xv6 MP3: Null pointers and Protected memory


In this machine problem, you'll be changing xv6 to support a feature virtually every modern OS does: causing an exception to occur when your program dereferences a null pointer, and adding the ability to change the protection levels of some pages in a process's address space.

Obtaining the repository

Claim your private repository using the GitHub invitation link found in the assignment list on the course homepage. Accept the invitation and clone your repository on the computer where you'll be doing your work.

Next, edit the "AUTHOR" file in your repository -- in it you will find an honor pledge that you should sign with your information and today's date. Do this NOW so you don't forget!

Null-pointer Dereference

In xv6, the VM system uses a simple two-level page table as discussed in class. As it currently is structured, user code is loaded into the very first part of the address space. Thus, if you dereference a null pointer, you will not see an exception (as you might expect); rather, you will see whatever code is the first bit of code in the program that is running. Try it and see!

Thus, the first thing you might do is create a program that dereferences a null pointer. It is simple! See if you can do it. Then run it on Linux as well as xv6, to see the difference.

Your job here will be to figure out how xv6 sets up a page table. Thus, once again, this part of the assignment is mostly about understanding the code, and not writing very much. Look at how exec() works to better understand how address spaces get filled with code and in general initialized.

You should also look at fork(), in particular the part where the address space of the child is created by copying the address space of the parent. What needs to change in there?

The rest of your task will be completed by looking through the code to figure out where there are checks or assumptions made about the address space. Think about what happens when you pass a parameter into the kernel, for example; if passing a pointer, the kernel needs to be very careful with it, to ensure you haven't passed it a bad pointer. How does it do this now? Does this code need to change in order to work in your new version of xv6?

One last hint: you'll have to look at the xv6 makefile as well. In there user programs are compiled so as to set their entry point (where the first instruction is) to 0. If you change xv6 to make the first page invalid, clearly the entry point will have to be somewhere else (e.g., the next page, or 0x1000). Thus, something in the makefile will need to change to reflect this as well.

Important: do NOT get rid of the -fno-delete-null-pointer-checks compiler flag — this flag makes it so GCC will automatically generate a exception if it notices a reference to address 0 in your code instead of relying on the OS to detect the invalid memory reference.

Read-only Code

In most operating systems, code is marked read-only instead of read-write. However, in xv6 this is not the case, so a buggy program could accidentally overwrite its own text. Try it and see!

In this portion of the xv6 project, you'll change the protection bits of parts of the page table to be read-only, thus preventing such over-writes, and also be able to change them back.

To do this, you'll be adding two system calls: int mprotect(void *addr, int len) and int munprotect(void *addr, int len).

Calling mprotect() should change the protection bits of the page range starting at addr and of len pages to be read only. Thus, the program could still read the pages in this range after mprotect() finishes, but a write to this region should cause a trap (and thus kill the process). The munprotect() call does the opposite: sets the region back to both readable and writeable.

Also required: the page protections should be inherited on fork(). Thus, if a process has mprotected some of its pages, when the process calls fork, the OS should copy those protections to the child process.

There are some failure cases to consider: if addr is not page aligned, or addr points to a region that is not currently a part of the address space, or len is less than or equal to zero, return -1 and do not change anything. Otherwise, return 0 upon success.

Hint: after changing a page-table entry, you need to make sure the hardware knows of the change. On 32-bit x86, this is readily accomplished by updating the CR3 register (what we generically call the page-table base register in class). When the hardware sees that you overwrote CR3 (even with the same value), it guarantees that your PTE updates will be used upon subsequent accesses. The lcr3() function will help you in this pursuit.

Handling Illegal Accesses

In both the cases above, you should be able to demonstrate what happens when user code tries to (a) access a null pointer or (b) overwrite an protected region of memory. In both cases, xv6 should trap and kill the process (this will happen without too much trouble on your part, if your implementation is sensible).

Testing and Scoring

As with the previous machine problem, we include a test suite which you can run with the command make test-mp3. The test source files (which you should not alter --- we'll be using our own pristine copies in any case) can be found in the "tests/mp3/ctests" directory if you're interested.

If a test fails, it'll stop immediately with an error report. If you want to continue running all tests even after hitting a failure, do make test-mp3-cont instead.

There are a total of 7 tests worth 10 points each, for a maximum of 70 points. If your code fails to build, you will not earn any points at all. If you do not pass a test, you will not earn any points for it.


Make sure that you edited the "AUTHOR" file in your repository and signed the honor pledge with your student information. If you don't do this we won't be able to associate your submission with you!

To submit your work, simply commit all your changes (adding new files as needed) and push your work to your GitHub repository.