xv6: An introduction

If you are like me, a low level pc programmer, it’s hard not to have heard of xv6. xv6, for those who haven’t really heard of it, is a UNIX version 6 clone, designed at MIT to help teach operating systems.

The reasoning behind doing this was fairly simple: Up until that point, MIT had used John Lions’ famous commentary on the Sixth Edition of UNIX. But V6 was challenging due to a number of reasons. To begin with, it was written in a near ancient version of C (pre K&R), and apart from that, it contained PDP-11 assembly (a legendary machine for us UNIX lovers, but ancient nonetheless), which didn’t really help the students that had to study both PDP-11 and the (more common) x86 architecture to develop another (exokernel) operating system on.

So, to make things much more simpler, professors there decided to roll with a clone of UNIX version 6, that was x86 specific, written in ANSI C and supported multiprocessor machines.

For a student (or a programmer interested in operating systems), xv6 is a unique opportunity to introduce himself to kernel hacking and to the architecture of UNIX like systems. At about 15k lines of code (iirc), including the (primitive) libraries, the userland and the kernel, it’s very easy (or well, at least easier than production scale UNIX like systems) to grok, and it’s also very easy to expand on. It also helps tremendously that xv6 as a whole has magnificent documentation, not only from MIT, but from other universities that have adopted xv6 for use in their operating systems syllabus.

An introduction to Ensidia: my very personal xv6 fork

When I first discovered xv6 I was ecstatic. For the reasons mentioned above I couldn’t lose on the opportunity to fork xv6 and use it as a personal testbed for anything I could feel like exploring or testing out.

As a matter of fact, when I first discovered xv6, I had just finished implementing (the base of) my own UNIX like operating system, named fotix, and the timing of my discovery was great. xv6 had done what I had done, and also implemented most of what I was planning to work on fotix (for example, elf file loading), and it was a solid base for further development. It also had a userland, which fotix at the time didn’t have.

After I forked xv6, I spent some time familiriazing myself with the code. I also cleaned up the source code quite a bit, structuring the code in a BSD like folder structure, instead of having all of the code in the same folder and made various small scale changes.

After that for quite some time, I had left ensidia alone and didn’t touch it much. However, I always felt like I wanted to develop it a bit more and get to play with its code in interesting ways. I was trying to think of a great way to get started with kernel hacking on it, in a simple way, to get more acquainted with the kernel, and found an interesting pdf with interesting project ideas for it. One of them was to add a system call. I figured out that would be an interesting and quick hack, so hey, why not?

Getting started with kernel hacking on xv6: Adding the system call.

The system call I decided to introduce was the suggested one. It was fairly simple sounding too. You have to introduce a new system call that returns the number of total system calls that have taken place so far. So let’s see how I went about implementing it:

An introduction to system calls in xv6

First of all, we should provide some context about what system calls are, how they are used, and how they are implemented in xv6.

A system call is a function that a userspace application will use, so as to ask for a specific service to be provided by the operating system. For instance with an sbrk(n) system call, a process can ask the kernel to grow its heap space by n bytes. Another example is the well known fork() system call in the UNIX world, that’s used to create a new process by cloning the caller process.

The way applications signal the kernel that they need that service is by issueing a software interrupt. An interrupt is a signal generated that notifies the processor that it needs to stop what its currently doing, and handle the interrupt. This mechanism is also used to notify the processor that information it was seeking from the disks is in some buffer, ready to be extracted and processed, or, that a key was pressed in the keyboard. This is called a hardware interrupt.

Before the processor stops to handle the interrupt generated, it needs to save the current state, so that it can resume the execution in this context after the interrupt has been handled.

The code that calls a system call in xv6 looks like this:

# exec(init, argv)
 .globl start
   pushl $argv
   pushl $init
   pushl $0  // where caller pc would be
   movl $SYS_exec, %eax
   int $T_SYSCALL

In essence, it pushes the argument of the call to the stack, and puts the system call number (in the above code, that’s $SYS_exec) into %eax. The number is used to match the entry in an array that holds pointers to all the system calls. After that, it generates a software interrupt, with a code (in this case $T_SYSCALL) that’s used to index the interrupt descriptor tables and find the appropriate interrupt handler.

The code that is specific to find the appropriate interrupt handler is called trap() and is available in the file trap.c. If trap() check’s out the trapnumber in the generated trapframe (a structure that represents the processor’s state at the time that the trap happened) to be equal to T_SYSCALL, it calls syscall() (the software interrupt handler) that’s available in syscall.c

// This is the part of trap that
// calls syscall()
trap(struct trapframe *tf)
  if(tf->trapno == T_SYSCALL){
    proc->tf = tf;

syscall() is finally the function that checks out %eax to get the number of the system call (to index the array with the system call pointers), and execute the code corresponding to that system call.

The implementation of system calls in xv6 is under two files. The first one is sysproc.c, and is the one containing the implementation of system calls correspondent to processes, and sysfile.c that contains the implementation of system calls regarding the file system.

The specific implementation of the numcalls() system call

To implement the system call itself is simple. I did so with a global variable in syscall.c called syscallnum, that’s incremented everytime syscall(), calls a system call function, that is, the system call is valid.

Next we just need a function, the system call implementation that returns that number to the userspace program that asks for it. Below is the function itself, and syscall() after our change.

// return the number of system calls that have taken place in
// the system
    return syscallnum;
// The syscall() implementation after
// our change
  int num;

  num = proc->tf->eax;
  if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {
    syscallnum++; // increment the syscall counter
    proc->tf->eax = syscalls[num]();
  } else {
    cprintf("%d %s: unknown sys call %d\n",
            proc->pid, proc->name, num);
    proc->tf->eax = -1;

After that was done, the next few things that were needed to be done were fairly straight forward. We had to add an index number for the new system call in syscall.h, expose it to user proccesses via user.h, and add a new macro to usys.S that defines an asm routine that calls that specific system call, and change the makefile to facilitate our change . After doing so we had to write a userspace testing program to test our changes.

The result after doing all this is below :)

cpu1: starting
cpu0: starting
init: starting sh
$ ls
.              1 1 512
..             1 1 512
README         2 2 2209
cat            2 3 9725
echo           2 4 9254
forktest       2 5 5986
grep           2 6 10873
init           2 7 9579
kill           2 8 9246
ln             2 9 9240
ls             2 10 10832
mkdir          2 11 9315
rm             2 12 9308
sh             2 13 16600
stressfs       2 14 9790
usertests      2 15 37633
wc             2 16 10207
zombie         2 17 9028
syscallnum     2 18 9144
console        3 19 0
$ syscallnum
The total number of syscalls so far is 643
$ syscallnum
The total number of syscalls so far is 705
$ syscallnum
The total number of syscalls so far is 767
$ syscallnum
The total number of syscalls so far is 829


I usually end my blog posts with an epilogue. Although this is a post that doesn’t necesarilly need one, I wanted to write one just to say to you that you should try kernel hacking, that is programming jargon for programming an operating system kernel, because it’s an experience that undoubtedly will teach you a great deal of things about how your computer actually works.

Last but not least, take a look at the ongoing work on Ensidia, my fork of xv6. To see this particular work, take a look at the syscall branch.


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