shikshan/syscall

Implement a Linux System Call

Implement a Linux System Call

setup

This project contains a Vagrantfile that runs a Fedora 31 VM.

./setup.sh

Steps

Get the linux kernel source

# Since Fedora 31 uses kernel v5.3.7, let's get the closest patch version 5.3.14
[vagrant@localhost ~]$ curl -OJ https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.3.14.tar.xz
[vagrant@localhost ~]$ unxz -v linux-5.3.14.tar.xz

# Optional - verify the distribution
[vagrant@localhost ~]$ curl -OJ https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.3.14.tar.sign
[vagrant@localhost ~]$ gpg --verify linux-5.3.14.tar.sign

# Fetch the keys if they are missing. <KEY> is the key mentioned in above step.
[vagrant@localhost ~]$ gpg --recv-keys <KEY>

[vagrant@localhost ~]$ tar xvf linux-5.3.14.tar
[vagrant@localhost ~]$ cd linux-5.3.14

Make the code changes

The code change steps assume the current directory is ~/linux-5.3.14.

• The implementation of the system call is in sys_greet_o.c file and the build configuration is in Makefile in code/greet_o directory. This directory will be available as /vagrant/code/greet_o in the VM. Copy it under the linux kernel source root directory.

  [vagrant@localhost ~]$ cp -r /vagrant/code/greet_o .

• Add the system call function header at the end of include/linux/syscalls.h file before the ending #endif.

  asmlinkage long sys_greet_o(const char __user *str,
                              const size_t str_len,
                              char __user *buf,
                              size_t buf_len);

• Add the system call number to the end of the arch/x86/entry/syscalls/syscall_64.tbl file. I'm using 999 as system call number.

  999     64      greet_o                 __x64_sys_greet_o

• Update the following line in the Makefile in the kernel source root directory to add greet_o directory:

  core-y += kernel/ certs/ mm/ fs/ greet_o/ ipc/ security/ crypto/ block/

Build the kernel

The kernel build steps assume the current directory is ~/linux-5.3.14.

# copy the configuration from existing kernel
[vagrant@localhost linux-5.3.14]$ cp /boot/config-$(uname -r) .config

[vagrant@localhost linux-5.3.14]$ make menuconfig
# The above command will pop open the config dialog. Just save and exit.

# Optionally update CONFIG_LOCALVERSION in .config file to to build kernel with some suffix.
# Here is the example that pleases my vanity :) - CONFIG_LOCALVERSION="-omkar"

# $(nproc) is the number of cpus available. For this VM, it's 4.

[vagrant@localhost linux-5.3.14]$ sudo make -j $(nproc)
[vagrant@localhost linux-5.3.14]$ sudo make modules_install -j $(nproc)
[vagrant@localhost linux-5.3.14]$ sudo make install -j $(nproc)

Once the build is successful, restart the virtual machine.

# Exit the VM
[vagrant@localhost linux-5.3.14]$ exit

# Restart the VM and connect to it
vagrant reload && vagrant ssh

Verify

• Check the kernel version.

  [vagrant@localhost ~]$ uname -mrs
  Linux 5.3.14-omkar x86_64
  
  # See that's my own linux ;)!

• Compile and run the /vagrant/code/test_driver.c file to see the custom syscall in action.

  [vagrant@localhost ~]$ cd /vagrant/code
  [vagrant@localhost code]$ gcc test_driver.c -o test_driver
  [vagrant@localhost code]$ ./test_driver
  greet_o system call message: Namaste Omkar!
  
  # Yay, it's working!

• Check the kernel logs.

  [vagrant@localhost code]$ dmesg
  [  151.368231] greet_o system call invoked.
  [  151.368428] greet_o system call message: Namaste Omkar!

• Finally let's actually trace the system calls usage using strace tool.

  [vagrant@localhost code]$ strace ./test_driver
  ...
  ...
  ...
  syscall_0x3e7(0x7ffc0d747b80, 0x6, 0x7ffc0d747b40, 0x40, 0x7feb91c84f30, 0x7ffc0d747c88) = 0
  ...
  exit_group(0)                           = ?
  +++ exited with 0 +++

  syscall_0x3e7 is our own system call, 0x3e7 being the hex value of 999. The trace shows that the call successfully completed with return value of 0.