Kernel Development Learning Pipeline

A3 - Add a new syscall 🤫

Add a new system call to the kernel

Outcomes:

• Get comfortable with the concept of a syscall from both kernelspace and userspace
  • Learn how userspace programs make systemcalls
  • Learn how those calls are handled by the kernel
• Understand how C relates to assembly languge
  • Write a C program to call your systemcall
  • Write the same program in the assembly language of your platform

Procedure:

1. Using the appropriate SYSCALL_DEFINE* macro, define a function that behaves as follows:

   a) The function takes two arguments, a userspace pointer to a buffer of type char *, an integer specifying the size of the buffer of type size_t.

c) The function returns the length of the string copied upon sucess. If there is an issue with the size of the provided buffer, checked by proxy of the `size_t` argument, then the function returns `-EFAULT` to the user.

d) The function should take care to prevent any possibility of a buffer overflow.

1. Add an asmlinkage declaration of your syscall in include/linux/syscalls.h. There are numerous examples of other syscalls in that file for ample student inspiration.

2. Add an entry to the syscall table for your architecture. On x86_64, this is arch/x86/entry/syscalls/syscall_64.tbl and on aarch64 this is include/uapi/asm-generic/unistd.h. Take inspiration from the other syscalls.

3. Write an assembly language program that invokes your syscall and prints the string that is placed in the buffer by the kernel to standard output using the write syscall.

4. Write a C program that does exactly the same thing as the previous step.

5. Compile and reboot into the newly modified kernel. Make sure your C and assembly language programs work correctly. Include the output in your cover letter.

The following pages will be of interest to a student:

• https://elixir.bootlin.com/linux/latest/ident/get_task_comm

Conventions:

1. Assembly files that must be pre-processed use the extension *.S

2. Assembly files that are ready to be assembled use the extension *.s

Refer to the Linux kernel documentation for further guidance.

What to submit:

• A patch which adds a directory named firstname_lastname to the assignment repo.
  • The first patch should create this directory and add your Makefile.
  • Make sure to have compiler warnings enabled (at least -Wall but ideally -Wextra and -Wpedantic too, or even -Weverything if you use clang) and that your code doesn’t have any warnings or errors.
• The second patch should add your kernel patches as patches to our repo. Generate a patch from your commit to the Linux kernel (make sure to use --base=HEAD^ on git format patch so we can tell what commit your work was based on), and then take this file, put it in your named directory, and add the email patch itself to the staged git files and commit this. Then, generate a patch from this commit. This is the second patch of the assignment.
• The third patch adds your assembly program to your named directory.
• The fourth patch adds your C program to your named directory.
• Don’t forget a cover letter.