CSC414 “Introduction to UNIX/ Linux” Lecture 2

Schedule 1. Introduction to Unix/ Linux 2. Kernel Structure and Device Drivers. 3. System and Storage Structure. 4. Processes and Inter- Process Communication. 5. Shell Programming. 6. User Management and Disk Quota. 7. System Management. 8. Network Management. 9. Security. 10. Advanced System Administration. 2015Qassim University, College of Computer, 9th level 2

Contents: Linux Kernel Overview Structure. modules Device Drivers Overview Types attributes. Interfacing Device Drivers with the Kernel 2015Qassim University, College of Computer, 9th level 3

Linux Kernel  The kernel hides the system’s hardware underneath an abstract, high-level programming interface.  Written mostly in C, with a sprinkling of assembly language to help it interface with hardware- or chip- specific functions that are not accessible through normal compiler directives. 2015Qassim University, College of Computer, 9th level 4

Cont.  the kernel creates all the following concepts from lower-level hardware features: Processes (time-sharing, protected address spaces) Signals and semaphores Virtual memory (swapping, paging, mapping) The filesystem (files, directories, namespace) General input/output (specialty hardware, keyboard, mouse, USB) Interprocess communication (pipes and network connections) 2015Qassim University, College of Computer, 9th level 5

How the Kernel knows about the system's hardware?. Informed by the system admin/ developer about the hardware it should expect to find.. Prospects for many devices on its own, either at boot time or dynamically (once the system is running) 2015Qassim University, College of Computer, 9th level 6

Kernel Structure Almost all Unix platforms run monolithic kernels. The entire operating system runs in kernel space, a section of memory reserved for privileged operating system functions. Services such as device drivers, interprocess communication, virtual memory, and scheduling run in the same address space. 2015Qassim University, College of Computer, 9th level 7

Modules: Modern monolithic kernels support on-demand loading of modules. Incorporate device drivers and other kernel functions as needed without rebuilding the kernel and rebooting. Drivers, filesystems, and new system calls are all commonly implemented as modules. The memory used by a module is allocated and freed as the code is loaded or removed, using the insmod and rmmod commands. Useful for embedded systems with limited memory. 2015Qassim University, College of Computer, 9th level 8

How to load a kernel module? Two ways: 1. Use the insmod command to manually insert the it into the kernel. 2. Load the module as it is needed; this is known as demand loading. To view the status of modules in the Linux Kernel: lsmod To view a module information use the commend modinfo (module name) Demand loaded kernel modules are normally kept in /lib/modules/kernel-version 2015Qassim University, College of Computer, 9th level 9

Cont. When the kernel discovers the need for a module, it will request that the kernel daemon (kerneld) attempts to load it. The kernel daemon is a normal user process with super user privileges. When it is started up, usually at system boot time, it opens up an Inter-Process Communication (IPC) channel to the kernel. This link is used by the kernel to send messages to the kerneld asking for various tasks to be performed. Kerneld does not perform these tasks itself, it runs the neccessary programs such as insmod to do the work. 2015Qassim University, College of Computer, 9th level 10

Drivers and Device Files A device driver is a program that manages the system’s interaction with a particular type of hardware. Translates between the hardware commands understood by the device and the stylized programming interface used by the kernel. It keeps the kernel reasonably device independent. It is part of the kernel, can be accessed both from within the kernel and from user space Device drivers are system specific 2015Qassim University, College of Computer, 9th level 11

Devices Types: 2015Qassim University, College of Computer, 9th level 12 Character devices: are read and written directly without buffering, for example the system's serial ports /dev/cua0 and /dev/cua1. Block devices: can only be written to and read from in multiples of the block size, typically 512 or 1024 bytes. Block devices can be accessed via their device special file but more commonly they are accessed via the file system. Only a block device can support a mounted file system. Network devices: are accessed via the BSD socket

Attributes 2015Qassim University, College of Computer, 9th level 13 kernel code: Device drivers are part of the kernel and, like other code within the kernel, if they go wrong they can seriously damage the system. A badly written driver may even crash the system, possibly corrupting file systems and losing data. Kernel interfaces: Device drivers must provide a standard interface to the Linux kernel or to the subsystem that they are part of. For example, the terminal driver provides a file I/O interface to the Linux kernel and a SCSI device driver provides a SCSI device interface to the SCSI subsystem which, in turn, provides both file I/O and buffer cache interfaces to the kernel. Kernel mechanisms and services: Device drivers make use of standard kernel services such as memory allocation, interrupt delivery and wait queues to operate.

Cont. Loadable: Most of the Linux device drivers can be loaded on demand as kernel modules when they are needed and unloaded when they are no longer being used. This makes the kernel very adaptable and efficient with the system's resources. Configurable: Linux device drivers can be built into the kernel. Which devices are built is configurable when the kernel is compiled. Dynamic: As the system boots and each device driver is initialized it looks for the hardware devices that it is controlling. It does not matter if the device being controlled by a particular device driver does not exist. In this case the device driver is simply redundant and causes no harm apart from occupying a little of the system's memory. 2015Qassim University, College of Computer, 9th level 14

Where to find the device files? Most devices have a corresponding file in /dev. Files in /dev each have a major and minor device number associated with them. The kernel uses these numbers to map device-file references to the corresponding driver. The major device number identifies the driver with which the file is associated (in other words, the type of device). The minor device number usually identifies which particular instance of a given device type is to be addressed. The minor device number is sometimes called the unit number. 2015Qassim University, College of Computer, 9th level 15

Where to find the device files? You can see the major and minor number of a device file with ls -l: This example shows the first SCSI disk on a Linux system. It has a major number of 8 and a minor number of Qassim University, College of Computer, 9th level 16

Interfacing Device Drivers with the Kernel The Linux kernel must be able to interact with the drivers in standard ways. Each class of device driver, character, block and network, provides common interfaces that the kernel uses when requesting services from them. The kernel can treat often very different devices and their device drivers absolutely the same. For example, SCSI and IDE disks behave very differently but the Linux kernel uses the same interface to both of them. 2015Qassim University, College of Computer, 9th level 17

Interfacing Device Drivers with the Kernel Every time a Linux kernel boots it may encounter different physical devices and thus need different device drivers. Linux allows to include device drivers at kernel build time via its configuration scripts. When these drivers are initialized at boot time they may not discover any hardware to control. Other drivers can be loaded as kernel modules when they are needed. Device drivers register themselves with the kernel as they are initialized. Linux maintains tables of registered device drivers as part of its interfaces with them. These tables include pointers to routines and information that support the interface with that class of devices. 2015Qassim University, College of Computer, 9th level 18

Reading Chapter 13 form “ Unix® and Linux® System Administration Handbook, Fourth Edition, Video Enhanced Edition”. The Linux Kernel on the tld Project ( 2015Qassim University, College of Computer, 9th level 19