Add a Syscall
Add a syscall to kernel and replace linux kernel of RPi.
Prepare:
Reference:
Firstly, get the latest kernel:
git clone https://github.com/raspberrypi/linux
Assume that the kernel directory is 'linux' and you have already installed cross-compiler tool.
Secondly, modify the source to add a simple syscall.
1. linux/arch/arm/kernel/sys_arm.c
Add syscall definition.
In order to simplify the process, we add the definition directly in the source that exists, so that we need not to modify the Makefile. And sys_arm.c is what we need.
Add the following definition to sys_arm.c
asmlinkage long sys_mysyscall(int num)
{
printk("My syscall with argument: %d\n",num);
return 0;
}
There is a second file(Call.S) in the same directory that we need to modify. But, in order to make the process more clear, we change another file first.
2.linux/arch/arm/include/unistd.h
Add macro define of our syscall to this file.
In this file, __NR_SYSCALL_BASE define the base address of syscall. And we will use this macro to define the address of our own function. Like this:
#define __NR_mysyscall (__NR_SYSCALL_BASE+223)
We use the 223th address, because this address is unused.
3.linux/arch/arm/kernel/call.S
Bind the definition and the address of our syscall function.
We have function definition in sys_arm.c and function address in unistd.h. Then we should tell the system, these two is associated.
Add this line in the file:
CALL(sys_mysyscall)
Be sure that this line is added in the 223th entry.
4. linux/include/linux/syscalls.h
Add the declaration of the syscall.
We must let system know 'ther is' a syscall 223. As we usually do, add the feclaration of the function to *.h:
asmlinkage long sys_mysyscall(int num);
Now the syscall is added in the linux kernel. Begin to compile the kernel.
Compile kernel
Personally, I create a new directory kernel-build for output.
That is /home/darren/opt/raspberry/kernel-build. It is not necessary.
Clean
Firstly, clean the project.
#Do you know that who is Mr.Proper? Ha...
make mrproper
Configure
Secondly, configure for your Raspberry.
There are some differences between RPi1 and RPi2. But luckily, the official offer us a template. We need not to do this by our own.
#RPi1
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-
O=/home/darren/opt/raspberry/kernel-build bcm_defconfig
#RPi2
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-
O=/home/darren/opt/raspberry/kernel-build bcm2709_defconfig
Okay, that is so easy...
Make
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-
O=/home/darren/opt/raspberry/kernel-build -j 12
Here '-j n' is the number of thread (Is it right? ). To speed up, let it be the 1.5 * the number of processors of your pc.
And you may know the nomber of processor by
cat /proc/cpuinfo | grep processor | wc -l
Install
Insert the sd card to computer. You may get two directories--root and boot.
Denote them like these two:
/media/boot/
/media/root/
Then run this command:
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- O=/home/darren/opt/raspberry/kernel-build -j 12 INSTALL_MOD_PATH=/media/root/ modules
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- O=/home/darren/opt/raspberry/kernel-build -j 12 INSTALL_MOD_PATH=/media/root/ modules_install
Your could replace the kernel.img(or kernel7.img for RPi2) with linux/arch/arm/boot/Image
cp linux/arch/arm/boot/Image /media/boot/
Reboot and all is well.
Syscall
Finally, write a function to call our function:
void inline_asm(int num)
{
asm volatile (
"mov r7, #223\n" //系统调用号
"mov r0, %[value]\n" //参数
"svc #0\n" //监督调用
:: [value] "r" (num) //return 留空,并将 num 作为传入参
);
}
int main()
{
int num = 10;
syscall(223, num); //直接使用 223 号系统调用
num = num << 2;
inline_asm(num);
return 0;
}
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