----sys_open:linux\fs\open.c----asmlinkage int sys_open(const char * filename,int flags,int mode){char * tmp;int fd, error;//获取打开文件描述符(进程打开文件结构指针数组的下标)fd = get_unused_fd();if (fd < 0)return fd;//从用户空间拷贝文件名到内核空间(分配一物理页),tmp指向内核空间的文件名字符串error = getname(filename, &tmp);if (!error) {//打开文件,current->files->fd[fd]指向打开的文件结构error = do_open(tmp,flags,mode, fd); putname(tmp);if (!error)return fd;}put_unused_fd(fd);return error;}----do_open:linux\fs\open.c----/* * Note that while the flag value (low two bits) for sys_open means: * 00 - read-only * 01 - write-only * 10 - read-write * 11 - special * it is changed into * 00 - no permissions needed * 01 - read-permission * 10 - write-permission * 11 - read-write * for the internal routines (ie open_namei()/follow_link() etc). 00 is * used by symlinks. */static int do_open(const char * filename,int flags,int mode, int fd){struct inode * inode;struct file * f;int flag,error;//从first_file链表获取空闲的file结构，必要时调用grow_files()填充该链表f = get_empty_filp();if (!f)return -ENFILE;f->f_flags = flag = flags;f->f_mode = (flag+1) & O_ACCMODE;if (f->f_mode)flag++;if (flag & O_TRUNC)flag |= 2;//通过文件路径名搜寻获取文件的i节点,由inode指向error = open_namei(filename,flag,mode,&inode,NULL);if (error)goto cleanup_file;//如果以写方式打开文件，检查是否有拒绝写的文件映射内存区域(mpnt->vm_flags & VM_DENYWRITE),有则出错;//否则inode->i_writecount++if (f->f_mode & FMODE_WRITE) {error = get_write_access(inode);if (error)goto cleanup_inode;}//file结构初始化f->f_inode = inode;f->f_pos = 0;f->f_reada = 0;f->f_op = NULL;if (inode->i_op)f->f_op = inode->i_op->default_file_ops;if (f->f_op && f->f_op->open) {error = f->f_op->open(inode,f);if (error)goto cleanup_all;}f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);//设置进程的打开文件指针数组项(fd为索引)current->files->fd[fd] = f;return 0;cleanup_all:if (f->f_mode & FMODE_WRITE)put_write_access(inode);cleanup_inode:iput(inode);cleanup_file:f->f_count--;return error;}----open_namei:linux\fs\open.c----/* * open_namei() * * namei for open - this is in fact almost the whole open-routine. * * Note that the low bits of "flag" aren't the same as in the open * system call - they are 00 - no permissions needed * 01 - read permission needed * 10 - write permission needed * 11 - read/write permissions needed * which is a lot more logical, and also allows the "no perm" needed * for symlinks (where the permissions are checked later). */int open_namei(const char * pathname, int flag, int mode, struct inode ** res_inode, struct inode * base){const char * basename;int namelen,error;struct inode * dir, *inode;mode &= S_IALLUGO & ~current->fs->umask;mode |= S_IFREG;error = dir_namei(pathname, &namelen, &basename, base, &dir);if (error)return error;if (!namelen) {/* special case: '/usr/' etc */if (flag & 2) {iput(dir);return -EISDIR;}/* thanks to Paul Pluzhnikov for noticing this was missing.. */if ((error = permission(dir,ACC_MODE(flag))) != 0) {iput(dir);return error;}*res_inode=dir;return 0;}dir->i_count++;/* lookup eats the dir */if (flag & O_CREAT) {down(&dir->i_sem);error = lookup(dir, basename, namelen, &inode);if (!error) {if (flag & O_EXCL) {iput(inode);error = -EEXIST;}} else if (IS_RDONLY(dir))error = -EROFS;else if (!dir->i_op || !dir->i_op->create)error = -EACCES;else if ((error = permission(dir,MAY_WRITE | MAY_EXEC)) != 0); /* error is already set! */else {dir->i_count++;/* create eats the dir */if (dir->i_sb && dir->i_sb->dq_op)dir->i_sb->dq_op->initialize(dir, -1);error = dir->i_op->create(dir, basename, namelen, mode, res_inode);up(&dir->i_sem);iput(dir);return error;}up(&dir->i_sem);} elseerror = lookup(dir, basename, namelen, &inode);if (error) {iput(dir);return error;}error = follow_link(dir,inode,flag,mode,&inode);if (error)return error;if (S_ISDIR(inode->i_mode) && (flag & 2)) {iput(inode);return -EISDIR;}if ((error = permission(inode,ACC_MODE(flag))) != 0) {iput(inode);return error;}if (S_ISFIFO(inode->i_mode) || S_ISSOCK(inode->i_mode)) {/** 2-Feb-1995 Bruce Perens <Bruce@Pixar.com>* Allow opens of Unix domain sockets and FIFOs for write on* read-only filesystems. Their data does not live on the disk.** If there was something like IS_NODEV(inode) for* pipes and/or sockets I'd check it here.*/ flag &= ~O_TRUNC;}else if (S_ISBLK(inode->i_mode) || S_ISCHR(inode->i_mode)) {if (IS_NODEV(inode)) {iput(inode);return -EACCES;}flag &= ~O_TRUNC;} else {if (IS_RDONLY(inode) && (flag & 2)) {iput(inode);return -EROFS;}}/** An append-only file must be opened in append mode for writing* Additionally, we must disallow O_TRUNC -- cevans*/if (IS_APPEND(inode) && (((flag & FMODE_WRITE) && !(flag & O_APPEND)) || (flag & O_TRUNC))) {iput(inode);return -EPERM;}if (flag & O_TRUNC) {if ((error = get_write_access(inode))) {iput(inode);return error;}/** Refuse to truncate files with mandatory locks held on them*/error = locks_verify_locked(inode);if (error) {iput(inode);return error;}if (inode->i_sb && inode->i_sb->dq_op)inode->i_sb->dq_op->initialize(inode, -1);error = do_truncate(inode, 0);put_write_access(inode);if (error) {iput(inode);return error;}} elseif (flag & FMODE_WRITE)if (inode->i_sb && inode->i_sb->dq_op)inode->i_sb->dq_op->initialize(inode, -1);*res_inode = inode;return 0;}
