Linux – What does mkinitrd change when it adds kernel modules to the initrd

initrdkernel-moduleslinux

When creating an initrd using mkinitrd (CentOS 5.5), the kernel modules it adds to the initrd get modified in the process. For example, the initrd's /lib/sata_via.ko is not binary identical to /lib/modules/2.6.18-194.32.1.el5/kernel/drivers/ata/sata_via.ko.

I am just curious as to what happens when mkinitrd includes a kernel module – does it link in dependencies, or what is it that makes the module change?

Best Answer

You have a /lib/sata_vio.ko in your initrd? Is (or was) one of your file systems (e.g. / = "root") on a SATA drive that would need that driver? Does an entry for it appear in /etc/modules or /etc/mkinitrd/modules?

On my Ubuntu system, the module is in the same location inside the initrd image, e.g. <extracted initrd>/lib/modules/<uname -r>/drivers/ata/sata_vio.ko.

What does file say? What does strings | grep '\<ver' say? Maybe it's from a different driver or a different kernel version?

Obviously you could use ls -l or du to get an idea if /lib/sata_vio.ko is larger, and run nm -D against both files to see if there is any difference in symbols (e.g. using diff).

The whole process should be documented in man mkinitrd, in particular, it should say what scripts your system runs, perhaps something in /usr/share/initrd-tools/scripts or /etc/mkinitrd/scripts?

Related Solutions

Linux – Will a “customized” initrd survive a kernel update via yum

No, your changes won't be in the new initrd. The CentOS kernel packages have a post-script that runs /sbin/new-kernel-pkg --package kernel --mkinitrd --depmod --install 2.6.18-238.1.1.el5 (an example from the RHEL5 kernel I have installed).

The command will run mkinitrd, which will build a new initrd, and the changes that you made to the previous initrd won't be created there, unless you've also changed the mkinitrd script or its files (or you patched nash or something like that).

Linux – Why do user space apps need kernel headers

Because those programs are build to use things defined in the kernel headers:

busybox-1.22.1]$ egrep -RHn '^#include <linux'
modutils/modutils-24.c:194:#include <linux/elf-em.h>
include/fix_u32.h:17:#include <linux/types.h>
libbb/loop.c:11:#include <linux/version.h>
console-tools/openvt.c:23:#include <linux/vt.h>
console-tools/kbd_mode.c:23:#include <linux/kd.h>
console-tools/showkey.c:19:#include <linux/kd.h>
util-linux/blockdev.c:36:#include <linux/fs.h>
util-linux/mkfs_ext2.c:50:#include <linux/fs.h>
util-linux/mkfs_vfat.c:28:#include <linux/hdreg.h> /* HDIO_GETGEO */
util-linux/mkfs_vfat.c:29:#include <linux/fd.h>    /* FDGETPRM */
....

For each specific tool, you'd need to read the source of the tool and the relevant kernel header to figure out exactly what.

You can see a few things are commented to make it easy.

For example, mkfs_vfat includes linux/fd.h to get FDGETPRM:

$ egrep -RHn FDGETPRM util-linux/mkfs_vfat.c
util-linux/mkfs_vfat.c:29:#include <linux/fd.h>    /* FDGETPRM */
util-linux/mkfs_vfat.c:351:         int not_floppy = ioctl(dev, FDGETPRM, &param);

You could probably remove the relevant #include and watch for compiler errors to make it easier, you'll get warnings that some things are not defined. Those things likely come from the kernel headers.

Best Answer

Related Solutions

Linux – Will a “customized” initrd survive a kernel update via yum

Linux – Why do user space apps need kernel headers

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