How can I create a custom built-in system call in Linux?

Below is an outline for adding a new (built-in) system call into the Linux kernel. Note that modifying the kernel requires caution, a deep understanding of kernel internals, and you should only do this in a safe development or testing environment.

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1. Define Your System Call

Create the implementation of your system call. For example, you can add a new source file (or include it in an existing one):

CCopied
// File: kernel/mysyscall.c

#include <linux/kernel.h>
#include <linux/syscalls.h>

SYSCALL_DEFINE0(mysyscall)
{
    printk(KERN_INFO "My new system call was invoked!\n");
    return 0;  // Return an integer value
}

The macro SYSCALL_DEFINE0 declares a system call that takes no arguments. There are variants like SYSCALL_DEFINE1, SYSCALL_DEFINE2, etc., if your system call needs parameters.

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2. Add a Declaration to the Header

Put the prototype in the appropriate header file so that it’s visible to other kernel parts. For instance, you might add a prototype in an existing header (such as include/linux/syscalls.h):

CCopied
extern long sys_mysyscall(void);

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3. Update the System Call Table

Every architecture has its own system call table. For x86-64, you would add an entry in the file typically located at arch/x86/entry/syscalls/syscall_64.tbl. For example:

332    64    mysyscall    __x64_sys_mysyscall

• 332 is an example syscall number; pick an unused number available in your kernel version.
• 64 signifies the syscall bitness.
• mysyscall is the symbolic name used from user space.
• __x64_sys_mysyscall is the kernel symbol for your system call.

For other architectures, check the corresponding syscall table under the arch/ directory.

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4. Rebuild and Install the Kernel

After making changes:

1. Rebuild the kernel:
   • Run the proper kernel build commands (e.g., make -j<number_of_cores>).
2. Install it and update your bootloader configuration.
3. Reboot into your newly built kernel.

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5. Test Your New System Call

Write a simple user-space program to invoke the new system call. Here’s an example:

CCopied
// File: test_mysyscall.c
#include <stdio.h>
#include <sys/syscall.h>
#include <unistd.h>

#ifndef __NR_mysyscall
#define __NR_mysyscall 332  // Must match the number used in syscall table
#endif

int main(void)
{
    long ret = syscall(__NR_mysyscall);
    printf("System call returned: %ld\n", ret);
    return 0;
}

Compile and run your program to see the effects (and check kernel logs via dmesg for your printk).

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Points to Note

• Safety: Always work in a controlled environment. Kernel modifications can render a system unbootable or unstable.
• Version Differences: The steps and file locations may slightly differ between kernel versions.
• Documentation: Consult the relevant kernel documentation and your architecture’s guidelines (for example, Documentation/syscalls/syscall.rst where available).

By following these steps, you'll have integrated a new built-in system call into the Linux kernel.