𝗣𝗗𝗙 | An Introductory observation focusing on SLAB Allocator, Process Scheduler and I/O Scheduler. LINUX INTERNALS. Peter Chubb and Etienne Le Sueur [email protected] A LITTLE BIT OF HISTORY. • Ken Thompson and Dennis Ritchie in – Linux Kernel. Development. Third Edition. Robert Love. Upper Saddle River, NJ • Boston • Indianapolis • San Francisco. New York • Toronto • Montreal • London.

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Professional Linux kernel architecture / Wolfgang Mauerer. p. cm. Includes pleasure in documenting and explaining Linux kernel internals. He is also the. The third edition of Understanding the Linux Kernel takes you on a guided tour Understanding the Linux Kernel will acquaint you with all the inner workings of. Linux Kernel Internals . Processing kernel command line . . Kernel Support for Loading.

Waiting The process is waiting for an external event.

Only after this has occurred will it continue its work. Return from system call This state is automatically adopted after every system call and after some interrupts. At this point checks are made as to whether the scheduler needs to be called and whether there are signals to process.

The scheduler can switch the process to the 'Ready' state and activate another process. Processes and threads In many modern operating systems a distinction is made between processes and threads.

A thread is a sort of independent 'strand' in the course of a program which can be processed in parallel with other threads.

As opposed to processes, threads work on the same main memory and can therefore influence each other. Linux does not make this distinction. In the kernel, only the concept of a task exists which can share resources with other tasks for example, the same memory. Thus, a task is a generalization of the usual thread concept. More details can be found in Section 3. Understanding these structures and how they interact is a necessary foundation for understanding the following chapters. The first components of the structure are also accessed from assembler routines.

This access is not made, as it usually is in C, via the names of the components, but via their offsets relative to the start of the structure. This means that the start of the task structure must not be modified without first checking all the assembler routines and modifying them if necessary. The keyword volatile indicates that this component can also be altered asynchronously from interrupt routines.

The scheduler uses the counter value to select the next process, counter thus represents something like the dynamic priority of a process, while priority holds the static priority of a process. The scheduling algorithm see Section 3. Removing this limitation would require modifications at various points in the kernel. A higher value will apply for the port to Alpha machines. Interested readers will find further information on this system call in Section 5. The errno variable holds the error code for the last faulty system call.

On return from the system call, this is copied into the global variable errno see Section 3. The debugreg variable contains the 80x86's debugging registers.

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These are at present used only by the system call ptrace. This completes the hard-coded part of the task structure. The following components of the task structure are considered in groups for the sake of simplicity. In a UNIX system, processes do not exist independently of each other. To enable a process to access all its child processes, the task structure holds the Figure 3.

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The scheduler uses a list of all processes that apply for the processor. Further information is given in Chapter 4. When a process is operating in System Mode, it needs its own stack differing from that for the User Mode. Process ID Every process has its own process ID number, pid, and is assigned to a process group, pgrp, and a session, session.

Every session has a leader process, Leader. These are inherited by the child process from the parent process when a new process is created by the fork system call see Section 3. However, for the actual access control the effective user ID, euid, and the effective group ID, eg id, are used. This is used whenever identification is required by the file system. Exceptions arise for so-called set-UID programs, where the values of euid and fsuid, or those of eg id and fsgid, are set to the user ID and the group ID for the owner of the executable file.

This makes a controlled distribution of privileges possible. This means that daemons can limit their rights when accessing file systems with setfsuid to the rights of the user for whom they are providing services , but they will retain their privileges.

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The reason this was introduced was a security gap in the NFS daemon. To limit its rights for file system access, this had set euid to the user ID of the requesting user.

The file access then worked as expected, but it also allowed the user to send Linux kernel internails - 35 - signals to the NFS daemon. This was not the desired result, and changes have now been made.

Similar considerations apply for the component fsgid and the system call setfsgid. These groups are considered when checking the access permissions to files. The values set using umask are also stored in the component umask.

Under UNIX, every process has a current directory, pwd,4 which is required when resolving relative pathnames and can be changed by means of the system call chdir. Every process has in addition its own root directory - root - which is used in resolving absolute pathnames. This root directory can only be changed by the superuser system call chroot. As this is only rarely used for example, in anonymous FTP , this fact is not well known. The count variable is reserved for future expansions.

A process opening a file with open or creat is given a file descriptor by the kernel to use in referencing the file in future. File descriptors are small 4 The abbreviation pwd most probably derives from the UNIX command pwd - Print Working Directory - which outputs the name of the current directory. Linux kernel internails - 36 - integers. This locates the file pointer assigned to the file descriptor, and with its help the file itself can then be accessed.

Again, count is used as a reference counter.

Standards documents applicable to the Linux kernel

Timing Various times are measured for each process. These ticks are generated by a timer chip every 10 milliseconds and counted by the timer interrupt. In Sections 3. The utime and stime variables hold the time the process has spent in User Mode and System Mode, respectively, while cutime and cstime contain the totals of the corresponding times for all child processes.

These values can be polled by means of the times system call. UNIX supports a number of process-specific timers.

Newer UNIX systems also support interval timers see system calls setitimer and getitmer on page More information on this is given in the description of the timer interrupt in Section 3. Among other things, this provides semaphores.

A process can occupy a semaphore, thereby blocking it. If other processes also wish to occupy this semaphore, they are halted until the semaphore is released.

Miscellaneous The following components do not fit any of the above groups. When a process terminates, it signals this to its parent process via this queue. There is more on this in the section on wait queues see Section 3. This is specified in the sigaction structure see page These are stored in the rl im structure. These data can be polled by a parent process after completion of the child process. This name is used in debugging. A rather obscure semantic in the POSIX standard requires, when calling setpgid, to distinguish whether a process is still running the original program or whether it has loaded a new program with the system call execve.

Another important component in the task structure is binfmt. This describes the functions responsible for loading the program. All the processor registers are saved here to enable them to be restored on return to User Mode. LINUX supports several scheduling algorithms. Thus, for each task the kernel needs to know on which processor the task is running. Once the system has been booted, this is only responsible for the use of unclaimed system time the idle process.

For this reason, it is rather in a class of its own and should not be regarded as a normal task. Many of the algorithms in the kernel have to take note of every individual task. The static size of the process table is an anachronism in modern UNIX operating systems. It is simpler to reserve a field than to use dynamic memory management.

Linux Kernel Internals

However, development within LINUX tends towards the removal of static limitations, such as the maximum number of processes. It is now no longer necessary to hold all the process entries in a table. Linux kernel internails - 41 - 3.

The inode structure describes a file, which gives the term 'inode' a number of meanings. Both the data structure in the kernel and the data structure on the hard disk describe files each from their own viewpoint , and are therefore called inodes. In the following, we will always be referring to the data structure in memory.

Inodes contain information such as the file's owner and access rights. There is exactly one inode entry in the kernel for each file used in the system. File structures that is, data structures of the struct file type , on the other hand, contain the view of a process on these files represented by inodes.

A number of file descriptors may refer to the same file structure. As these are inherited through the system call fork, the same file Chapter 3 Introduction to the kernel structure may also be referenced from different processes. The file structure is only released once there is no longer any process referring to it.

Each of these file systems implements accesses in a different way. The idea is that the functions operating on the file system are not called directly, but via a function specific to the file system. The file-system-specific operations are part of the file or inode structure, which corresponds to the principle of virtual functions in object-oriented programming languages.

Comprehensive information on the VFS is given in Section 6. Many of the components of this structure can be polled via the system call stat. The dev, ino pair thus provides an identification of the file which is unique throughout the system. Further information on inodes is given in Section 6. One page contains bytes.

If dma is not equal to 0, memory is requested that can be addressed by the DMA component. This is important 'or two reasons. Firstly, some parts of the kernel expect freshly requested Linux kernel internails - 44 - memory to be initialized to zero for example, the system call exec.

Secondly, this is a security measure: if the page has already been used it may contain another user's data for example, passwords , which should not be made available to the current process.

TUTORIALS :: Linux Kernel Internals :: Linux Kernel Boot Process

C programmers will as a rule be accustomed to using malloc and free to manage memory. For example, the system call read has to wait until the data have been loaded into the process's area of memory from the hard disk, or a parent process is using wait to wait for the end of a child process.

In each of these cases it is not known how long the process will have to wait. A wait queue is nothing other than a cyclical list containing as its elements pointers to the process table. They should therefore only be modified using one of the two functions below. By blocking the interrupts, these make sure that the wait queue is not modified from an interrupt routine at the same time.

Consistency is thus quaranteed. A process wishing to wait for a specific event now enters itself in a wait queue of this type and relinquishes control. There is a wait queue for every possible event.

When the relevant event occurs, all the processes in its wait queue are reactivated and can resume operation. The process then voluntarily relinquishes control. This kind of timer is obsolete and only used for certain device drivers. The component expires gives the time at which the function function is to be called with the argument data.

In version 1. The kernel uses signals to inform processes about certain events. The Linux kernel internails - 48 - user typically uses signals to abort processes or to switch interactive programs to a defined state. This book is intended to cover everything that is needed to know in order to properly build, customize, and install the Linux kernel. No programming experience is needed to understand and use this book.

Some familiarity with how to use Linux, and some basic command-line usage is expected of the reader. This book is not intended to go into the programming aspects of the Linux kernel; there are many other good books listed in the Bibliography that already cover this topic.

I want this book to help bring more people into the Linux kernel development fold.

The act of building a customized kernel for your machine is one of the basic tasks needed to become a Linux kernel developer. The more people that try this out, and realize that there is not any real magic behind the whole Linux kernel process, the more people will be willing to jump in and help out in making the kernel the best that it can be. That means that you are free to download and redistribute it. The development of the book was made possible, however, by those who download a copy from O'Reilly or elsewhere.

The book is current as of the 2. However the main concepts in the book still remain for any kernel version released. The book is available for download in either PDF or DocBook format for the entire book, or by the individual chapter. The entire history of the development of the book you too can see why the first versions of the book were pages long can be downloaded in a git repository.

Linux Kernel in a Nutshell chapter files: To clone this tree, run:But the chaos is apparent in the detail. Linux kernel internails - 28 - Chapter 9. To understand the object-oriented design concept behind the X Toolkit and program using the Xt Intrinsics. These are inherited by the child process from the parent process when a new process is created by the fork system call see Section 3.

Every 2. More details can be found in Section 3.

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