+ * Attempt to atomically look up, create and open a file from a negative
+ * dentry.
+ *
+ * Returns 0 if successful. The file will have been created and attached to
+ * @file by the filesystem calling finish_open().
+ *
+ * Returns 1 if the file was looked up only or didn't need creating. The
+ * caller will need to perform the open themselves. @path will have been
+ * updated to point to the new dentry. This may be negative.
+ *
+ * Returns an error code otherwise.
+ */
+static int atomic_open(struct nameidata *nd, struct dentry *dentry,
+ struct path *path, struct file *file,
+ const struct open_flags *op,
+ bool *want_write, bool need_lookup,
+ int *opened)
+{
+ struct inode *dir = nd->path.dentry->d_inode;
+ unsigned open_flag = open_to_namei_flags(op->open_flag);
+ umode_t mode;
+ int error;
+ int acc_mode;
+ int create_error = 0;
+ struct dentry *const DENTRY_NOT_SET = (void *) -1UL;
+
+ BUG_ON(dentry->d_inode);
+
+ /* Don't create child dentry for a dead directory. */
+ if (unlikely(IS_DEADDIR(dir))) {
+ error = -ENOENT;
+ goto out;
+ }
+
+ mode = op->mode & S_IALLUGO;
+ if ((open_flag & O_CREAT) && !IS_POSIXACL(dir))
+ mode &= ~current_umask();
+
+ if (open_flag & O_EXCL) {
+ open_flag &= ~O_TRUNC;
+ *opened |= FILE_CREATED;
+ }
+
+ /*
+ * Checking write permission is tricky, bacuse we don't know if we are
+ * going to actually need it: O_CREAT opens should work as long as the
+ * file exists. But checking existence breaks atomicity. The trick is
+ * to check access and if not granted clear O_CREAT from the flags.
+ *
+ * Another problem is returing the "right" error value (e.g. for an
+ * O_EXCL open we want to return EEXIST not EROFS).
+ */
+ if ((open_flag & (O_CREAT | O_TRUNC)) ||
+ (open_flag & O_ACCMODE) != O_RDONLY) {
+ error = mnt_want_write(nd->path.mnt);
+ if (!error) {
+ *want_write = true;
+ } else if (!(open_flag & O_CREAT)) {
+ /*
+ * No O_CREATE -> atomicity not a requirement -> fall
+ * back to lookup + open
+ */
+ goto no_open;
+ } else if (open_flag & (O_EXCL | O_TRUNC)) {
+ /* Fall back and fail with the right error */
+ create_error = error;
+ goto no_open;
+ } else {
+ /* No side effects, safe to clear O_CREAT */
+ create_error = error;
+ open_flag &= ~O_CREAT;
+ }
+ }
+
+ if (open_flag & O_CREAT) {
+ error = may_o_create(&nd->path, dentry, op->mode);
+ if (error) {
+ create_error = error;
+ if (open_flag & O_EXCL)
+ goto no_open;
+ open_flag &= ~O_CREAT;
+ }
+ }
+
+ if (nd->flags & LOOKUP_DIRECTORY)
+ open_flag |= O_DIRECTORY;
+
+ file->f_path.dentry = DENTRY_NOT_SET;
+ file->f_path.mnt = nd->path.mnt;
+ error = dir->i_op->atomic_open(dir, dentry, file, open_flag, mode,
+ opened);
+ if (error < 0) {
+ if (create_error && error == -ENOENT)
+ error = create_error;
+ goto out;
+ }
+
+ acc_mode = op->acc_mode;
+ if (*opened & FILE_CREATED) {
+ fsnotify_create(dir, dentry);
+ acc_mode = MAY_OPEN;
+ }
+
+ if (error) { /* returned 1, that is */
+ if (WARN_ON(file->f_path.dentry == DENTRY_NOT_SET)) {
+ error = -EIO;
+ goto out;
+ }
+ if (file->f_path.dentry) {
+ dput(dentry);
+ dentry = file->f_path.dentry;
+ }
+ goto looked_up;
+ }
+
+ /*
+ * We didn't have the inode before the open, so check open permission
+ * here.
+ */
+ error = may_open(&file->f_path, acc_mode, open_flag);
+ if (error)
+ fput(file);
+
+out:
+ dput(dentry);
+ return error;
+
+no_open:
+ if (need_lookup) {
+ dentry = lookup_real(dir, dentry, nd->flags);
+ if (IS_ERR(dentry))
+ return PTR_ERR(dentry);
+
+ if (create_error) {
+ int open_flag = op->open_flag;
+
+ error = create_error;
+ if ((open_flag & O_EXCL)) {
+ if (!dentry->d_inode)
+ goto out;
+ } else if (!dentry->d_inode) {
+ goto out;
+ } else if ((open_flag & O_TRUNC) &&
+ S_ISREG(dentry->d_inode->i_mode)) {
+ goto out;
+ }
+ /* will fail later, go on to get the right error */
+ }
+ }
+looked_up:
+ path->dentry = dentry;
+ path->mnt = nd->path.mnt;
+ return 1;
+}
+
+/*
+ * Look up and maybe create and open the last component.
+ *
+ * Must be called with i_mutex held on parent.
+ *
+ * Returns 0 if the file was successfully atomically created (if necessary) and
+ * opened. In this case the file will be returned attached to @file.
+ *
+ * Returns 1 if the file was not completely opened at this time, though lookups
+ * and creations will have been performed and the dentry returned in @path will
+ * be positive upon return if O_CREAT was specified. If O_CREAT wasn't
+ * specified then a negative dentry may be returned.
+ *
+ * An error code is returned otherwise.
+ *
+ * FILE_CREATE will be set in @*opened if the dentry was created and will be
+ * cleared otherwise prior to returning.