Rodrigo Gobbi

Some software experiments ;)

For some architectures, kernel has changed the generation of some inner headers for syscall tables. Now at kernel 6.1.y (Pi fork) there are some utility scripts like syscallnr.sh which acts over a table file called syscall.tbl. Apparently, for arm64, this is not the case since we still have to add the entries manually like the old days.

At linux-rpi-6.1.y folder, change the following headers:

// include/linux/syscalls.h
asmlinkage long sys_my_pi_syscall(void);

// include/uapi/asm-generic/unistd.h
#define __NR_my_pi_syscall 451
__SYSCALL(__NR_my_pi_syscall, sys_my_pi_syscall)

// add a 'plus one' at total number...
#define __NR_syscalls 452

// usr/include/asm-generic/unistd.h
#define __NR_my_pi_syscall 451
__SYSCALL(__NR_my_pi_syscall, sys_my_pi_syscall)

// add a 'plus one' at total number...
#define __NR_syscalls 452

// arch/arm64/include/asm/unistd32.h
#define __NR_my_pi_syscall 451
__SYSCALL(__NR_my_pi_syscall, sys_my_pi_syscall)

// for syscall implementation: use an existing file or create a new one.
// i've used the kernel/sys.c...
SYSCALL_DEFINE0(my_pi_syscall)
{
    printk(KERN_ERR "my pi syscall called for arm64!!!");
    return 0;
}

There is a document about this called adding-syscalls.rst inside the tree, this info is also there. After updating the kernel image and installing the new headers:

sudo make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- INSTALL_HDR_PATH=/mnt/rootfs_sdcard/usr/ headers_install

You can call the my_pi_syscall with syscall function from libc.