Diagram 15.1. Using diff to produce a patch file.
Objectives| Introduction| Kernel Responsibilities of the Systems Administrator| Valid Kernels| Configuring the Kernel| Modifying the Kernel| Compiling the Kernel| Conclusions| Review Questions
The UNIX operating system (sometimes referred to as the kernel) is actually a conglomeration of functions, procedures and data structures that perform a variety of tasks. The only way in which someone may use the operating system is via a number of function calls called system calls.
The specific responsibilities of the operating system kernel are to
The interface provided by the operating system is the set of system calls it understands. System calls are basically function calls that are used by user programs to access the services provided by the operating system.
The system calls that are understood by a UNIX operating system will be listed in the second section of the manual page (remember man 2 kill). All a user program knows about the operating system is the set of system calls it will understand.
Exercise 15.1. Examine the kernel code for your system. List some of the different functionality included in the kernel.
Exercise 15.2. Count the number of separate source files and directories that make up the kernel source code. HINT: use a combination of the find and wc commands.
Exercise 15.3. Discover the list of system calls supported by your system. (Hint: examine the manual page for the man command and in particular look for mention of a shell variable MANPATH.)
These types of mistakes may cause the kernel to freeze during initialisation. If the only bootable version of the kernel freezes during initialisation it can be very difficult to get the operating system functioning.
There should always be at least one backup method of starting the system including a backup kernel. The reserve kernel is used in cases where the primary boot method has failed for a variety of reasons.
In most cases the reserve kernel will be kept on
Exercise 15.5. Use the reserve kernel to reboot your system.
The kernel does not contain code alone. It also includes a number of data structures used by the operating system.
For example:
Under Linux information about every process is stored in an array
containing elements of the following structure.
struct task_struct {
/* these are hardcoded - don't touch */
volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
long counter;
long priority;
unsigned long signal;
struct sigaction sigaction[32];
.
/* file system info */
int link_count;
int tty; /* -1 if no tty, so it must be signed */
unsigned short umask;
struct inode * pwd;
struct inode * root;
struct inode * executable;
struct vm_area_struct * mmap;
.
}
The size of this array is controlled by the following macro.
By changing this value a large system can handle more processes.
/*
* This is the maximum nr of tasks - change it if you need to
*/
#define NR_TASKS 256
Configuring the kernel is the act of deciding
This command runs a shell script that asks a number of questions about what should or should not be included into the kernel.
kernel math emulation? used on systems without a math co-processor (386) normal harddisk support? Normal means one of ide, rll, mfm, esdi, etc. SCSI is not normal. Xt hard disk support? For MFM, RLL, ESDI Type drives on an 8 bit controller. Not needed if you have a 16 bit card, or only IDE or SCSI. Networking support? Needed if you are going to connect to a LAN or use serial line TCP/IP protocols like SLIP or PPP. As a suggestion it should be all the time. Limit Memory to below 16M? Answer yes if your physical memory is below 16Mb OR your motherboard design prevents you from caching any RAM above 16Mb (systems with 64Kb cache). If your machine has 128Kb or 256Kb cache and more than 16Mb of RAM answer no. SysV IPC? YES Includes interprocess communication primitives that originated in SysV based UNIX. Includes shared memory and semaphores. use -m486 flag? Performs optimisations for 486 based processes. Answer yes if you have any type of 486 processor (including Pentiums).Network Questions.
TCP/IP ? Answer yes to this all the time. Even if you aren't on a network you can use the loopback device to practice with networking software. SCSI support? Answer yes if you have a SCSI interface of any sort (including SCSI-2 and SCSI-3). If you do have a SCSI interface you will be faced with a range of SCSI options. Network device support? Yes if you plan to use Ethernet cards to connect to a LAN. Dummy net driver? Not really necessary but can be useful for network programming. SLIP? Do you plan to use SLIP (serial line internet protocol) to connect to a network? CSLIP? CSLIP is compressed SLIP, same answer as you gave to the SLIP question. PPP? PPP (point-to-point protocol) is another serial line internet protocol. Load balancing? Very experimental and should be left out.The next set of questions will ask whether or not to include drivers for specific network cards. Basically if you have the card you include the driver.
After the network cards it will ask a number of questions related to CD-ROM drives. The following is the list of CD-ROM drives Linux supports.
Sony? Mitsumi? Matushita/Panasonic/(Some Creative Labs too)After CD-ROMs come the file systems questions. If you want your kernel to support a different file system (MS-DOS for example) you must compile into the kernel the code for that particular file system. File systems supported by Linux include:
minix? extended? second extended? The major file system used by Linux. xiafs? msdos? /proc? nfs? NFS is the network file system. iso9660? The standard CD-ROM file system. os/2 hpfs? system V and coherent? parallel printer support? Yes if you want to use a parallel printer.The next questions are related to drivers for various "non-standard" mice. You include them if you have one.
logitech bus mouse? ps/2 mouse? microsoft busmouse (This refers to the bus mouse not the normal serial mouse) Atixl busmouse? selection? Allows you to cut and paste on the normal text screens. I've never used it and so don't include it. It appears to cause problems with bus mice. QIC-02? Qic 117? Both specify a type of tape-drive. If you don't have one there is no need to include it. Sound card support? Yes if you have a sound card. You will have to answer a number of questions about various sound card options if you do. kernel profiling? Only really used when you are developing kernel code. When turned on will generally slow the system down.Exercise 15.6. Perform the necessary operations to configure the kernel on your system.
If a vendor discovers these errors there are two ways in which the problem can be fixed
Patching the kernel means applying patches to the kernel source tree to produce a slightly modified version of the kernel source.
Diagram 15.1. Using diff to produce a patch file.
Diagram 15.2. Applying a patch file.
Exercise 15.7. Create a file called abc.doc that contains a number of lines of text. Create another file called def.doc that contains a modified version of what is in abc.doc. Produce and use a patch file to transform abc.doc so that it is exactly the same as def.doc.
The make command is used to compile and link a large number of source files according to some user specified rules. These rules are usually laid down in a file called a Makefile (it usually uses that as a filename).
The make command will take a number of parameters that will effect the way it works. In the previous section you saw how the command make config (using the Makefile in /usr/src/linux) runs a configuration script. The Linux kernel Makefile will also accept a number of different command line parameters.
These parameters include
The main reason being is that their root partition is not always guaranteed to be the same as yours. So when the kernel goes looking for the root partition and the files it is supposed to contain it will end up being disappointed.
Exercise 15.8. Compile a new kernel for your system. (This will typically take quite sometime).
Exercise 15.9. Carry out the necessary tasks to reboot your system using the new kernel.
User programs access the kernel's functions by using system calls.
The type of operations a Systems Administrator might perform on a kernel include
15.2. What are some of the kernel related responsibilities that a Systems Administrator must fulfil?
15.3. What is the purpose of the patch and diff commands?
David Jones (author)