Assignment 1: Compiling Linux Kernel and Adding Custom System Calls

Assignment 1: 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** ![image](https://hackmd.io/_uploads/rkmobCrAT.png) 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".** ```c 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:** ```c #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:** ![image](https://hackmd.io/_uploads/BkbnZCrCp.png) ### `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".** ```c 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:** ![image](https://hackmd.io/_uploads/HJn3WAHR6.png)