![图片描述](https://dn-simplecloud.shiyanlou.com/courses/uid1393219-20200520-1589951394969) ![图片描述](https://dn-simplecloud.shiyanlou.com/courses/uid1393219-20200520-1589951400119) ![图片描述](https://dn-simplecloud.shiyanlou.com/courses/uid1393219-20200520-1589951403392) 实验过程： ``` include/linux/sys.h: 添加声明： extern int sys_iam(); extern int sys_whoam(); sys_call_table数组的最后增加两个元素： ..., sys_iam, sys_whoami} ``` ``` include/unistd.h: 添加两个宏 #define __NR_iam 72 #define __NR_whoami 73 声明供用户调用的函数： int iam(const char *name); int whoami(char *name, unsigned int size); ``` ``` kernel/system_call.s: //nr_system_calls = 72 nr_system_calls = 72 ``` ``` kernel/who.c 实现 #define __LIBRARY__ #include #include #include #define MAX 23 char usnm[MAX+1] = { 0 }; int sys_iam(const char* name){ int count = 0; int index = 0; while( get_fs_byte(name+count) != '\0' && count < MAX+1 ) count++; if( count > 23 ) return -EINVAL; for( index =0; index <= count; index++ ) usnm[index]=get_fs_byte( name + index ); return count; } int sys_whoami(char* name,unsigned int size){ int count = 0; int index = 0; while( usnm[count] != '\0'&&count < MAX +1 ) count++; if( count > MAX + 1 ) return -EINVAL; for( index =0; cnt<=result; index++) put_fs_byte( usnm[index], name + index ); return count; } } ``` iam.c实现 ``` #include #define __LIBRARY__ //_syscall1(int,iam,const char *,name)必须的头文件 #include _syscall1(int,iam,const char *,name) //API，EAX = 72，int 0x80，进入内核，根据EAX里面的的系统调用号找到 int iam(const cahr* name); int main(int argc, char *argv[]) { if(argc<=1) { printf("error: please input your name \n "); return -1; } if(iam(argv[1])==-1) { printf("error: length limit is 23. \n"); return -1; } return 0; } ``` whoami.c实现 ``` #include #define __LIBRARY__ #include _syscall2(int,whoami,char *,name,unsigned int,size) int main(int argc, char *argv[]) { char name[24]; if(whoami(name, 23)==-1) { printf("error while reading name..."); return -1; } printf("%s\n", name); return 0; } ``` 问题: 从 Linux 0.11 现在的机制看，它的系统调用最多能传递几个参数？你能想出办法来扩大这个限制吗？用文字简要描述向 Linux 0.11 添加一个系统调用 foo() 的步骤。答：传递三个，EAX 存放系统调用号，剩下三个EBX，ECX，EDX用来存放参数。添加foo的步骤： ``` 第一步： linux/kernel/foo.c #define __LIBRARY__ #include #include #include ... ... void sys_foo() { ... } 第二步：然后，nclude/linux/sys.h: 添加声明： extern void sys_foo(); sys_call_table数组的最后增加两个元素： ..., sys_iam, sys_whoami, sys_foo} 第三步： include/unistd.h: 添加宏 #define __NR_foo 74 声明供用户调用的函数： void foo(); 第四步： kernel/system_call.s: nr_system_calls = 73 第四步：软件层调用： #include #define __LIBRARY__ //_syscall1(int,iam,const char *,name)必须的头文件 #include ... _syscall0(void,foo) //API调用 int main(int argc, char *argv[]) { ... return integer_number; } ```

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