Linux Initial RAM Disk (initrd) Overview

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Using the Initial RAM Disk to Boot the System

Now that we understand how to build and use a custom initial RAM disk, this section will explore how the kernel identifies initrd and mounts it as the root file system. We will introduce several key functions in the boot chain and explain what operations are performed.

Bootloaders, such as GRUB, define the kernel to be loaded and copy the kernel image along with the associated initrd into memory. Much of this functionality can be found in the ./init subdirectory of the Linux kernel source directory.

After the kernel and initrd images are decompressed and copied into memory, the kernel is invoked. It performs various initialization operations, eventually leading to the init/main.c:init() (subdir/file:function) function. This function performs a large number of subsystem initialization operations. Here, a call to init/do_mounts.c:prepare_namespace() is executed, which prepares the namespace (mounting the dev file system, RAID or md, devices, and finally the initrd). Loading the initrd is achieved by calling init/do_mounts_initrd.c:initrd_load().

The initrd_load() function calls init/do_mounts_rd.c:rd_load_image(), which determines which RAM disk to load by calling init/do_mounts_rd.c:identify_ramdisk_image(). This function checks the image file's magic number to determine if it is in minux, ext2, romfs, cramfs, or gzip format. Before returning to initrd_load_image, it also calls init/do_mounts_rd:crd_load(). This function is responsible for allocating space for the RAM disk, calculating the Cyclic Redundancy Check (CRC), then decompressing the RAM disk image and loading it into memory. Now, we have the initrd image in a block device suitable for mounting.

Now, this block device is mounted as the root file system using an init/do_mounts.c:mount_root() call. It creates the root device and calls init/do_mounts.c:mount_block_root(). Here, init/do_mounts.c:do_mount_root() is called, which in turn calls fs/namespace.c:sys_mount() to actually mount the root file system, and then chdir into it. This is where we see the familiar message VFS: Mounted root (ext2 file system). as shown in Listing 6.

Finally, control returns to the init function, and init/main.c:run_init_process is called. This leads to a call to execve to start the init process (which is /linuxrc in this example). linuxrc can be an executable program or a script (provided a script interpreter is available).

The call hierarchy of these functions is shown in Listing 7. Although not all functions involved in copying and mounting the initial RAM disk are listed here, this is sufficient to provide a rough framework of the overall process.

Listing 7. Hierarchy of Key Functions Used During initrd Loading and Mounting

init/main.c:init
  init/do_mounts.c:prepare_namespace
    init/do_mounts_initrd.c:initrd_load
      init/do_mounts_rd.c:rd_load_image
        init/do_mounts_rd.c:identify_ramdisk_image
        init/do_mounts_rd.c:crd_load
          lib/inflate.c:gunzip
    init/do_mounts.c:mount_root
      init/do_mounts.c:mount_block_root
         init/do_mounts.c:do_mount_root
           fs/namespace.c:sys_mount
  init/main.c:run_init_process
    execve