Compiling Linux Kernel and Adding Custom System Calls

• Assignment 1: Compiling Linux Kernel and Adding Custom System Calls
  • Compiling Linux Kernel
    • Change kernel suffix
  • Adding custom system calls
    • sys_hello
    • sys_revstr

Part 1: Compiling Linux Kernel & Change kernel suffix

Step 1: Download and Extract the Linux kernel source code

wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.19.12.tar.xz

tar -xJf linux-5.19.12.tar.xz

cd linux-5.19.12

Step 2: Install essential tools and libraries required for compiling the kernel

sudo apt update
sudo apt upgrade
sudo apt-get install flex
sudo apt-get install bison
sudo apt install build-essential
sudo apt install libelf-dev libssl-dev
sudo apt-get install libncurses5-dev gcc make git exuberant-ctags bc libssl-dev

Step 3: Copy current kernel config file, then set it as default configuration

cp /boot/config-`uname -r`* .config

make defconfig

Step 4: Modify the config file

vim .config

Find CONFIG_LOCALVERSION change value to -os-312551002

Step 5: Compile the kernel (wait 20~50 mins)

make -j4

Step 6: Install the kernel modules

sudo make modules_install install

Step 7: Update GRUB, the bootloader

sudo update-grub2

Step 8: Reboot the system

reboot

Press Esc button when system boot > Advanced options for Ubuntu > choose Linux 5.19.12-os-312551002 to start

Step 9: Check the running kernel version

uname -a

The screenshot of changed kernel suffix

Part 2: Adding custom system calls

sys_hello

For the implementation of the sys_hello system calls, I modified 4 kernel source files and added 1 new C code to verify if the system calls work correctly. The modified and added files are:

1. linux-5.19.12/kernel/sys.c
2. linux-5.19.12/arch/x86/include/generated/uapi/asm/unistd_64.h
3. linux-5.19.12/arch/x86/entry/syscalls/syscall_64.tbl
4. linux-5.19.12/include/linux/syscalls.h
5. sys_hello.c

Step 1: In the sys.c file, add the following code to defines a new system call named "hello".

SYSCALL_DEFINE0(hello){
	printk(KERN_INFO "Hello, world! \n");
	printk(KERN_INFO "312551002 \n");
	return 0;
}

Step 2: In the unistd_64.h file, add the following code to defines the system call number for "hello".

#define __NR_hello 548

Step 3: In the syscall_64.tbl file, add the following entry to map system call number to its corresponding function.

548	common	hello	sys_hello

Step 4: In the syscalls.h file, add the following code to add the new system call in the header file.

asmlinkage long sys_hello(void);

Step 5: Create new C code file named sys_hello.c and add the following code:

#include <assert.h>
#include <unistd.h>
#include <sys/syscall.h>

#define __NR_hello 548

int main(int argc, char *argv[]) {
    int ret = syscall(__NR_hello);
    assert(ret == 0);

    return 0;
}

Step 6: Rebuild the kernel to compile the modified kernel source code.

make

Step 7: Reinstall the kernel

sudo make modules_install install

Step 8: Reboot the kernel

reboot

Step 9: Compile the sys_hello.c file to test if the system call operates correctly.

gcc sys_hello.c -o sys_hello

./sys_hello

Step 10: Display the output messages produced by the sys_hello system call.

dmesg | tail

The screenshot of the results of executing sys_hello system call:

sys_revstr

For the implementation of the sys_hello system calls, I modified 4 kernel source files and added 1 new C code to verify if the system calls work correctly. The modified and added files are:

1. linux-5.19.12/kernel/sys.c
2. linux-5.19.12/arch/x86/include/generated/uapi/asm/unistd_64.h
3. linux-5.19.12/arch/x86/entry/syscalls/syscall_64.tbl
4. linux-5.19.12/include/linux/syscalls.h
5. sys_revstr.c

Step 1: In the sys.c file, add the following code to defines a new system call named "revstr".

SYSCALL_DEFINE2(revstr, int, len, char __user *, usr_str) {
    char k_str[256];
    char k_revstr[256];
    int start = 0;
    int end = len - 1;

    if(len >= sizeof(k_str))
        return -EINVAL;
    if(copy_from_user(k_str, usr_str, len))
        return -EFAULT;

    k_str[len] = '\0';
    printk(KERN_INFO "The origin string: %s", k_str);

    while(start < end) {
        char temp = k_str[start];
        k_str[start] = k_str[end];
        k_str[end] = temp;
        start++;
        end--;
    }

    strncpy(k_revstr, k_str, sizeof(k_str));
    printk(KERN_INFO "The reversed string: %s", k_revstr);

    return 0;
}

Step 2: In the unistd_64.h file, add the following code to defines the system call number for "revstr".

#define __NR_revstr 549

Step 3: In the syscall_64.tbl file, add the following entry to map system call number to its corresponding function.

549	common	revstr	sys_revstr

Step 4: In the syscalls.h file, add the following code to add the new system call in the header file.

asmlinkage long sys_revstr(int len, char __user *usr_str);

Step 5: Create new C code file named sys_revstr.c and add the following code:

#include <assert.h>
#include <unistd.h>
#include <sys/syscall.h>

#define __NR_revstr 549

int main(int argc, char *argv[]) {  
    int ret1 = syscall(__NR_revstr, 5, "hello");
    assert(ret1 == 0);

    int ret2 = syscall(__NR_revstr, 11, "5Y573M C411");
    assert(ret2 == 0);
    return 0;
}

Step 6: Rebuild the kernel to compile the modified kernel source code.

make

Step 7: Reinstall the kernel

sudo make modules_install install

Step 8: Reboot the kernel

reboot

Step 9: Compile the sys_revstr.c file to test if the system call operates correctly.

gcc sys_revstr.c -o sys_revstr

./sys_revstr

Step 10: Display the output messages produced by the sys_ revstr system call.

dmesg | tail

The screenshot of the results of executing sys_ revstr system call: