添加系统系统到内核，测试环境huawei u9508手机，基于内核代码huawei u9508 kernel

1. 修改 arch/arm/kernel/calls.S

   diff —git a/arch/arm/kernel/calls.S b/arch/arm/kernel/calls.S
   index 80f7896..100320f 100644
   —- a/arch/arm/kernel/calls.S
   +++ b/arch/arm/kernel/calls.S
   @@ -385,6 +385,7 @@

   CALL(sys_syncfs)
   CALL(sys_sendmmsg)
   

   / 375 / CALL(sys_setns)

   • CALL(sys_hello)

     ifndef syscalls_counted

     .equ syscalls_padding, ((NR_syscalls + 3) & ~3) - NR_syscalls

     define syscalls_counted

2. 修改 include/linux/syscalls.h

   diff —git a/include/linux/syscalls.h b/include/linux/syscalls.h
   index 8c03b98..ca05033 100644
   —- a/include/linux/syscalls.h
   +++ b/include/linux/syscalls.h
   @@ -847,4 +847,5 @@ asmlinkage long sys_open_by_handle_at(int mountdirfd,

   struct file_handle __user *handle,
   int flags);
   

   asmlinkage long sys_setns(int fd, int nstype);
   +asmlinkage long sys_hello(void);

   endif

3. 修改 arch/arm/include/asm/unistd.h

   diff —git a/arch/arm/include/asm/unistd.h b/arch/arm/include/asm/unistd.h
   index 2c04ed5..fa6cc69 100644
   —- a/arch/arm/include/asm/unistd.h
   +++ b/arch/arm/include/asm/unistd.h
   @@ -402,6 +402,7 @@

   define NR_syncfs (NR_SYSCALL_BASE+373)

   define NR_sendmmsg (NR_SYSCALL_BASE+374)

   define NR_setns (NR_SYSCALL_BASE+375)

   +#define NR_sys_hello (NR_SYSCALL_BASE+376)

   /*

   • The following SWIs are ARM private.

4. 在kernel下添加hello目录，并添加hello.c, Makefile

   hello/
   ├── hello.c
   └── Makefile

vim hello.c

#include <linux/kernel.h>
asmlinkage long sys_hello(void) 
{
    printk("hello world\n");
    return 0;
}

vim Makefile

obj-y := hello.o

1. 修改kernel下Makefile

   diff —git a/Makefile b/Makefile
   index 9651e1c..3e54a62 100755
   —- a/Makefile
   +++ b/Makefile
   @@ -719,7 +719,7 @@ export mod_strip_cmd

 ifeq ($(KBUILD_EXTMOD),)
-core-y         += kernel/ mm/ fs/ ipc/ security/ crypto/ block/
+core-y         += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ hello/

 vmlinux-dirs   := $(patsubst %/,%,$(filter %/, $(init-y) $(init-m) \
                     $(core-y) $(core-m) $(drivers-y) $(drivers-m) \

1. 编译内核

   export CROSS_COMPILE=arm-linux-androideabi-
   export ARCH=arm
   make distclean
   make hisi_k3v2oem1_defconfig
   make zImage

2. modify boot.img with our build zImage
   mkboot boot.img boot
   ….
   mkboot boot boot_new.img
   adb reboot bootloader
   fastboot flash boot boot_new.img
   fastboot reboot

3. 测试添加的系统调用

   jni/
   ├── Android.mk
   └── sys_hello.cpp

vim sys_hello.cpp

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>

#define __NR_hello 376

long hello_syscall(void)
{
    return syscall(__NR_hello);
}

int main(int argc, char **argv)
{
    long int a = hello_syscall();
    printf("System call returned %ld\n", a);
    return 0;
}

vim Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_SRC_FILES += sys_hello.cpp

LOCAL_MODULE := hello
include $(BUILD_EXECUTABLE)

ndk-build

StaticLibrary  : libstdc++.a
Executable     : hello
Install        : hello => libs/armeabi/hello

adb push libs/armeabi/hello /data/local/tmp/
adb shell

root@android:/data/local/tmp # ./hello                                         
System call returned 0
root@android:/data/local/tmp # dmesg                                           
<4>[5494.1, hello] [ 1911.446306] hello world

以上针对arm平台添加的系统调用，若为x86平台，则需要将1,2,3步修改为x86目录下相应的文件即可;
参考： How To Add A System Call and Recompile the Linux Kernel