linux-kernel

Modified Version of Linux Kernel

Installation

Before Installation

ajmi@burner:~$ uname -mrs
Linux 5.13.0-1031-azure x86_64

Install Requirements

sudo apt update
sudo apt upgrade
sudo apt install build-essential libncurses-dev bison flex libssl-dev libelf-dev dwarves zstd

Clone Linux Kernel Repository

git clone https://github.com/JameelKaisar/linux-kernel.git

Change Directory

cd linux-kernel

Copy Config File

cp -v /boot/config-$(uname -r) .config

Modify Config File

sudo make menuconfig

• Save and Exit

Edit Config File

sudo nano .config

• Set CONFIG_SYSTEM_TRUSTED_KEYS to ""
• Set CONFIG_SYSTEM_REVOCATION_KEYS to ""

Compile Linux Kernel (takes time)

sudo make -j$(nproc)

Secret Command (skip this)

sleep 5 && echo start > ~/start.txt && sudo make -j$(nproc) > ~/logs.txt 2> ~/errors.txt && echo success > ~/success.txt || echo fail > ~/fail.txt & disown

• Turns out we can use screen window manager instead of using the so-called Secret Command. screen can be installed using sudo apt install screen.

Install Linux Kernel Modules

sudo make modules_install -j$(nproc)

Install the Linux Kernel

sudo make install -j$(nproc)

Update GRUB Config

sudo update-initramfs -c -k 5.19.0-rc3
sudo update-grub

Reboot Device

sudo reboot

After Installation

ajmi@burner:~$ uname -mrs
Linux 5.19.0-rc3 x86_64

System Calls

retint System Call (451)

Return number from a system call

retint.c

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>

int main()
{
    long int n = syscall(451);
    printf("System Call sys_retint Returned: %ld\n", n);
    return 0;
}

Output

System Call sys_retint Returned: 2022

swpnum System Call (452)

Swap numbers using a system call

swpnum.c

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>

int main()
{
    int a = 1;
    int b = 2;
    printf("Before Swapping: a = %d, b = %d\n", a, b);
    syscall(452, &a, &b);
    printf("After Swapping: a = %d, b = %d\n", a, b);
    return 0;
}

Output

Before Swapping: a = 1, b = 2
After Swapping: a = 2, b = 1

revstr System Call (453)

Reverse string using a system call

revstr.c

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>

int main()
{
    char s[10] = "abcdefg";
    printf("Before Reversing: %s\n", s);
    syscall(453, s, 10);
    printf("After Reversing: %s\n", s);
    return 0;
}

Output

Before Reversing: abcdefg
After Reversing: gfedcba

cpyarr System Call (454)

Copy array using a system call

cpyarr.c

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>

int main()
{
    int a[7] = {1, 2, 3, 4, 5, 6, 7};
    int b[7];
    printf("Initial Array: ");
    for (int i=0; i<7; i++)
        printf("%d ", a[i]);
    printf("\n");
    syscall(454, a, b, 7);
    printf("Copied Array: ");
    for (int i=0; i<7; i++)
        printf("%d ", b[i]);
    printf("\n");
    return 0;
}

Output

Initial Array: 1 2 3 4 5 6 7 
Copied Array: 1 2 3 4 5 6 7 

swpsrt System Call (455)

Swap members of structure using a system call

swpsrt.c

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>

struct swap_srt {
    int a;
    int b;
};

int main()
{
    struct swap_srt s;
    s.a = 1;
    s.b = 2;
    printf("Before Swapping: s.a = %d, s.b = %d\n", s.a, s.b);
    syscall(455, &s);
    printf("After Swapping: s.a = %d, s.b = %d\n", s.a, s.b);
    return 0;
}

Output

Before Swapping: s.a = 1, s.b = 2
After Swapping: s.a = 2, s.b = 1

revsrt System Call (456)

Reverse array in structure using a system call

revsrt.c

#include <stdio.h>
#include <stdlib.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>

struct reverse_srt {
    int *arr;
    int n;
};

int main()
{
    struct reverse_srt s;
    s.n = 7;
    s.arr = malloc(s.n * sizeof(int));
    for (int i=0; i<s.n; i++)
        s.arr[i] = i+1;
    printf("Before Reversing: ");
    for (int i=0; i<7; i++)
        printf("%d ", s.arr[i]);
    printf("\n");
    syscall(456, &s);
    printf("After Reversing: ");
    for (int i=0; i<7; i++)
        printf("%d ", s.arr[i]);
    printf("\n");
    return 0;
}

Output

Before Reversing: 1 2 3 4 5 6 7 
After Reversing: 7 6 5 4 3 2 1