extra_credit.txt

For extra credit:

We implemented bidirection pipe for inter process communication mentioned in the MP3 document, and the virtual memory extra credit functions from cp2


PIPE:

We added a file pipe.c which contains core functions and definitions for the pipe.
The pipeline is created through calling _pipe() syscall in user.
This is redirected to syspipe() function in kernel, further redirected to pipe_open() which opens a pipeline of 512B maximum
pipe_read, pipe_write, pipe_ioctl and pipe_close are functions that are compatible with the io_intf.
On syspipe, an pipeline io_intf is put to the iotab of the current process.
After this current process forks, all child (and parent) processes have access to this pipe.

We choose to implement this pipeline as a BUS (multi master and multi slave)
Any process that have access to it can attempt to write to (and read from) it, as long as it acquire the lock to the pipeline buffer successfully.

Steps to test:
1. 
make filesystem kfs.raw with the executable compiled from pipe_test.c

2. 
run pipe_test as an init user program in main.c

3. 
attach to two uart serials using screen

4. 
How pipe_test work is that the screen attached to serial1 is from the child process and the one attached to serial2 is from the parent process.
Initially child will be input(write to pipe) and parent will be output(read from pipe).
By entering "Enter" on the keyboard the role of child and parent switches. 
We call the screen that is allowing inputs(writing to pipe) be the writing screen.
We call the screen that is receiving be the reading screen.

5. Initially the child process will print a prompt asking for input but because we attach screen after that it won't show at first.
    We just need to find the screen corresponding to serial1 and type characters into it, then we can see characters appearing in the other screen.
    By pressing "Enter" the role of child and parent switches, and a prompt will show on both screen. Now just type into the screen where it says "write line"

Note: If accidentally type into the reading screen, it will show the characters but uart will store the characters in the buffer.
        By pressing enter in the "writing line" screen to switch roles, these characters will show. 
        Becareful that typing enter in the reading screen will make it immediately switch from writing screen to reading screen after next time it switch from reading to writing.

Virtual Memory:
We implement the two extra credit functions in virtmem: 'memory_validate_vptr_len' and 'memory_validate_vstr'. And we use them in syscall, in 'sysmsgout', 'sysread', and 'syswrite' specifically. System calls pass as parameters pointers in user memory. This is a vulnerability since the
 kernel could read or write from invalid memory. Using the virtual memory validation functions can protect the kernel from malicous user programs.