Add system calls by compiling the kernel

Ubuntu edition ：20.04 LTS Virtual machine configuration ： Hard disk ：40 GB Memory ：4 GB Kernel version ：5.4.0.26-generic VMware edition ：15.5.0

Preparation

You need some libraries to compile , So install it in advance ：

sudo apt-get install libncurses5-dev

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sudo apt-get install bison

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sudo apt-get install flex

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sudo apt-get install libssl-dev

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sudo apt-get install 

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sudo apt-get install build-essential

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sudo apt-install kernel-package

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So much ？？ ok , It doesn't have to be all , part Linux The distribution may come with , You can skip this step first , Do the next thing first , The premise is that you should do a good job first and show up halfway error To prepare ！ If an error occurs halfway, you can install it according to the prompt .

Check your kernel version

uname -a

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My is 5.4.0.26-generic

Download the kernel

Try to download a kernel similar to your own , Avoid incompatibility caused by too large version span . Here's the URL

http://cdn.kernel.org/pub/linux/kernel

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I downloaded it linux-5.3.9.tar.xz

Unzip the kernel file

First, move the above kernel file to /usr/src/ Under the table of contents

stay /usr/src/ Open the terminal in the directory

decompression

sudo xz -d linux-5.3.9.tar.xz

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Unpack

sudo tar -xvf linux-5.3.9.tar

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Add functions

After unpacking, a linux-5.3.9 Folder , Enter the folder kernel In the folder

cd linux-5.3.9/kernel

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edit sys.c file , Add the function at the end ：

asmlinkage long sys_hello(void){
    prink("hello syscall!");
    return 1;
}

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Add function declaration

Get into /usr/src/linux-5.3.9/arch/x86/include/asm

edit syscalls.h, Join us in sys_hello function ：

Assign a to our system call id

open /usr/src/linux-5.3.9/arch/x86/entry/syscalls/syscall_64.tbl,

Be sure not to conflict with what you already have

I added

548 64 hello sys_hello

Configure the kernel

Enter into /usr/src/linux-5.3.9 Under the table of contents , Execute sequentially

sudo make mrproper

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sudo make clean

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sudo make menuconfig

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If you execute sudo make menuconfig When the following error occurs

Full screen virtual machine （ Fill up the screen ）

Then the page appears , direct save, then exit.

Then we can compile our kernel with the following commands

sudo make

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The process takes about a few hours , Anyway, I put it backstage , Then go to bed first ~~.

however ！ When I woke up, I found , The computer went to sleep automatically ！ A wasted night , So remind to turn off the hibernation mode of the computer before compiling . In addition, the compilation process will produce a large number of intermediate files , It is recommended to give enough hard disk 40 GB

The above information indicates that the compilation is successful .

Generate modules

sudo make modules

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This process also takes some time .

install modules

sudo make modules_install

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Install the kernel into the system

sudo make install

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Restart and test

Restart the process to enter advanced options

Just press... When restarting ESC（ The Internet says yes ESC, But mine is a long press Shift）

Choose advanced options , Then select the new kernel

If the process of entering the kernel occurs kernel panic not syncing:System is deadlocked on memory error , Just increase the memory .

Create a new test source program testMyCall.c

#include<stdio.h>
#include<sys/syscall.h>
#include<unistd.h>
#include<stdio.h>
int main(){
    // syscall  You need to pass a system call number 
    long int myRetutn = syscall(548);
    printf("syscall return: %ld",myRetutn);
    return 0;
}

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Compile operation

gcc testMyCall.c -o test
./test
syscall return: 1

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We found no output hello syscall!, Because of the log output level problem , Just call dmesg Command is enough .