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OPERATING SYSTEMS, ASSIGNMENT 1 XV6 INTRODUCTION, SHELL, SYSTEM CALLS AND SIGNALS Introduction Throughout this course we will be using a simple, UNIX like, teaching operating system called xv6: http://pdos.csail.mit.edu/6.828/2011/xv6.html The xv6 OS is simple enough to cover and understand within a few weeks yet it still implements the important concepts and organizational structure of UNIX. To run it, you will have to compile the source files and use the QEMU processor emulator (installed on all CS lab computers). Tip: xv6 was (and still is) developed as part of MIT’s 6.828 Operating Systems Engineering course. You can find a lot of useful information and getting started tips there: http://pdos.csail.mit.edu/6.828/2011/overview.html Tip: xv6 has a very useful guide. It will greatly assist you throughout the course assignments: http://pdos.csail.mit.edu/6.828/2011/xv6/book-rev6.pdf Tip: you may also find the following useful: http://pdos.csail.mit.edu/6.828/2011/xv6/xv6-rev6.pdf In this assignment, we will start exploring xv6 and extend it to support signals. The last part will be devoted to writing a user program, which will test the OS modifications. Task 0: running xv6 Begin by downloading our revision of xv6, from the os122 svn repository: Open a shell, and traverse to the desired working directory. Execute the following command (in a single line): svn checkout http://bgu-os-122-xv6-rev6-1.googlecode.com/svn/trunk assignment1 This will create a new folder called assignment1 which will contain all project files. Build xv6 by calling: make Run xv6 on top of QEMU by calling: make qemu Task 1: warm up (“HelloXV6”) This part of the assignment is aimed at getting you started. It includes an extension to the xv6 shell – a simplistic implementation of the pwd command Note that in terms of writing code, the current xv6 implementation is limited: it does not support system calls you may use when writing on Linux and its standard library is very thin. Throughout all assignments, we will try to refrain from directing you to specific parts of xv6. Although this may be a challenge by itself, we believe that in time it will help you understand this OS better. Extending xv6 – current working directory in shell (pwd) In this task you will replace the xv6 shell's current prompt (the ‘$’ symbol) with the full path to the current working directory. This improved prompt is a simple form of a productivity tool. One can think of several different ways to implement this feature. The solution you will follow in this task is a naïve one and will not involve important file system concepts discussed at later parts of the course. It relies on the maintenance of a new variable holding the string representation of the current working directory. Start by finding the shell's source file. Look for the function which is in charge of handling the change directory command (i.e., “cd”). Add a variable that will be used by the shell to print the current working directory. This variable’s value should be revised after each call to “cd <val>” and reflect the new working directory. Assume that the maximal size of the prompt message does not exceed 256 characters. Remember that the command may fail and illegal arguments or a single dot (“cd .”) may be entered. Next, locate the code lines that are in charge of printing the prompt to the user. Change these lines so that the value of your new variable is printed instead. For example, after initialization the shell’s prompt should be: /> After invoking “cd temp” (and only if the temp folder exists) the prompt would change to: /temp/> Tip: your solution should only affect the shell's source file. Tip: don’t forget about relative changes to the working directory, such as “cd ..”. Task 2: Signals framework As seen in class, signals are a simple inter process form of communication currently not implemented in xv6. In this part of the assignment, you will add the framework that will enable the passing of signals from one process to the other. This implementation will cover the basic features needed for a signals framework, and despite its resemblance to the Linux framework it is far from being complete. 2.1 – updating the process data structure: The first step towards meeting this goal is to extend the ‘proc’ struct located at proc.h (line 61). The struct should contain a data word called signal each of whose bits represents a currently unhandled (pending) signal. For example, when this word’s value is 0x…2 (hex representation of the binary word 00…0010) then this process received a signal whose identifier is 2. For simplicity, you may assume that your implementation will never have to support more than 32 signals. Note that this representation follows many modern operating systems where multiple signals of the same type (having the same identifier) are ignored. Each signal must also be associated with some action. To support this, add an array of 32 entries where every entry is a pointer to a function (accepting no arguments and returning no value). By default all signals should be ignored, except for the ones defined in task 3. 2.2 – registering to alternate signal handlers: A process wishing to register a custom handler for a specific signal will use the following system call which you should add to xv6: int signal(int signum, sighandler_t handler) This system call will register a new handler (handler) for a given a signal number (signum). If successful, 0 is returned otherwise a -1 value is returned. The type sighandler_t should be defined as: typedef void (*sighandler_t)(void); Tip: Adding a system call requires some delicate work and proper registration. Be sure to add changes to syscall.c, syscall.h, usys.S, user.h and sysproc.c 2.3 – sending a signal: So far, we have allowed a process to prepare itself for an incoming signal. Next we will add the ability to send a signal to a different process. Add the following system call: int sigsend(int pid, int signum) Although “kill” is the standard name for the system call in charge of sending a signal, it is already used in xv6 for terminating processes. Given a process id (pid) and a signal number (signum), the sigsend system call will send process pid the desired signal number. Upon successful completion, 0 will be returned. A -1 value will be returned in case of a failure. 2.4 – getting the process to handle the signal: Finally, you are to implement some mechanism which will make sure that a process receiving a signal actually executes the relevant signal handler. In your extension of xv6, a signal is handled (if at all) whenever the scheduler returns to the process. That is, before the scheduler allocates a time slice to a process, it first checks for sent signals to that process and if needed it updates the instruction pointer (and stack pointer) by calling register_handler. This way, when the process receives its time slice, the code for the signal handler is executed. void register_handler(sighandler_t sighandler) { char* addr = uva2ka(proc->pgdir, (char*)proc->tf->esp); if ((proc->tf->esp & 0xFFF) == 0) panic("esp_offset == 0"); /* open a new frame */ *(int*)(addr + ((proc->tf->esp - 4) & 0xFFF)) = proc->tf->eip; proc->tf->esp -= 4; /* update eip */ proc->tf->eip = (uint)sighandler; } The register_handler function is not native to xv6 and was added by the OS122 team to support the present assignment (you can find it in proc.c). The function locates the current process’ stack and opens a new frame. It must also update the old instruction pointer so that when the new code (sighandler) is completed the process normally resumes its execution. The details of this function go beyond the scope of what you have seen so far and require better understanding of xv6’s memory management. 2.5 – inheriting handlers to a child process: Modify the fork system call so that a child process will have the same registered signal handlers as his parent. Task 3: A few simple signals Support the following signals: SIGINT (id=0), SIGUSR1 (id=1), SIGUSR2 (id=2) and SIGCHLD (id=3). The default behavior for these signals is specified below: SIGINT – terminates a process SIGUSR1 – reserved for user defined handlers, by default prints “SIGUSR1 <pid>” SIGUSR2 – reserved for user defined handlers, by default prints “SIGUSR2 <pid>” SIGCHLD – notifies the process’ parent of any state change. Specifically, whenever a process ends it should send a SIGCHLD to its parent. Support the generation of a SIGINT signal to the currently executing process from the keyboard. This signal will be generated whenever the user presses Ctrl+C (‘^C’). Notice that the signal could be sent when the current process sleeps, or even when the shell awaits for the user's input. Find a way in which you would properly handle these situations. Tip: start by finding out where xv6 handles other special key combinations (e.g. ^p) Task 4: Testing In this section you will add a user application which tests the impact of your new signals framework. Creating a sanity test Similar to several built-in user space programs in xv6 (e.g., ls, grep, echo, etc.), you can add your own user space programs to xv6. Add a program called sanity. This program will fork 3 child processes and will then offer the user the following simple command line interface. Enter a child id (0 – 2): <input> Which signal to send: <input> The program should terminate after all child processes have terminated.

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