View source for OS: Pengertian Kernel Modul

Sumber: http://en.wikipedia.org/wiki/Kernel_module

Dalam dunia komputasi, '''Loadable Kernel Module''' (atau'''LKM''') adalah sebuah [[object file]] yang berisi code yang memperluas fungsi [[kernel]] yang berjalan, atau biasa disebut ''base kernel'', dari sebuah [[sistem operasi]]. LKM biasanya digunakan untuk dukungan bagi [[hardware]] baru atau [[filesystem]] baru, atau menambahkan [[system call]]. Saat fungsi yang diberikan oleh LKM tidak lagi dibutuhkan, kita dapat meng-unload LKM tersebut untuk membebaskan [[memory]] maupun sumber daya lainnya.

Sebagian besar sistem [[Unix-like]] yang baru dan [[Microsoft Windows]] mendukung loadable kernel modules, meskipun mereka akan menggunakan nama lain, seperti '''kernel loadable module''' ('''kld''') di [[FreeBSD]], '''kernel extension''' ('''kext''') di [[OS X]] dan '''kernel-mode driver''' di Windows NT. Dia juga sering dikenali sebagai '''Kernel Loadable Module''' (atau '''KLM'''), atau hanya '''Kernel Module''' ('''KMOD''').

== Keuntungan ==

Tanpa loadable kernel module, sebuah sistem operasi harus memasukan semua fungsi yang mungkin nantinya akan digunakan ke dalam base kernel. Sebagian besar fungsi tersebut akan mengambil memory tanpa kita gunakan untuk apa-apa. Dengan kata lain, memboroskan memory, dan membutuhkan user membuat ulang dan [[reboot]] base kernel setiap kali user membutuhkan fungsi baru. Kebanyakan sistem operasi mendukung loadable kernel module akan memasukan modul untuk mendukung fungsi yang paling di butuhkannya.

== Kerugian ==

Sedikit kritik bagi mereka yang lebih suka pada kernel yang modular daripada kernel statik adalah ''Fragmentation Penalty''.
''base kernel'' selalu masuk ke dalam memory (RAM) tanpa putus / contiguous oleh routine setup-nya; 
oleh karena-nya ''base kernel'' tidak pernah terfragmentasi / terputus.
Saat sistem mengijinkan bahwa modul dapat di masukan / di insert, sangat mungkin bahwa kernel code yang di insert / di masukan
akan menyebabkan kernel menjadi fragmented / terputus, ini kemungkinan akan menyebabkan pengorbanan / penalty performance.

== Implementasi di berbagai sistem operasi ==

=== Linux ===

Loadable kernel module di Linux  di load (dan di unload) menggunakan perintah <tt>[[modprobe]]</tt>.  Sejak versi 2.6, kernel modul berlokasi di <tt>/lib/modules</tt> dan mempunyai extension <tt>.ko</tt> ("kernel object"), versi sebelumnya menggunakan extension <tt>.o</tt>). 
Perintah <tt>[[lsmod]]</tt> memperlihatkan kernel modul yang di lad.
Pada saat emergency, saat sistem gagal di booting karena, misalnya, modul rusak, modul tertentu dapat di enable atau disable
dengan cara memodifikasi daftar parameter boot kernel
(contoh, jika menggunakan [[GRUB]], dengan menekan 'e' di GRUB start menu, kemudian mengedit kalimat kernel parameter).

==== License issues ====

In the opinion of Linux maintainers, LKM are [[derived work]]s of the kernel. The Linux maintainers tolerate the distribution of [[Proprietary software|proprietary]] modules,{{Citation needed|date=April 2012}} but allow symbols to be marked as only available to [[GNU General Public License]] (GPL) modules.

Loading a proprietary or non-GPL-compatible LKM will set a 'taint' flag in the running kernel—meaning that any problems or [[Software bug|bugs]] experienced will be less likely to be investigated by the maintainers. LKMs effectively become part of the running kernel, so can corrupt kernel data structures and produce bugs that may not be able to be investigated if the module is indeed proprietary.

===== Linuxant controversy =====

In 2004, Linuxant—a consulting company that releases proprietary [[device driver]]s as loadable kernel modules—attempted to bypass GPLONLY symbol restrictions by abusing a [[Null character|NULL terminator]] in their <code>MODULE_LICENSE</code>:

<source lang="text">
MODULE_LICENSE("GPL\0for files in the \"GPL\" directory; for others, only LICENSE file applies");
</source>

The string comparison code used by the kernel at the time tried to determine whether the module was GPLed stopped when it reached a null character (<tt>\0</tt>), so it would be fooled into thinking that the module was declar
ing its license to be just "GPL".

=== FreeBSD ===
Kernel modules for [[FreeBSD]] are stored within <tt>/boot/kernel/</tt> for modules distributed with the [[Operating system|OS]], or usually <tt>/boot/modules/</tt> for modules installed from [[FreeBSD ports]] or [[FreeBSD ports#Packages|FreeBSD packages]], or for proprietary or otherwise binary-only modules.  FreeBSD kernel modules usually have the extension <tt>.ko</tt>.  Once the machine has booted, they may be loaded with the <tt>kldload</tt> command, unloaded with <tt>kldunload</tt>, and listed with <tt>kldstat</tt>.  Modules can also be loaded from the loader before the kernel starts, either automatically (through <tt>/boot/loader.conf</tt>) or by hand.

=== OS X ===
Some loadable kernel modules in OS X can be loaded automatically.  Loadable kernel modules can also be loaded by the <tt>kextload</tt> command.  They can be listed by the <tt>kextstat</tt> command.  Loadable kernel modules are located in [[application bundle]]s with the extension <tt>.kext</tt>.  Modules supplied with the operating system are stored in the <tt>/System/Library/Extensions</tt> directory; modules supplied by third parties are in various other directories.

== Binary compatibility ==
Linux does not provide a stable [[API]] or [[Application binary interface|ABI]] for kernel modules.  This means that there are differences in internal structure and function between different kernel versions, which can cause compatibility problems.  In an attempt to combat those problems, symbol versioning data is placed within the <tt>.modinfo</tt> section of loadable [[Executable and Linkable Format|ELF]] modules.  This versioning information can be compared with that of the running kernel before loading a module; if the versions are incompatible, the module will not be loaded.

Other operating systems, such as [[Solaris (operating system)|Solaris]], [[FreeBSD]], [[OS X]], and [[Microsoft Windows|Windows]] keep the kernel [[API]] and [[Application binary interface|ABI]] relatively stable, thus avoiding this problem.  For example, [[FreeBSD]] kernel modules compiled against kernel version 6.0 will work without recompilation on any other FreeBSD 6.x version, e.g. 6.4.  However, they are not compatible with other major versions and must be recompiled for use with FreeBSD 7.x, as API and ABI compatibility is maintained only within a branch.

== Security ==
While loadable kernel modules are a convenient method of modifying the running kernel, this can be abused by attackers on a compromised system to prevent detection of their [[Process (computing)|processes]] or [[Computer file|file]]s, allowing them to maintain control over the system.  Many [[rootkits]] make use of LKMs in this way. Note that on most operating systems modules do not help [[privilege elevation]] in any way, as elevated privilege is required to load a LKM; they merely make it easier for the attacker to hide the break-in.

=== Linux ===
Linux allows disabling module loading via [[sysctl]] option <code>/proc/sys/kernel/modules_disabled</code>.  An initramfs system may load specific modules needed for a machine at boot and then disable module loading.  This makes the security very similar to a monolithic kernel.  If an attacker can change the initramfs, they can change the kernel binary.

=== OS X ===
On OS X, a loadable kernel module in a kernel extension bundle can be loaded by non-root users if the OSBundleAllowUserLoad property is set to True in the bundle's property list. However, if any of the files in the bundle, including the executable code file, are not owned by root and group wheel, or are writable by the group or "other", the attempt to load the kernel loadable module will fail.

== External links ==
* [http://www.ibm.com/developerworks/linux/library/l-lkm/index.html?ca=dgr-lnxw07LinuxLKM&S_TACT=105AGX59&S_CMP=GR IBM DeveloperWorks in-depth article on LKM]
* [http://www.osx86.net/ Online kext database for OSX / Hackintosh]
* [http://en.tldp.org/LDP/lkmpg/index.html Ori Pomerantz, Linux Kernel Module Programming Guide]
* [http://www.linux.org/docs/ldp/howto/Module-HOWTO/copyright.html Copyright Considerations With LKMs]{{dead link|date=October 2011}}
* [http://developer.apple.com/mac/library/documentation/Darwin/Conceptual/KEXTConcept/KEXTConceptIntro/introduction.html "Kernel Extension Programming Topics" document for OS X]
* [http://www.web-manual.net/linux-3/what-is-a-loadable-kernel-module-lkm/ What Is A Loadable Kernel Module (LKM)?]


==Referensi==

* http://en.wikipedia.org/wiki/Kernel_module