so that could mean that it is not an archive. The absence of an archive extension could mean that it is an image file, which would never be used for a ramfs but could be used for a ramdisk (or initrd).

AFAIK an image (or archive) for the initrd is loaded by the boot program (ref: Using the initial RAM disk (initrd)) rather than linked in.

Check the kernel command line used with that kernel image.

Are there parameters that specify a ramdisk, e.g. initrd=... and/or root=/dev/ram0?

Be sure that you do not conflate initramfs and initrd. See Documentation/filesystems/ramfs-rootfs-initramfs.txt and
The difference between initrd and initramfs?

how to make a custom kernel which boots a custom initrd.img file

Check out this link. Here exactly your requirement is explained where it prints hello world at the boot by creating initrd image.

What is contained in Android's init on initrd.img?

Here's a brief explanation of android bootup and init

• On android bootup , the bootloader locates and loads the linux
  kernel. The linux kernel boots into the userspace from the
  kernelspace using the initramfs functionality in the kernel.
• The ramdisk is essentially a disk image that contains everything
  associated with the root filesystem of the device, essentially the '/
  ' directory. The initramfs mounts the ramdisk into memory ( a
  temporary filesystem ) .
• Later, initramfs looks into the memory and loads and executes the
  init binary. The ramdisk.img has C files which configure what init
  does .
• Init is the first process executed in the userspace which is
  responsible for getting the core userspace functionality up and
  running.
• initrc files , the ones responsible for configuring what init
  actually does have set of instructions with scripts which run on
  certain event triggers ( such as on init, on fs, on boot).
• init keeps on running and monitors the system for certain changes in
  the properties (triggers). Instructions associated with these
  triggers are executed upon change in the properties. (Such as USB Debugging on/off)
• Additionally, init executes native processes called services
  which are essentially daemons such as installd, logd, ueventd etc. init also restarts services that may exit/killed. Daemon processes
  have limited ability to communicate to each other and use local
  sockets to read and write data from.

To read further you can refer here and this youtube video

Booting a newly compiled linux kernel

Most automatic grub setups include kernel options to hide the messages generated as the kernel attempts to boot. For example:

/boot/vmlinuz-x.x.x.x-generic root=UUID=something ro quiet splash

While on the grub menu line of the kernel you wish to boot, press 'e' to temporarily edit it, and delete options such as quiet and splash (but leave the ro).

Then boot the temporarily modified line, and see the progress messages in order to get an idea where the boot is failing.

---

Related Topics