c++ 调用程序接口_添加系统调用（返回文件信息）

终于把操作系统实验一：完成添加一个新的系统调用做完啦！一个什么也不懂的linux小白，从一开始觉得做完实验遥遥无期，甚至在想会不会挂科，到现在终于编译成功，心情也是一波三折。故以此篇记录第一次实验完成的情况。

要求：添加一个新的系统调用,返回指定文件的相关信息，如索引节点编号、硬连接数、文件所有者标识符、文件的字节数和访问方式等。

什么是系统调用

系统调用也叫程序接口，是应用程序请求OS内核完成某功能时的一种过程调用，是用户程序对OS内核功能进行调用的一种手段。

Linux中系统调用的处理过程如下：

1）首先有封装例程设置系统调用功能号和参数，并存到相应寄存器中；

2）执行封装例程中的int 0x80指令，系统产生软中断（或陷入），由中断硬件完成部分现场信息保护：PSW的值和PC寄存器的值；并通过中断向量转向一个中断处理程序system_call()完成其他CPU现场信息的保存：如陷入类型、参数表指针、其他CPU寄存器的值等；

3）使用系统调用功能号查找系统调用入口表，找到相应系统调用的服务例程的入口地址sys_xxx()；

4）执行系统调用服务例程sys_xxx()，完毕后返回执行成功与否以及成功时的执行结果给调用者；恢复被中断进程或新调度进程的CPU现场，返回被中断进程或新调度进程执行。

添加系统调用步骤

1、以下载的linux内核文件夹为base dir，首先在./arch/x86/entry/syscalls/syscall_64.tbl文件中添加一个系统调用号，包括系统调用名称、服务例程入口。

2、在./include/linux/syscalls.h文件中添加服务例程的原型声明。

3、在./kernel/sys.c中添加系统调用服务例程。

添加の源码

系统调用服务例程

SYSCALL_DEFINE6(checkfile,char __user*,filename,void __user*,innode,void __user*,lnnum,void __user*,owner,void __user*,size,void __user*,method){printk("checkfile:n");struct kstat* myfilestat;myfilestat =(struct kstat *) kmalloc(sizeof(struct kstat),GFP_KERNEL);vfs_stat(filename,myfilestat);printk("done,then copy to user...n");unsigned long ker_ino =myfilestat->ino;unsigned long ker_nlink=myfilestat->nlink;unsigned long ker_uid=(myfilestat->uid).val;unsigned long ker_size=myfilestat->size;unsigned short ker_mode=myfilestat->mode;copy_to_user(innode, &ker_ino, sizeof(ker_ino));copy_to_user(lnnum, &ker_nlink, sizeof(ker_nlink));copy_to_user(owner,&ker_uid,sizeof(ker_uid));copy_to_user(size,&ker_size,sizeof(ker_size));copy_to_user(method,&ker_mode,sizeof(ker_mode));return 0;
}

测试程序

#include<stdio.h>
#include<unistd.h>
#include<sys/syscall.h>
#include<stdlib.h>#define _SYSCALL_MYNUM 336
int modecode[16];
void print_short(unsigned short *a){for(int i=0;i<16;i++){int k=(*a)&(1<<(15-i))?1:0;printf("%d",k);if(i==3||i==6||i==9||i==12) printf(" ");modecode[i]=k;}printf("n");
}int main(){unsigned long innode,lnlink,owner,size;unsigned short method;char array[16];char s[50];printf("Want to checkfile for you? Just input the path!");scanf("%s",s);printf("The infomation of the file:%s",s);syscall(_SYSCALL_MYNUM,s,&innode,&lnlink,&owner,&size,&method);printf("n");printf("innode: %lun",innode);printf("lnlink: %lun",lnlink);printf("uid: %lun",owner);printf("size: %lubytesn",size);printf("mode: %un",method);printf("mode(binary code 16bits):");print_short(&method);printf("access:");for(int j=7;j<16;j++){if(j%3==1)printf("%c",modecode[j]==1?'r':'-');else if(j%3==2)printf("%c",modecode[j]==1?'w':'-');else if(j%3==0)printf("%c",modecode[j]==1?'x':'-');}printf("n");printf(" donen");return 0;
}

所使用的内核源码解释

源码版本4.16.3

参考服务例程代码

SYSCALL_DEFINE2(stat64, const char __user *, filename,struct stat64 __user *, statbuf)
{struct kstat stat;int error = vfs_stat(filename, &stat);if (!error)error = cp_new_stat64(&stat, statbuf);return error;
}

正常时，返回0

参考文档

include/linux/stat.h —— kstat结构体定义

struct kstat {u32     result_mask;    /* What fields the user got */umode_t     mode;       //保护模式unsigned int    nlink;  //硬链接数uint32_t    blksize;    /* Preferred I/O size 系统块的大小*/u64     attributes;u64     attributes_mask;
#define KSTAT_ATTR_FS_IOC_FLAGS             (STATX_ATTR_COMPRESSED |            STATX_ATTR_IMMUTABLE |             STATX_ATTR_APPEND |                STATX_ATTR_NODUMP |                STATX_ATTR_ENCRYPTED               )/* Attrs corresponding to FS_*_FL flags */u64     ino;dev_t       dev;                //文件所在设备的iddev_t       rdev;               //设备号 针对设备文件kuid_t      uid;kgid_t      gid;                //group idloff_t      size;               //文件大小struct timespec atime;          //最近存取时间struct timespec mtime;          //最近修改时间struct timespec ctime;struct timespec btime;          /* File creation time */u64     blocks;                 //  文件所占块数
};

include/linux/fs.h

vfs_stat

static inline int vfs_stat(const char __user *filename, struct kstat *stat)
{return vfs_statx(AT_FDCWD, filename, AT_NO_AUTOMOUNT,stat, STATX_BASIC_STATS);
}

AT_FDCWD：相对于当前进程工作空间

AT_NO_AUTOMOUNT: 和挂载点有关，允许收集挂载点的属性。

STATX_BASIC_STATS：查询基本信息

fs/stat.c

vfs_statx

/*** vfs_statx - Get basic and extra attributes by filename* @dfd: A file descriptor representing the base dir for a relative filename表示相对文件名的基本目录的文件描述符* @filename: The name of the file of interest* @flags: Flags to control the query* @stat: The result structure to fill in.* @request_mask: STATX_xxx flags indicating what the caller wants** This function is a wrapper around vfs_getattr().  The main difference is* that it uses a filename and base directory to determine the file location.* Additionally, the use of AT_SYMLINK_NOFOLLOW in flags will prevent a symlink* at the given name from being referenced.* 这个函数是vfs_getattr()的包装器。主要区别在于，它使用文件名和基目录来确定文件的位置。* 此外，在标志中使用AT_SYMLINK_NOFOLLOW将防止给定名称处的符号链接被引用。* 0 will be returned on success, and a -ve error code if unsuccessful.*/
int vfs_statx(int dfd, const char __user *filename, int flags,struct kstat *stat, u32 request_mask)
{struct path path;int error = -EINVAL;//EINVAL表示 无效的参数，即为 invalid argument ，包括参数值、类型或数目无效等unsigned int lookup_flags = LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT;//根据flag 更新需要查找的flag值lookup_flagsif ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |AT_EMPTY_PATH | KSTAT_QUERY_FLAGS)) != 0)return -EINVAL;if (flags & AT_SYMLINK_NOFOLLOW)lookup_flags &= ~LOOKUP_FOLLOW;if (flags & AT_NO_AUTOMOUNT)lookup_flags &= ~LOOKUP_AUTOMOUNT;if (flags & AT_EMPTY_PATH)lookup_flags |= LOOKUP_EMPTY;retry://查找文件所在的patherror = user_path_at(dfd, filename, lookup_flags, &path);if (error)goto out;//根据path得到文件的属性信息并保存到stat中，返回给用户error = vfs_getattr(&path, stat, request_mask, flags);path_put(&path);//在查找到文件属性的正常情况下error 等于零if (retry_estale(error, lookup_flags)) {lookup_flags |= LOOKUP_REVAL;goto retry;}
out:return error;
}
EXPORT_SYMBOL(vfs_statx);

查找文件所在的path，填入path结构体

struct path {struct vfsmount *mnt;struct dentry *dentry;
} __randomize_layout;

vfsmount结构描述的是一个独立文件系统的挂载信息，每个不同挂载点对应一个独立的vfsmount结构，属于同一文件系统的所有目录和文件隶属于同一个vfsmount，该vfsmount结构对应于该文件系统顶层目录，即挂载目录；

dentry：目录的相关信息

详见 https://blog.csdn.net/dongcheng123456789/article/details/100125707

user_path_at

static inline int user_path_at(int dfd, const char __user *name, unsigned flags,struct path *path)
{return user_path_at_empty(dfd, name, flags, path, NULL);
}

user_path_at_empty

int user_path_at_empty(int dfd, const char __user *name, unsigned flags,struct path *path, int *empty)
{return filename_lookup(dfd, getname_flags(name, flags, empty),flags, path, NULL);
}
EXPORT_SYMBOL(user_path_at_empty);

filename_lookup

static int filename_lookup(int dfd, struct filename *name, unsigned flags,struct path *path, struct path *root)
{int retval;struct nameidata nd;if (IS_ERR(name))return PTR_ERR(name);if (unlikely(root)) {nd.root = *root;flags |= LOOKUP_ROOT;}set_nameidata(&nd, dfd, name);retval = path_lookupat(&nd, flags | LOOKUP_RCU, path);if (unlikely(retval == -ECHILD))retval = path_lookupat(&nd, flags, path);if (unlikely(retval == -ESTALE))retval = path_lookupat(&nd, flags | LOOKUP_REVAL, path);if (likely(!retval))audit_inode(name, path->dentry, flags & LOOKUP_PARENT);restore_nameidata();putname(name);return retval;
}

fs/stat.c

vfs_getattr()

/** vfs_getattr - Get the enhanced basic attributes of a file* @path: The file of interest* @stat: Where to return the statistics* @request_mask: STATX_xxx flags indicating what the caller wants* @query_flags: Query mode (KSTAT_QUERY_FLAGS)** Ask the filesystem for a file's attributes.  The caller must indicate in* request_mask and query_flags to indicate what they want.* 向文件系统询问文件的属性。调用者必须在request_mask和query_flags中指出他们需要什么。** If the file is remote, the filesystem can be forced to update the attributes* from the backing store by passing AT_STATX_FORCE_SYNC in query_flags or can* suppress the update by passing AT_STATX_DONT_SYNC.** Bits must have been set in request_mask to indicate which attributes the* caller wants retrieving.  Any such attribute not requested may be returned* anyway, but the value may be approximate, and, if remote, may not have been* synchronised with the server.** 0 will be returned on success, and a -ve error code if unsuccessful.*/
int vfs_getattr(const struct path *path, struct kstat *stat,u32 request_mask, unsigned int query_flags)
{int retval;retval = security_inode_getattr(path);if (retval)return retval;return vfs_getattr_nosec(path, stat, request_mask, query_flags);
}
EXPORT_SYMBOL(vfs_getattr);

security_inode_getattr

int security_inode_getattr(const struct path *path)
{if (unlikely(IS_PRIVATE(d_backing_inode(path->dentry))))return 0;return call_int_hook(inode_getattr, 0, path);
}#define call_int_hook(FUNC, IRC, ...) ({                           int RC = IRC;                                                  do {                                                            struct security_hook_list *P;                               list_for_each_entry(P, &security_hook_heads.FUNC, list) {   RC = P->hook.FUNC(__VA_ARGS__);                         if (RC != 0)                                           break;                                              }                                                           } while (0);                                                    RC;
})

vfs_getattr_nosec

d_backing_inode 函数会获得 inode

/*** vfs_getattr_nosec - getattr without security checks* @path: file to get attributes from* @stat: structure to return attributes in* @request_mask: STATX_xxx flags indicating what the caller wants* @query_flags: Query mode (KSTAT_QUERY_FLAGS)** Get attributes without calling security_inode_getattr.** Currently the only caller other than vfs_getattr is internal to the* filehandle lookup code, which uses only the inode number and returns no* attributes to any user.  Any other code probably wants vfs_getattr.*/
int vfs_getattr_nosec(const struct path *path, struct kstat *stat,u32 request_mask, unsigned int query_flags)
{//得到path 表示文件的inodestruct inode *inode = d_backing_inode(path->dentry);//清零stat，因为stat 会保存扩展属性返回给用户memset(stat, 0, sizeof(*stat));stat->result_mask |= STATX_BASIC_STATS;request_mask &= STATX_ALL;query_flags &= KSTAT_QUERY_FLAGS;//若文件的扩展属性可以保存到inode上，则通过inode 提供的函数返回给用户if (inode->i_op->getattr)return inode->i_op->getattr(path, stat, request_mask,query_flags);//如果inode上没有保存，则调用文件系统提供的通用函数获得文件的扩展属性generic_fillattr(inode, stat);return 0;
}
EXPORT_SYMBOL(vfs_getattr_nosec);

generic_fillattr

/*** generic_fillattr - Fill in the basic attributes from the inode struct* @inode: Inode to use as the source* @stat: Where to fill in the attributes** Fill in the basic attributes in the kstat structure from data that's to be* found on the VFS inode structure.  This is the default if no getattr inode* operation is supplied.*/
void generic_fillattr(struct inode *inode, struct kstat *stat)
{stat->dev = inode->i_sb->s_dev;stat->ino = inode->i_ino;stat->mode = inode->i_mode;stat->nlink = inode->i_nlink;stat->uid = inode->i_uid;stat->gid = inode->i_gid;stat->rdev = inode->i_rdev;stat->size = i_size_read(inode);stat->atime = inode->i_atime;stat->mtime = inode->i_mtime;stat->ctime = inode->i_ctime;stat->blksize = i_blocksize(inode);stat->blocks = inode->i_blocks;if (IS_NOATIME(inode))stat->result_mask &= ~STATX_ATIME;if (IS_AUTOMOUNT(inode))stat->attributes |= STATX_ATTR_AUTOMOUNT;
}
EXPORT_SYMBOL(generic_fillattr);