Init cut of xv6 VM project

vm-xv6-intro/README.md

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+# 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.
+
+## 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 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 [code 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.
+
+## 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.
+
+## 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 mprotected
+region of memory. In both cases, xv6 should trap and kill the process (this
+will happen without too much trouble on your part, if you do the project in a
+sensible way).
+
+
+
+