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

"font-size:12px;">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

"font-size:12px;">#define get_unused_fd_flags(flags) alloc_fd(0, (flags))

"font-size:12px;">int alloc_fd(unsigned start, unsigned flags)
{
    struct files_struct *files = current->files;//获取当前进程的files_struct
    unsigned int fd;
    int error;
    struct fdtable *fdt;

    spin_lock(&files->file_lock);
repeat:
    fdt = files_fdtable(files);//获取进程的fdtable
    fd = 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);
    else
        FD_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);
    }
#endif

out:
    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：

"font-size:12px;">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fdtable
    spin_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fdtable
    if (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fdtable
        rcu_assign_pointer(files->fdt, new_fdt);//用新的fdtable替换旧的fdtable
        if (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

"font-size:12px;">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 file
    if (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filp
    filp = 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;
}