Kernel Modules




Kernel Modules

The Linux kernel has the ability to load and unload arbitrary sections of kernel code on demand. These loadable kernel modules run in privileged kernel mode and as a consequence have full access to all the hardware capabilities of the machine on which they run.

Kernel Modules

In theory, there is no restriction on what a kernel module is allowed to do; typically, a module might implement a device driver, a file system, or a networking protocol.

Topics You May Be Interested In
Layered Architecture Of Operating System Deadlock Avoidance
System Programs And Calls Copy On Write
Threads Overview Xds-940
Mass Storage Structure Overview What Is The Security Problem?
Operating System Services What Is Ibm Os/360?

Kernel modules are convenient for several reasons. Linux's source code is free, so anybody wanting to write kernel code is able to compile a modified kernel and to reboot to load that new functionality; however, recompiling, relinking, and reloading the entire kernel is a cumbersome cycle to undertake when you are developing a new driver. If you use kernel modules, you do not have to make a new kernel to test a new driver—the driver can be compiled on its own and loaded into the already-running kernel. Of course, once a new driver is written, it can be distribttted as a module so that other users can benefit from it without having to rebuild their kernels. This latter point has another implication. Because it is covered by the GPL license, the Linux kernel cannot be released with proprietary components added to it, unless those new components are also released under the GPL and the source code for them is made available on demand. The kernel's module interface allows third parties to write and distribute, on their own terms, device drivers or file systems that could not be distributed under the GPL.

Kernel modules allow a Linux system to be set up with a standard, minimal kernel, without any extra device drivers built in. Any device drivers that the user needs can be either loaded explicitly by the system at startup or loaded automatically by the system on demand and unloaded when not in use. For example, a CD-ROM driver might be loaded when a CD is mounted and unloaded from memory when the CD is dismounted from the file system. The module support under Linux has three components:

1. The module management allows modules to be loaded into memory and to talk to the rest of the kernel.

2. The driver registration allows modules to tell the rest of the kernel that a new driver has become available.

Topics You May Be Interested In
Network Operating System File Access Methods
Computer System Organization File Replication
Disk Structure File System-recovery
File Sharing Revocation Of Access Rights
Operating System Services Introduction To Memory Management

3. A conflict-resolution mechanism allows different device drivers to reserve hardware resources and to protect those resources from accidental use by another driver

Module Management

Loading a module requires more than just loading its binary contents into kernel memory. The system must also make sure that any references the module makes to kernel symbols or entry points are updated to point to the correct locations in the kernel's address space. Linux deals with this reference updating by splitting the job of module loading into two separate sections: the management of sections of module code in kernel memory and the handling of symbols that modules are allowed to reference.

Linux maintains an internal symbol table in the kernel. This symbol table does not contain the full set of symbols defined in the kernel during the latter's compilation; rather, a symbol must be exported explicitly by the kernel.

Topics You May Be Interested In
Time Sharing Operating Systems Thread Scheduling
Threads Overview Communication Protocols
Disk Structure Streams
Systems Analysis And Design: Core Concepts The Operating System
User Os Interface, Command Interpreter, And Graphical User Interfaces Mutual Exclusion

The set of exported symbols constitutes a well-defined interface by which a module can interact with the kernel. Although exporting symbols from a kernel function requires an explicit request by the programmer, no special effort is needed to import those symbols into a module. A module writer just uses the standard external linking of the C language: Any external symbols referenced by the module but not declared by it are simply marked as unresolved in the final module binary produced by the compiler. When a module is to be loaded into the kernel, a system utility first scans the module for these unresolved references. All symbols that still need to be resolved are looked up in the kernel's symbol table, and the correct addresses of those symbols in the currently running kernel are substituted into the module's code.

Only then is the module passed to the kernel for loading. If the system utility cannot resolve any references in the module by looking them up in the kernel's symbol table, then the module is rejected.

The loading of the module is performed in two stages. First, the moduleloader utility asks the kernel to reserve a continuous area of virtual kernel memory for the module. The kernel returns the address of the memory allocated, and the loader utility can use this address to relocate the module's machine code to the correct loading address. A second system call then passes the module, plus any symbol table that the new module wants to export, to the kernel. The module itself is now copied verbatim into the previously allocated space, and the kernel's symbol table is updated with the new symbols for possible use by other modules not yet loaded. The final module-management component is the module requestor.

The kernel defines a communication interface to which a module-management program can connect. With this connection established, the kernel will inform the management process whenever a process requests a device driver, file system, or network service that is not currently loaded and will give the manager the opportunity to load that service. The original service request will complete once the module is loaded. The manager process regularly queries the kernel to see whether a dynamically loaded module is still in use and unloads that module when it is no longer actively needed.

Topics You May Be Interested In
Operating System Generation I/o Performance
Swap Space Management Example: The Intel Pentium
Atomicity System And Network Threats
Network Topology What Is Election Algorithms ?
Design Principles Explain Reaching Agreement.

 

Driver Registration

Once a module is loaded, it remains no more than an isolated region of memory until it lets the rest of the kernel know what new functionality it provides. The kernel maintains dynamic tables of all known drivers and provides a set of routines to allow drivers to be added to or removed from these tables at any time. The kernel makes sure that it calls a module's startup routine when that module is loaded and calls the module's cleanup routine before that module is unloaded: These routines are responsible for registering the module's functionality. A module may register many types of drivers and may register more than one driver if it wishes. For example, a device driver might want to register two separate mechanisms for accessing the device. Registration tables include the following items:

• Device drivers. These drivers include character devices (such as printers, terminals, and mice), block devices (including all disk drives), and network . interface devices. • File systems. The file system may be anything that implements Linux's virrual-file-system calling routines. It might implement a format for storing files on a disk, but it might equally well be a network file system, such as NFS, or a virtual file system whose contents are generated on demand, such as Linux's /proc file system. • Network protocols. A module may implement an entire networking protocol, such as IPX, or simply a new set of packet-filtering rules for a network firewr all. • Binary format. This format specifies a way of recognizing, and loading, a new type of executable file. In addition, a module can register a new set of entries in the sysctl and /proc ' tables, to allow that module to be configured dynamically

Topics You May Be Interested In
Operations On Process File System-efficiency And Performance
Semaphore In Operation System Firewalling To Protect Systems And Networks
Disk Structure Xds-940
Special Purpose Systems What Is Compression In Mutimdedia?
Thread Libraries Event Ordering

Conflict Resolution

Commercial UNIX implementations are usually sold to run on a vendor's own hardware. One advantage of a single-supplier solution is that the software vendor has a good idea about what hardware configurations are possible. IBM PC hardware, however, comes in a vast number of configurations, with large numbers of possible drivers for devices such as network cards, SCSI controllers, and video display adapters. The problem of managing the hardware configuration becomes more severe when modular device drivers are supported, since the currently active set of devices becomes dynamically variable. Linux provides a central conflict-resolution mechanism to help arbitrate access to certain hardware resources. Its aims are as follows:

• To prevent modules from clashing over access to hardware resources

• To prevent autoprobes—device-driver probes that auto-detect device configuration—from interfering with existing device drivers j

Topics You May Be Interested In
Multithreading Models Application I/o Interface
Critical Section Problems Afs - Andrew File System
Os Examples Disk Scheduling In Multimedia Systems
User Os Interface, Command Interpreter, And Graphical User Interfaces Overview Of Mass Storage Structure
Communication Protocols What Is Special Purpose System?

• To resolve conflicts among multiple drivers trying to access the same j hardware—for example, as when both the parallel printer driver and the i: parallel-line IP (PLIP) network driver try to talk to the parallel printer port .,* To these ends, the kernel maintains lists of allocated hardware resources. E 1 The PC has a limited number of possible I/O ports (addresses in its hardware I" I/O address space), interrupt lines, and DMA channels; when any device driver I wants to access such a resource, it is expected to reserve the resource with I the kernel database first. This requirement incidentally allows the system ;i administrator to determine exactly which resources have been allocated by % which driver at any given point. :;

• A module is expected to use this mechanism to reserve in advance any f hardware resources that it expects to use. If the reservation is rejected because i the resource is not present or is already in use, then it is up to the module

The Linux System to decide how to proceed. It may fail its initialization and request thatnt be unloaded if it cannot continue, or it may carry on, using alternative hardware resources.



Frequently Asked Questions

+
Ans: Atomicity We introduced the concept of an atomic transaction, which is a program unit that must be executed atomically. That is, either all the operations associated with it are executed to completion, or none are performed. When we are dealing with a distributed system, ensuring the atomicity of a transaction becomes much more complicated than in a centralized system. This difficulty occurs because several sites may be participating in the execution of a single transaction. The failure of one of these sites, or the failure of a communication link connecting the sites, may result in erroneous computations. Ensuring that the execution of transactions in the distributed system preserves atomicity is the function of the transaction coordinator. Each site has its own local transaction coordinator, which is responsible for coordinating the execution of all the transactions initiated at that site. view more..
+
Ans: Deadlock Characterization In a deadlock, processes never finish executing, and system resources are tied up, preventing other jobs from starting. Before we discuss the various methods for dealing with the deadlock problem, we look more closely at features that characterize deadlocks. view more..
+
Ans: System Model A system consists of a finite number of resources to be distributed among a number of competing processes. The resources are partitioned into several types, each consisting of some number of identical instances. Memory space, CPU cycles, files, and I/O devices (such as printers and DVD drives) are examples of resource types. If a system has two CPUs, then the resource type CPU has two instances. Similarly, the resource type printer may have five instances. If a process requests an instance of a resource type, the allocation of any instance of the type will satisfy the request. If it will not, then the instances are not identical, and the resource type classes have not been defined properly. view more..
+
Ans: Kernel Modules The Linux kernel has the ability to load and unload arbitrary sections of kernel code on demand. These loadable kernel modules run in privileged kernel mode and as a consequence have full access to all the hardware capabilities of the machine on which they run. In theory, there is no restriction on what a kernel module is allowed to do; typically, a module might implement a device driver, a file system, or a networking protocol. Kernel modules are convenient for several reasons. Linux's source code is free, so anybody wanting to write kernel code is able to compile a modified kernel and to reboot to load that new functionality; however, recompiling, relinking, and reloading the entire kernel is a cumbersome cycle to undertake when you are developing a new driver. If you use kernel modules, you do not have to make a new kernel to test a new driver—the driver can be compiled on its own and loaded into the already-running kernel. view more..
+
Ans: Disk Attachment Computers access disk storage in two ways. One way is via I/O ports (or host-attached storage); this is common on small systems. The other way is via a remote host in a distributed file system; this is referred to as network-attached storage. view more..
+
Ans: Memory-Mapped Files Consider a sequential read of a file on disk using the standard system calls openQ, readO, and writeQ. Each file access requires a system call and disk access. Alternatively, we can use the virtual memory techniques discussed so far to treat file I/O as routine memory accesses. This approach, known as memory mapping a file, allows a part of the virtual address space to be logically associated with the file. view more..
+
Ans: Efficiency and Performance Now that we have discussed various block-allocation and directorymanagement options, we can further consider their effect on performance and efficient disk use. Disks tend to represent a major bottleneck in system performance, since they are the slowest main computer component. In this section, we discuss a variety of techniques used to improve the efficiency and performance of secondary storage. view more..
+
Ans: Recovery Files and directories are kept both in main memory and on disk, and care must taken to ensure that system failure does not result in loss of data or in data inconsistency. view more..
+
Ans: Log-Structured File Systems Computer scientists often find that algorithms and technologies originally used in one area are equally useful in other areas. Such is the case with the database log-based recovery algorithms described in Section 6.9.2. These logging algorithms have been applied successfully to the problem of consistency checking. The resulting implementations are known as log-based transaction-oriented (or journaling) file systems. view more..
+
Ans: Example: The WAFL File System Disk I/O has a huge impact on system performance. As a result, file-system design and implementation command quite a lot of attention from system designers. Some file systems are general purpose, in that they can provide reasonable performance and functionality for a wide variety of file sizes, file types, and I/O loads. Others are optimized for specific tasks in an attempt to provide better performance in those areas than general-purpose file systems. view more..
+
Ans: Network Structure There are basically two types of networks: local-area networks (LAN) and wide-area networks (WAN). The main difference between the two is the way in which they are geographically distributed. Local-area networks are composed of processors distributed over small areas (such as a single building? or a number of adjacent buildings), whereas wide-area networks are composed of a number of autonomous processors distributed over a large area (such as the United States). These differences imply major variations in the speed and reliability of the communications network, and they are reflected in the distributed operating-system design. view more..
+
Ans: Network Topology The sites in a distributed system can be connected physically in a variety of ways. Each configuration has advantages and disadvantages. We can compare the configurations by using the following criteria: • Installation cost. The cost of physically linking the sites in the system • Communication cost. The cost in time and money to send a message from site A to site B 16.4 Network Topology 621 • Availability. The extent to which data can be accessed despite the failure of some links or sites view more..
+
Ans: Revocation of Access Rights In a dynamic protection system, we may sometimes need to revoke access rights to objects shared by different users view more..
+
Ans: We survey two capability-based protection systems. These systems vary in their complexity and in the types of policies that can be implemented on them. Neither system is widely used, but they are interesting proving grounds for protection theories view more..
+
Ans: Robustness A distributed system may suffer from various types of hardware failure. The failure of a link, the failure of a site, and the loss of a message are the most common types. To ensure that the system is robust, we must detect any of these failures, reconfigure the system so that computation can continue, and recover when a site or a link is repaired. view more..
+
Ans: Design Issues Making the multiplicity of processors and storage devices transparent to the users has been a key challenge to many designers. Ideally, a distributed system should look to its users like a conventional, centralized system. The1 user interface of a transparent distributed system should not distinguish between local and remote resources. That is, users should be able to access remote resources as though these resources were local, and the distributed system should be responsible for locating the resources and for arranging for the appropriate interaction. view more..
+
Ans: Design Principles Microsoft's design goals for Windows XP include security, reliability, Windows and POSIX application compatibility, high performance, extensibility, portability, and international support. view more..
+
Ans: Input and Output To the user, the I/O system in Linux looks much like that in any UNIX system. That is, to the extent possible, all device drivers appear as normal files. A user can open an access channel to a device in the same way she opens any other file—devices can appear as objects within the file system. The system administrator can create special files within a file system that contain references to a specific device driver, and a user opening such a file will be able to read from and write to the device referenced. By using the normal file-protection system, which determines who can access which file, the administrator can set access permissions for each device. Linux splits all devices into three classes: block devices, character devices, and network devices. view more..




Rating - 3/5
469 views

Advertisements