User-level programs

Our System/161 simulator can run normal C programs if they are compiled with a cross-compiler, os161-gcc. A cross compiler runs on a host (e.g., a Linux x86 machine) and produces MIPS executables; it is the same compiler used to compile the OS/161 kernel. Various user-level programs already exist in userland/bin, userland/testbin, and userland/sbin. Note: that only a small subset these programs will execute successfully due to OS/161 only supporting a small subset of the system call interface.

To create new user programs (for testing purposes), you need to edit the Makefile in bin, sbin, or testbin (depending on where you put your programs) and then create a directory similar to those that already exist. Use an existing program and its Makefile as a template. In assignment 1, we did this for you to produce the starting state of test programs such as rot13 and boxes.

Design

In the beginning, you should tackle this assignment by producing a DESIGN. The design should clearly reflect the development of your solution. The design should not merely be what you programmed. If you try to code first and design later, or even if you design hastily and rush into coding, you will most certainly end up in a software "tar pit". Don't do it! Plan everything you will do. Don't even think about coding until you can precisely explain to your partner what problems you need to solve and how the pieces relate to each other. Note that it can often be hard to write (or talk) about new software design, you are facing problems that you have not seen before, and therefore even finding terminology to describe your ideas can be difficult. There is no magic solution to this problem; but it gets easier with practice. The important thing is to go ahead and try. Always try to describe your ideas and designs to someone else. In order to reach an understanding, you may have to invent terminology and notation, this is fine. If you do this, by the time you have completed your design, you will find that you have the ability to efficiently discuss problems that you have never seen before. Why do you think that CS is filled with so much jargon? To help you get started, we have provided the following questions as a guide for reading through the code to comprehend what is already provided.

To get a feel for what problems you need to solve, review the design questions and design document section later in this specification.

Setup

We assume after ASST0 and ASST1 that you now have some familiarity with setting up for OS/161 development. If you need more detail, refer back to ASST0.

You will soon be able to clone the ASST2 source repository from gitlab.cse.unsw.edu.au, replacing the XXX with your 3 digit group number:

% cd ~/cs3231
% git clone https://zNNNNNNN@gitlab.cse.unsw.edu.au/COMP3231/25T2/grpXXX-asst2.git asst2-src

The above requires a group number! If you haven't been issued a group yet, see the group nomination site here.

If you're still waiting for your group gitlab repository, you can fetch the assignment 2 sources prior to your group being formed via a read-only link as in assignment 1:

% cd ~/cs3231
% git clone https://www.cse.unsw.edu.au/~cs3231/25T2/assignments/asst2-src

Notes:

• The nominations to group ID and group ID to gitlab project scripts run from time to time. There might be a modest delay between nominating your partner and receiving an email about a gitlab repository. If you have been waiting for much more than 24h, ask on the forum.
• The gitlab repository above is shared between you and your partner. You can both push and pull changes to and from the repository to cooperate on the assignment. If you are not familiar with cooperative software development and git you should consider spending a little time familiarising yourself with git.

Building and Testing the Provided Code

% cd ~/cs3231/asst2-src
% ./configure

% cd ~/cs3231/asst2-src
% bmake
% bmake install

% cd ~/cs3231/asst2-src/kern/conf
% ./config ASST2

% cd ../compile/ASST2
% bmake depend
% bmake
% bmake install

% cd ~/cs3231/root
% wget http://cgi.cse.unsw.edu.au/~cs3231/25T2/assignments/asst2/sys161.conf

Unknown syscall 55
Unknown syscall 55
Unknown syscall 55
Unknown syscall 55
:
:
Unknown syscall 55
Unknown syscall 55
Unknown syscall 3
exit() was called, but it's unimplemented.
This is expected if your user-level program has finished.
panic: Can't continue further until sys_exit() is implemented

OS/161 kernel [? for menu]: p /testbin/asst2
Operation took 0.000212160 seconds
OS/161 kernel [? for menu]:

**********
* File Tester
**********
* write() works for stdout
**********
* write() works for stderr
**********
* opening new file "test.file"
* open() got fd 3
* writing test string
* wrote 45 bytes
* writing test string again
* wrote 45 bytes
* closing file
**********
* opening old file "test.file"
* open() got fd 3
* reading entire file into buffer
* attempting read of 500 bytes
* read 90 bytes
* attempting read of 410 bytes
* read 0 bytes
* reading complete
* file content okay
**********
* testing lseek
* reading 10 bytes of file into buffer
* attempting read of 10 bytes
* read 10 bytes
* reading complete
* file lseek  okay
* closing file
exit() was called, but it's unimplemented.
This is expected if your user-level program has finished.
panic: Can't continue further until sys_exit() is implemented

The Assignment Task: File System Calls

Of the full range of system calls that is listed in kern/include/kern/syscall.h, your task is to implement the following file-based system calls: open, read, write, lseek, close, dup2, and document your design. You need to perform this assignment in three steps, see the specs of Parts 1-3 below. As always, it's important to read the entire specification document before you get started.

Note: You will be writing the kernel code that implements part of the system call functionality within the kernel. You are not writing the C stubs that user-level applications call to invoke the system calls. The userland stubs are automatically generated when you build OS/161 in build/userland/lib/libc/syscalls.S which you should not modify.

It's crucial that your syscalls handle all error conditions gracefully (i.e., without crashing OS/161.) no matter what an application requests. Your code should also be memory leak free. You should consult the OS/161 man pages (also included in the distribution) and understand the system calls that you must implement. Your system calls must return the correct value (in case of success) or an appropriate error code (in case of failure) as specified in the man pages. Some of the auto-marking scripts rely on the return of error codes, however, we are lenient as to the specific code in the case of potential ambiguity as to which error code would be most appropriate. It's also not necessary to generate all error codes listed in the man pages.

The file userland/include/unistd.h contains the user-level interface definition of the system calls. This interface is different from that of the kernel functions that you will define to implement these calls. You need to design the kernel side of this interface. The function prototype for your interface can be put it in kern/include/syscall.h. The integer codes for the calls are defined in kern/include/kern/syscall.h.

Part 1: Implement the base system calls

Implement open(), close(), read() and write() as described above.

Once these are working, OS/161 can run various simple programs. The asst2 test we have provided you with should succeed in producing output and testing read and write operations, up until the point at which it attempts lseek.

Once this simple test passes, you have finished Part 1.

Once you have finished Part 1, commit your work using git. You will need to refer back to this commit in Part 3. Make sure you have committed everything that you have done. It would be a good idea if this commit message mentioned that you had finished Part 1.

You will need to find this revision number again for Part 3. We recommend you name the current revision as an additional branch, for instance like this:

% # add a branch "part1_done" at the current revision.
% git branch part1_done
% # list your branches. you should still be on the "main" branch.
% git branch -v

Part 2: Implement the unusual system calls

Implement dup2() and lseek() as described above.

The asst2 test will do some simple tests of lseek(). We do not provide a test for dup2(), and you might want to add one.

Once you are happy that these system calls are working as well, you have finished Part 2. Just like Part 1, make sure that your work is committed using git. If you plan to make further changes, e.g. in the advanced parts, make sure that you take a note of this revision number in the same way.

It is quite likely that you will have to change some of your Part 1 work to get the Part 2 system calls working, or that you will find a bug in your Part 1 work that you want to fix. That is OK, and there is no need to change the Part 1 work that you recorded above.

Part 3: Documenting your solution

This is a compulsory component of this assignment.

You must submit a small design document discussing how you have gone about the assignment. This part of the assignment is done individually, and this part does not need to be added to git or any bundle. If you are working with a partner, both you and your partner should be able to write this short document yourselves.

Your design document should be written as a markdown file named part3.md. We expect it to be 500 to 1500 words, i.e. short and to the point.

In this document, we expect you to discuss:

• What significant data structures did you add (in Part 1 or 2)? What function do they perform?
• What data structures are per-process and what structures are shared between processes?
• What are the main issues related to transferring data to and from applications?
• If you were to implement fork() (although you haven't yet) and introduce concurrency, where would concurrency issues appear in your implementation?
• How did your design change from Part 1 to Part 2? More on this below.

In particular, we are interested in how your design evolved from Part 1 to Part 2. Did you anticipate the requirements of Part 2 entirely during Part 1, or did you have to revise your design?

Recall the revision number you saved at the end of Part 1? You can view the difference in your sources between then and now with git diff:

% git diff -r part1_done..HEAD

The above assumes you saved the branch name part1_done in Part 1, and have your Part 2 work checked out now (HEAD). You can compute the change between different revision points in your work as well.

We have included an example markdown file with some example relevant syntax to get you started. Markdown is mostly a text format, and you don't need to use any fancy features. However, you should use at least one text snippet taken from a git diff command to describe the difference between your Part 1 state and your Part 2 state. An example is included.

If you view a markdown file on gitlab, it will be rendered into an HTML document. This is how a marker will inspect your document, and we recommend you check that any markdown features you have used render correctly. On that point, recall that you are not meant to be working with your partner? We recommend that you put this file in a private repository (they are easy to create) or otherwise manage the markdown->HTML rendering.

FAQ, Gotchas, etc

FAQ and other guides to come.

We recommend discussing any high-level questions on the forum.

Basic Assignment Submission

The standard submission instructions are available on the Wiki . Like ASST0 and ASST1, you will be submitting the git repository bundle via CSE's give system. For ASST2, the submission system will do a test build and run a simple test to confirm your bundle at least compiles. For the basic submission, you will also include a part3.md document separately.

Submit your bundle and your Part 3 document via give:

% cd ~
% give cs3231 asst2 asst2.bundle part3.md

You're now done.

Even though the generated bundle should represent all the changes you have made to the supplied code, occasionally students do something "ingenious". So always push your changes back to gitlab (and keep your git repository) so that you may recover your assignment should something go wrong.

Part A-1: Forking additional processes

Marks: worth the equivalent of 10% of this assignment

Implement the fork() and getpid() system calls.

This system call (together with execv() below) is probably the most difficult part of the whole assignment, but also the most rewarding. They enable multiprogramming and make OS/161 a much more useful entity. fork() is your mechanism for creating new processes. It should make a copy of the invoking process and make sure that the parent and child processes each observe the correct return value (that is, 0 for the child and the newly created pid for the parent).

You will want to think carefully through the design of fork. If you plan on doing Part A-4, consider it together with execv() to make sure that each system call is performing the correct functionality. More on that below.

Part A-2: Copying Data within the Kernel

Marks: worth the equivalent of 5% of this assignment

Part A-2 is unrelated to Part A-1.

Your task is to implement the copy_splice() system call, which is a hybrid of Linux's splice() and copy_file_range() system calls. The Linux variants specify which file objects they can be used on, splice() only for pipes and copy_file_range() only for regular files. Your copy_splice() should support both regular files and devices such as the console device "con:".

The intended signature is:

int copy_splice(int fd_in, int fd_out, size_t len);

This signature omits the offsets and flags in the Linux signatures for splice() etc, but has otherwise the same intention. The user program wants to copy up to len bytes from the input file to the output file without those bytes needing to be copied to and from user memory.

This is not a complex concept, and requires less fiddly user-space copies than the read() and write() system calls from the main part. However, there are some snags, and the Linux man pages mention buggy implementations. Think through all the possible cases before you start implementing.

Part A-3: Ending Processes

Marks: worth the equivalent of 5% of this assignment

This part depends on Part A-1.

Part A-4: Running new binaries

Marks: worth the equivalent of 5% of this assignment

This part depends on Part A-1.

Design Questions

Here are some additional questions and thoughts to aid in your design. They are not, by any means, meant to be a comprehensive list of all the issues you will want to consider. Your system must allow user programs to receive arguments from the command line. By the end of Parts A-3 and A-4, you should be capable of executing lines (in user programs) such as:

char *filename = "/bin/cp";
char *args[4];
pid_t pid;
args[0] = "cp";
args[1] = "file1";
args[2] = "file2";
args[3] = NULL;
pid = fork();
if (pid == 0)
execv(filename, argv);

which will load the executable file cp, install it as a new process, and execute it. The new process will then find file1 on the disk and copy it to file2. You can test your implementation using OS/161's shell, /bin/sh.

Some questions to think about:

• Passing arguments from one user program, through the kernel, into another user program, is a bit of a chore. What form does this take in C? This is rather tricky, and there are many ways to be led astray. You will probably find that very detailed pictures and several walk-throughs will be most helpful.
• How will you determine: (a) the stack pointer initial value; (b) the initial register contents; (c) the return value; (d) whether you can exec the program at all?
• What new data structures will you need to manage multiple processes?
• What relationships do these new structures have with the rest of the system?
• How will you manage file accesses? When we invoke the cat command, and it starts to read file1, what will happen if the shell also tries to read file1? What would you like to happen?
• How will you keep track of running processes. For which system calls is this useful?
• How will you implement the execv system call. How is the argument passing in this function different from that of other system calls?

Advanced Assignment Submission

Submission for the advanced assignment is similar to the basic assignment, except the advance component is given to a distinguished assignment name: asst2_adv. Again, you need to generate a bundle based on your repository. Note: Our marking scripts will switch to the asst2_adv branch prior to testing the advanced assignment.

We've added a directory to the repository called "advanced". It just contains some text files for parts 1-4. For each advanced part you have attempted, adjust that file to remove the "not attempted" message and include a short (100 - 500 word) description of your approach. If you are working with a partner, you can write these descriptions together (unlike part3.md above). If you are working with a partner, you should both submit the advanced part bundle, to avoid confusion, even though the contents will be the same.

Submit your solution by doing:

% cd ~
% give cs3231 asst2_adv asst2_adv.bundle