Linux虚拟文件系统--open()
open()系统调用用来打开一个文件，本文就VFS层，对open系统调用的过程进行一个简单的分析。
SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode){long ret;if (force_o_largefile())flags |= O_LARGEFILE;ret = do_sys_open(AT_FDCWD, filename, flags, mode);/* avoid REGPARM breakage on x86: */asmlinkage_protect(3, ret, filename, flags, mode);return ret;}force_o_largefile()用来判断系统是否为32位的，如果不是32位，也就是说为64位，则将O_LARGEFILE置位，主体工作由do_sys_open()来做
long do_sys_open(int dfd, const char __user *filename, int flags, int mode){char *tmp = getname(filename);//拷贝文件名字符串到内核空间int fd = PTR_ERR(tmp);if (!IS_ERR(tmp)) {fd = get_unused_fd_flags(flags);//为文件分配一个文件描述符if (fd >= 0) {//实际的OPEN操作处理struct file *f = do_filp_open(dfd, tmp, flags, mode, 0);if (IS_ERR(f)) {put_unused_fd(fd);fd = PTR_ERR(f);} else {fsnotify_open(f->f_path.dentry);fd_install(fd, f);}}putname(tmp);}return fd;}
open操作是特定于某个进程进行的，因此涉及到了VFS中特定于进程的结构，这里简单的介绍下
struct files_struct { /* * read mostly part */atomic_t count;struct fdtable *fdt;struct fdtable fdtab; /* * written part on a separate cache line in SMP */spinlock_t file_lock ____cacheline_aligned_in_smp;int next_fd;struct embedded_fd_set close_on_exec_init;struct embedded_fd_set open_fds_init;struct file * fd_array[NR_OPEN_DEFAULT];};count表示共享该结构的进程数
fdtable是该进程的文件描述符数组
fdt指向fdtable
next_fd表示最大文件描述符号+1
embedded_fd_set是一个位图结构，用来标记文件描述符，close_on_exec_init用来标记那些执行exec时要关闭的文件的文件描述符，open_fds_init用来标记已经分配出去了的文件描述符
fd_array用来存储进程打开的文件的struct file指针
do_sys_open()的一个重要任务就是调用get_unused_fd_flags()为即将打开的文件分配一个文件描述符
#define get_unused_fd_flags(flags) alloc_fd(0, (flags))
int alloc_fd(unsigned start, unsigned flags){struct files_struct *files = current->files;//获取当前进程的files_structunsigned int fd;int error;struct fdtable *fdt;spin_lock(&files->file_lock);repeat:fdt = files_fdtable(files);//获取进程的fdtablefd = start;if (fd < files->next_fd)fd = files->next_fd;if (fd < fdt->max_fds)fd = find_next_zero_bit(fdt->open_fds->fds_bits, fdt->max_fds, fd);//从位图中获取一个空闲位error = expand_files(files, fd);//这里根据需要扩充文件描述符数组if (error < 0)goto out;/* * If we needed to expand the fs array we * might have blocked - try again. */if (error)//之前进行了扩充操作，重新进行一次空闲bit的搜索goto repeat;if (start <= files->next_fd)files->next_fd = fd + 1;FD_SET(fd, fdt->open_fds);//在open_fds的位图上置位if (flags & O_CLOEXEC)//如果设定了O_CLOEXEC，则在close_on_exec位图上将相应位置位FD_SET(fd, fdt->close_on_exec);elseFD_CLR(fd, fdt->close_on_exec);error = fd;#if 1/* Sanity check */if (rcu_dereference(fdt->fd[fd]) != NULL) {printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);rcu_assign_pointer(fdt->fd[fd], NULL);}#endifout:spin_unlock(&files->file_lock);return error;}
int expand_files(struct files_struct *files, int nr){struct fdtable *fdt;fdt = files_fdtable(files);/* * N.B. For clone tasks sharing a files structure, this test * will limit the total number of files that can be opened. */ /*如果nr大于进程允许的最大打开文件数，则返回错误*/if (nr >= current->signal->rlim[RLIMIT_NOFILE].rlim_cur)return -EMFILE;/*nr小于最大文件描述符，则不用进行fdtable的扩展，直接返回*/if (nr < fdt->max_fds)return 0;/*扩展的话不能超过sysctl_nr_opend的上限*/if (nr >= sysctl_nr_open)return -EMFILE;/* 到这里表示确实需要进行扩充，进行实际的扩展操作 */return expand_fdtable(files, nr);}
实际的扩充操作：
static int expand_fdtable(struct files_struct *files, int nr)__releases(files->file_lock)__acquires(files->file_lock){struct fdtable *new_fdt, *cur_fdt;spin_unlock(&files->file_lock);new_fdt = alloc_fdtable(nr);//根据nr重新创建一个新的fdtablespin_lock(&files->file_lock);if (!new_fdt)return -ENOMEM;/* * extremely unlikely race - sysctl_nr_open decreased between the check in * caller and alloc_fdtable(). Cheaper to catch it here... */ /*这里为了防止因为竞争，在alloc_fdtable调用之前systl_nr_open减小了新创建的fdtable小于nr*/if (unlikely(new_fdt->max_fds <= nr)) {free_fdarr(new_fdt);free_fdset(new_fdt);kfree(new_fdt);return -EMFILE;}/* * Check again since another task may have expanded the fd table while * we dropped the lock */cur_fdt = files_fdtable(files);//获取旧的fdtableif (nr >= cur_fdt->max_fds) {//新的nr必须大于旧的fdtable的大小/* Continue as planned */copy_fdtable(new_fdt, cur_fdt);//将旧的fdtable中的内容拷贝至新的fdtablercu_assign_pointer(files->fdt, new_fdt);//用新的fdtable替换旧的fdtableif (cur_fdt->max_fds > NR_OPEN_DEFAULT)free_fdtable(cur_fdt);//释放旧的fdtable} else {/* Somebody else expanded, so undo our attempt */free_fdarr(new_fdt);free_fdset(new_fdt);kfree(new_fdt);}return 1;}
到此为止，分配新的fd的工作完成，如果分配fd成功，接下来do_sys_open()就要通过do_filp_open()函数查找文件并执行相应的打开操作
do_filp_open的工作针对两种情况进行：
1.flag中未标识O_CREAT，也就是只进行单纯的搜索打开，如果没有搜索到目标文件的话，不会进行创建，这种情况处理起来比较简单，主要工作就是通过路径解析来查找文件，查找到了的话再根据文件系统定义的open方式进行打开
2.flag中标识了O_CREAT，也就是说如果没找到目标文件要进行创建。这种情况要先查找目标文件的父目录(通过将LOOKUP_PARENT标识置位然后进行路径解析来实现)，因为假如没查找到目标文件的话，创建工作需要在父目录下完成；然后再查找最后一个文件分量，也就是目标文件，并进行打开操作，其中涉及到的许多部分在前面几篇文章中也都已经分析过了
struct file *do_filp_open(int dfd, const char *pathname,int open_flag, int mode, int acc_mode){struct file *filp;struct nameidata nd;int error;struct path path;struct dentry *dir;int count = 0;int will_write;int flag = open_to_namei_flags(open_flag);if (!acc_mode)acc_mode = MAY_OPEN | ACC_MODE(flag);/* O_TRUNC implies we need access checks for write permissions */if (flag & O_TRUNC)acc_mode |= MAY_WRITE;/* Allow the LSM permission hook to distinguish append access from general write access. */if (flag & O_APPEND)acc_mode |= MAY_APPEND;/* * The simplest case - just a plain lookup. *//*如果没有设置O_CREAT，则在未找到文件的情况下不用创建文件，直接通过查找来打开文件*/if (!(flag & O_CREAT)) {error = path_lookup_open(dfd, pathname, lookup_flags(flag), &nd, flag);if (error)return ERR_PTR(error);goto ok; //成功查找到了目标文件的话，就跳转到ok去执行后续操作}/* * Create - we need to know the parent. */ /*如果需要creat,那么就要知道目标文件的父目录，因此需要设置LOOKUP_PARENT标识*/error = path_init(dfd, pathname, LOOKUP_PARENT, &nd);if (error)return ERR_PTR(error);/*进行路径名的解析，父目录将保存到nd中*/error = path_walk(pathname, &nd);if (error) {if (nd.root.mnt)path_put(&nd.root);return ERR_PTR(error);}if (unlikely(!audit_dummy_context()))audit_inode(pathname, nd.path.dentry);/* * We have the parent and last component. First of all, check * that we are not asked to creat(2) an obvious directory - that * will not do. */error = -EISDIR;/*这里要先保证路径名的最后一个分量是普通文件名(不为.和..)，并且长度不为0*/if (nd.last_type != LAST_NORM || nd.last.name[nd.last.len])goto exit_parent;error = -ENFILE;filp = get_empty_filp();//分配一个struct fileif (filp == NULL)goto exit_parent;/*将打开文件的信息保存在nd.intent中*/nd.intent.open.file = filp;nd.intent.open.flags = flag;nd.intent.open.create_mode = mode;dir = nd.path.dentry;//获取父目录nd.flags &= ~LOOKUP_PARENT;//取消LOOKUP_PARENT标识nd.flags |= LOOKUP_CREATE | LOOKUP_OPEN;//设置CREATE和OPEN标识if (flag & O_EXCL)nd.flags |= LOOKUP_EXCL;mutex_lock(&dir->d_inode->i_mutex);//lookup_hash进行最终分量的查找，先查找dentry缓存，没找到的话再通过特定于文件系统的lookup方式从磁盘查找path.dentry = lookup_hash(&nd);path.mnt = nd.path.mnt;do_last:error = PTR_ERR(path.dentry);//检查目标dentry是否有效if (IS_ERR(path.dentry)) {mutex_unlock(&dir->d_inode->i_mutex);goto exit;}if (IS_ERR(nd.intent.open.file)) {//检查file是否有效error = PTR_ERR(nd.intent.open.file);goto exit_mutex_unlock;}/* Negative dentry, just create the file */if (!path.dentry->d_inode) {//dentry没有对应上inode，创建之，可能的情况就是该文件被删除了/* * This write is needed to ensure that a * ro->rw transition does not occur between * the time when the file is created and when * a permanent write count is taken through * the 'struct file' in nameidata_to_filp(). */error = mnt_want_write(nd.path.mnt);if (error)goto exit_mutex_unlock;/*__open_namei_create将会调用到父目录所属文件系统中定义的create方式创建文件*/error = __open_namei_create(&nd, &path, flag, mode);if (error) {mnt_drop_write(nd.path.mnt);goto exit;}/*nameidata_to_filp将会调用目标文件的inode对应的open函数进行打开操作*/filp = nameidata_to_filp(&nd, open_flag);if (IS_ERR(filp))ima_counts_put(&nd.path, acc_mode & (MAY_READ | MAY_WRITE | MAY_EXEC));mnt_drop_write(nd.path.mnt);if (nd.root.mnt)path_put(&nd.root);return filp;}/* * 下面的情况对应目标文件存在 */mutex_unlock(&dir->d_inode->i_mutex);audit_inode(pathname, path.dentry);error = -EEXIST;if (flag & O_EXCL)goto exit_dput;/*下面要做一些必要的检查*/if (__follow_mount(&path)) {//检测目标对象上是否挂载了文件系统error = -ELOOP;if (flag & O_NOFOLLOW)goto exit_dput;}error = -ENOENT;if (!path.dentry->d_inode)//检测目标对象的inode是否存在goto exit_dput;if (path.dentry->d_inode->i_op->follow_link)//检测目标对象是否为链接文件goto do_link;/*检查OK，将path保存至nd*/path_to_nameidata(&path, &nd);error = -EISDIR;if (path.dentry->d_inode && S_ISDIR(path.dentry->d_inode->i_mode))goto exit;ok:/* * Consider: * 1. may_open() truncates a file * 2. a rw->ro mount transition occurs * 3. nameidata_to_filp() fails due to * the ro mount. * That would be inconsistent, and should * be avoided. Taking this mnt write here * ensures that (2) can not occur. */will_write = open_will_write_to_fs(flag, nd.path.dentry->d_inode);if (will_write) {error = mnt_want_write(nd.path.mnt);if (error)goto exit;}/*may_open()会做一些检测*/error = may_open(&nd.path, acc_mode, flag);if (error) {if (will_write)mnt_drop_write(nd.path.mnt);goto exit;}//执行文件系统定义的打开操作，并保存信息至filpfilp = nameidata_to_filp(&nd, open_flag);if (IS_ERR(filp))ima_counts_put(&nd.path, acc_mode & (MAY_READ | MAY_WRITE | MAY_EXEC));/* * It is now safe to drop the mnt write * because the filp has had a write taken * on its behalf. */if (will_write)mnt_drop_write(nd.path.mnt);if (nd.root.mnt)path_put(&nd.root);return filp;exit_mutex_unlock:mutex_unlock(&dir->d_inode->i_mutex);exit_dput:path_put_conditional(&path, &nd);exit:if (!IS_ERR(nd.intent.open.file))release_open_intent(&nd);exit_parent:if (nd.root.mnt)path_put(&nd.root);path_put(&nd.path);return ERR_PTR(error);do_link://目标文件为符号链接的处理，前文已经分析过error = -ELOOP;if (flag & O_NOFOLLOW)goto exit_dput;/* * This is subtle. Instead of calling do_follow_link() we do the * thing by hands. The reason is that this way we have zero link_count * and path_walk() (called from ->follow_link) honoring LOOKUP_PARENT. * After that we have the parent and last component, i.e. * we are in the same situation as after the first path_walk(). * Well, almost - if the last component is normal we get its copy * stored in nd->last.name and we will have to putname() it when we * are done. Procfs-like symlinks just set LAST_BIND. */nd.flags |= LOOKUP_PARENT;error = security_inode_follow_link(path.dentry, &nd);if (error)goto exit_dput;error = __do_follow_link(&path, &nd);if (error) {/* Does someone understand code flow here? Or it is only * me so stupid? Anathema to whoever designed this non-sense * with "intent.open". */release_open_intent(&nd);if (nd.root.mnt)path_put(&nd.root);return ERR_PTR(error);}nd.flags &= ~LOOKUP_PARENT;if (nd.last_type == LAST_BIND)goto ok;error = -EISDIR;if (nd.last_type != LAST_NORM)goto exit;if (nd.last.name[nd.last.len]) {__putname(nd.last.name);goto exit;}error = -ELOOP;if (count++==32) {__putname(nd.last.name);goto exit;}dir = nd.path.dentry;mutex_lock(&dir->d_inode->i_mutex);path.dentry = lookup_hash(&nd);path.mnt = nd.path.mnt;__putname(nd.last.name);goto do_last;}