Adding NULL pages to xv6 address spaces

1. Intro To xv6 Virtual Memory
In this project, 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.
2. 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 want to do is to 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
project 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
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?



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