Custom System Call | Operating Systems Assignment

Developed a Custom System Call as a Mainline Kernel Function to compute determinants of 2x2 matrices in C programming language.

Problem Statement - Kernel 2D Memcpy()

Overview

The Kernel 2D Memcpy() exercise is designed to evaluate your understanding of system calls, particularly focusing on kernel memory copy operations. The task involves implementing a custom system call, kernel 2d memcpy(), responsible for copying one 2-D floating-point matrix to another within the kernel space. This exercise assesses your ability to utilize kernel functions such as copy_from_user() and copy_to_user() to transfer data between user space and kernel space.

Task Description

You are required to implement the kernel 2d memcpy() system call, which mimics the functionality of the standard memcpy() operation but relies on the kernel for the copy operations instead of user space. The primary goal is to understand and demonstrate how system calls function within the kernel environment.

Submission Requirements/Rubric

1. Kernel Modification Diff:

   • Provide a diff file showcasing the changes made to the stock kernel to incorporate your custom system call (kernel 2d memcpy()).
   • Successful compilation of the patched kernel is crucial, and no partial points will be awarded.

   Points: 50

2. Sample Test Program:

   • Develop a sample program to test the kernel 2d memcpy() system call.
   • Hard-wire the source 2-D matrix in the program; there is no need to take input from the user at runtime or via a file.
   • Ensure the program correctly calls the system call with appropriate parameters, successfully copying the matrix.

   Points:

   • Program correctly calls the system call: 10
   • Program compiles successfully, but copying is unsuccessful: 5

3. README/Write-up:

   • Provide a README or write-up explaining the logic used in the program to achieve the required functionality.
   • Limit the description to no more than one page.

   Points: 5

Deployment

1. Copy the linux kernel source (stock kernel with the custom config shared earlier) into another directory (would be used later to generate Diff file).

2. In the copied linux kernel directory, open syscall_64.tbl under arch/x86/entry/syscalls & add this entry -

451 common hello_system_call sys_hello

3. I’ve named my system call as hello_system_call, which does the job of copying data from one 2D matrix to another.

4. Next, create another directory named hello_syscall in the copied kernel source directory, and add 2 files in it -

hello_syscall.c
Makefile

5. In the Makefile, add the code -

obj-y := hello_syscall.o

6. This will build the hello_syscall.c file. Here, y refers to yes.

7. Now, we’ll open the root of the kernel source (of the copy) and edit the Makefile using vim command.

8. At line 1103 add hello_syscall/ at the end of the command. This builds our system call and gets it linked to the kernel.

9. Now Compilation & installation steps of the kernel -

make -j$(nproc)
sudo cp -v arch/x86_64/boot/bzImage /boot/vmlinuz-linux-5.19.8-special3
sudo make modules_install
sed s/linux/linux-5.19.8-special3/g </etc/mkinitcpio.d/linux.preset >/etc/mkinitcpio.d/linux-5.19.8-special3.preset
mkinitcpio -p linux-5.19.8-special3
grub-mkconfig -o /boot/grub/grub.cfg

10. Now we can boot into the new kernel & test the system call by compiling the demo.c program using the usual gcc command, and then running the generated executable file.