一、前言
最近想学一下kernel和hal,所以买了一块板子,带了个摄像头和屏幕,1100+,学习投资了。这个Android内核定一个系统调用感觉是真的麻烦，主要是有一层bionic C，一开始不熟悉的时候还是花了点时间去配置。
二、kernel修改
include/uapi/asm-generic/unistd.h
系統调用的定义里增加自己的定义，这里是直接集成到arm64了，我发现arm都没有，简单起见所以后面bionic就直接arm64生成代码了。
#define __NR_hello 449
__SYSCALL(__NR_hello, sys_helloworld)
#undef __NR_syscalls
#define __NR_syscalls 450
然后新增一个目录drivers/char/helloworld，新增以下文件，这里的配置是默认的，后面编译刷入就行，这里我不说明刷入了，只介绍代码流程。
├── Kconfig
├── Makefile
├── mysyscall.c
mysyscall.c
#include <linux/kernel.h>
#include <linux/syscalls.h>SYSCALL_DEFINE0(helloworld){printk("Hello, world my syscall!\n");return 0;
}Makefile
obj-$(CONFIG_helloworld) += mysyscall.o
Kconfig
config helloworldbool "helloworld suppore"default yhelphelloworld
这里配置完成之后需要上一层在char目录下的kConfig和Makefile中引用这里我也贴一下，分别新增
source "drivers/char/helloworld/Kconfig"
obj-y += helloworld/
kernel刷入后可以使用命令查看系统调用是否配置在kernel中，这里看到已经有了，第一个就是。
三、bionic修改
进入Android源码目录，今日bionic文件夹，下面的路径都是这里的相对路径了。
新增一个头文件libc/include/sys/myhello.h
/** Copyright (C) 2008 The Android Open Source Project* All rights reserved.** Redistribution and use in source and binary forms, with or without* modification, are permitted provided that the following conditions* are met:* * Redistributions of source code must retain the above copyright* notice, this list of conditions and the following disclaimer.* * Redistributions in binary form must reproduce the above copyright* notice, this list of conditions and the following disclaimer in* the documentation and/or other materials provided with the* distribution.** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF* SUCH DAMAGE.*/#pragma once/*** @file sys/sysinfo.h* @brief System information.*/#include <sys/cdefs.h>
#include <sys/types.h>__BEGIN_DECLS/*** just direct test*/
int hello();__END_DECLS
接下来修改libc/SYSCALLS.TXT
末尾新增
# <sys/myhello.h>
int hello() arm64
在libc/libc.map.txt新增hello，我这里直接在LIBC_P下面增加的，这里随便找的增加的，没有文档介绍，理论上到不要求版本的地方也行。这里的P也就是版本要求了。
LIBC_P { # introduced=Pglobal:__freading;__free_hook;__fseterr;__fwriting;__malloc_hook;__memalign_hook;__realloc_hook;aligned_alloc;endhostent;endnetent;endprotoent;epoll_pwait64;fexecve;fflush_unlocked;fgetc_unlocked;fgets_unlocked;fputc_unlocked;fputs_unlocked;fread_unlocked;fwrite_unlocked;getentropy;getnetent;getprotoent;getrandom;+ hello; #arm64...} LIBC_O;
接下来还需要在两个头文件下面增加定义
libc/include/bits/glibc-syscalls.h
#if defined(__NR_hello)#define SYS_hello __NR_hello
#endif
libc/kernel/uapi/asm-generic/unistd.h 这里要注意内核里定义的数字要和这里一致，剩下的两个内核里没有的直接注释
// #define __NR_futex_waitv 449
// #define __NR_set_mempolicy_home_node 450#define __NR_hello 449
到这里修改已经结束，进入bionic目录下面，然后mma即可，这里更新so也介绍一下
进入源码目录rk3568_android13/out/target/product/topeet_rk3568/system/apex/
adb push com.android.runtime /system/apex/ 这样就整体替换了。下面开始编写测试程序
四、测试程序编写
Android源码下随便新建一个目录，添加如下文件
├── Android.bp
└── hello.cppAndroid.bp
cc_binary {name: "hello",srcs: ["hello.cpp"],cflags: ["-Werror"],product_specific: true,
}hello.cpp
#include <cstdio>
#include <sys/myhello.h>int main()
{hello();return 0;
}写完之后make hello，我们把可执行文件push到Android目录下执行即可
topeet_rk3568:/data/local/tmp # ls
hello
topeet_rk3568:/data/local/tmp # ./hello
topeet_rk3568:/data/local/tmp #
接着查看内核日志 dmesg
可以看到日志已经成功打印，我们自定义的系统调用已经实现
