Project 1 - The Shell and System Calls

Assignment Goals

• To become familiar with the tools and skills needed to understand, modify, compile, install, and debug the Linux kernel.
• To understand the relationship between OS command interpreters (shells), system calls, and the kernel.
• To design and implement an extremely simple shell and system call.
• To dust off your C programming skills.

Background

The OS command interpreter is the program that people interact with in order to launch and control programs. On UNIX systems, the command interpreter is usually called the shell: it is a user-level program that gives people a command-line interface to launching, suspending, and killing other programs. sh, ksh, csh, tcsh, bash, ... are all examples of UNIX shells. (It might be useful to look at the manual pages of these shells, for example, type: "man csh".)

Every shell is structured as the following loop:

1. print out a prompt
2. read a line of input from the user
3. parse the line into the program name, and an array of parameters
4. use the fork() system call to spawn a new child process
   • the child process then uses the exec() system call to launch the specified program
   • the parent process (the shell) uses the wait() system call to wait for the child to terminate
5. once the child (i.e. the launched program) finishes, the shell repeats the loop by jumping to 1.

Although most of the commands people type on the prompt are the name of other UNIX programs (such as ls or more), shells recognize some special commands (called internal commands) which are not program names. For example, the exit command terminates the shell, and the cd command changes the current working directory. Shells directly make system calls to execute these commands, instead of forking a child process to handle them.

This assignment consists of two parts. In the first, you will design and implement an extremely simple shell that knows how to launch new programs, and also recognizes four internal commands (exit, uptime, freeram, and usercount), which we will describe below. The first three internal commands will work by calling existing system calls (exit and sysinfo); the fourth internal command will work by calling a new system call that you will design and implement. So, in the second part of this assignment, you will design and implement the usercount system call. This will involve making changes to the Linux kernel source code. The semantics of the usercount system call, and some hints on how to go about implementing it are also described below.

The Assignment

Write a shell program in C which has the following features:

• It should parse the parameters and pass in the parameters to the exec'ed program.
• It should recognize three internal commands exit and uptime and freeram. exit terminates the shell, ie, the shell calls the exit system call or returns from main. uptime and freeram use the sysinfo system call to get these information and print them out.

Assume that the full names like /bin/ls are given. Also, try to use the same prompt as given below; the output produced by your shell should look like the following:

CSE451Shell% /bin/date
Sat Jan 6 16:03:51 PST 2001
CSE451Shell% /bin/cat /etc/HOSTNAME /etc/motd
spinlock.cs.washington.edu
Pine, MH, and emacs RMAIL are the supported mailers on the
instructional systems.

Contact support@cs if you need assistance.

Note: The words in bold are output by the shell and the words underlined are typed in by the user.
Please take a look at the manual pages of execv, fork and wait.

To allow users to pass arguments to programs you will have to parse the input line into words separated by whitespace (spaces and '\t' tab characters) and place these words into an array of strings. You might try using strtok() for this (man strtok for a very good example of how to solve exactly this problem with strtok).

Then you'll need to pass the name of the command as well as the entire list of tokenized strings to one of the other variants of exec, such as execvp(). These tokenized strings will then end up as the argv[] argument to the main() function of the new program executed by the child process. Try man execv or man execvp for more details.
 
 

Write a system call which returns the number of currently logged in users.
There are many ways to achieve this goal. E.g. traversing internal structures for memory management or process management.

Warning 1: Remember that the Linux kernel should be allowed to access any memory location, while the calling application should be prevented from causing the kernel to unwittingly read/write addresses other than those in its own address space. Details about this are here.

Warning 2: Remember that it's inconceivable that this problem (warning 1) has never before been confronted in the existing kernel.

Warning 3: Remember that the kernel must never, ever trust the application to know what it's talking about when it makes a request, particularly with respect to parameters passed in from the application to the kernel.

Warning 4: Remember that you must be sure not to create security holes in the kernel with your code.

Warning 5: Remember that the kernel should not leak memory.

SOME HINTS

You should be using the C language whenever you alter or add to the Linux kernel.

The part of the user-level application you didn't learn in CSE 142 is this:

#include <sys/syscall.h>
#define __NR_usercount something
#include <unistd.h>
....

int ret = syscall(__NR_usercount, ...);

(One last detail: If we were really implementing a new system call, we'd put the #define above in  <sys/syscall.h>.  But, we're better off not monkeying with that file, as it's shared among all of us.)

Recommended Procedure

I suggest you wade, rather than dive, into this.  In particular, here's a suggested set of incremental steps:

Don't change any Linux code.  Figure out how to do a make of a new boot image, what file to move where so that you can boot the image you just created, how to tell the loader (LILO) that your image exists, and then how to boot your image.

Now put a "printk()" somewhere in the code, and figure out how to find its output.  (Hints: /var/log and "man dmesg").

Now implement a parameterless system call, whose body is just a printk() call.  Write a user-level routine that invokes it.  Check to make sure it was invoked.

Now write the full implementation.

Now that you have a working shell and an implementation of your new system call, it's time to integrate them; this should be very simple. Add a new internal command to your shell, called usercount. The usercount command should invoke the system call that you build in Part 2, and print out the number of users currently logged on.

You should print out and turn in the following:

1. The C source code to your shell, and a Makefile that compiles the shell.
2. The names of all of the Linux kernel source files that you modified in order to add your new system call, and a verbal description of what you did to them and why you needed to do it (i.e. why was it necessary to modify this particular file).
3. The interface to the new system call (i.e. a miniature man page for it)
4. The complete source code of the routine that implements the new system all in the kernel (i.e. just the new code you wrote, not the source code that was already in the kernel that got control to your new routine).
5. A printout showing you using your new shell to invoke the /bin/date program, the /bin/cat program, and the four internal commands supported by your shell. The /usr/bin/script program might come in handy to generate this printout (as always, do man script to find out how to use the command).
6. A brief (less than a page) discussion of any important design decisions that you made while implementing your system call and/or shell.