[O_TMPFILE] it's still short a few helpers, but infrastructure should be OK now...

Signed-off-by: Al Viro <[e-mail]>

arch/alpha/include/uapi/asm/fcntl.h

···
 #define O_SYNC		(__O_SYNC|O_DSYNC)
 
 #define O_PATH		040000000
+#define O_TMPFILE	0100000000
 
 #define F_GETLK		7
 #define F_SETLK		8

arch/parisc/include/uapi/asm/fcntl.h

···
 #define O_INVISIBLE	004000000 /* invisible I/O, for DMAPI/XDSM */
 
 #define O_PATH		020000000
+#define O_TMPFILE	040000000
 
 #define F_GETLK64	8
 #define F_SETLK64	9

arch/sparc/include/uapi/asm/fcntl.h

···
 #define O_SYNC		(__O_SYNC|O_DSYNC)
 
 #define O_PATH		0x1000000
+#define O_TMPFILE	0x2000000
 
 #define F_GETOWN	5	/*  for sockets. */
 #define F_SETOWN	6	/*  for sockets. */

fs/dcache.c

···
 	goto again;
 }
 
+void d_tmpfile(struct dentry *dentry, struct inode *inode)
+{
+	inode_dec_link_count(inode);
+	BUG_ON(dentry->d_name.name != dentry->d_iname ||
+		!hlist_unhashed(&dentry->d_alias) ||
+		!d_unlinked(dentry));
+	spin_lock(&dentry->d_parent->d_lock);
+	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
+	dentry->d_name.len = sprintf(dentry->d_iname, "#%llu",
+				(unsigned long long)inode->i_ino);
+	spin_unlock(&dentry->d_lock);
+	spin_unlock(&dentry->d_parent->d_lock);
+	d_instantiate(dentry, inode);
+}
+EXPORT_SYMBOL(d_tmpfile);
+
 /**
  * find_inode_number - check for dentry with name
  * @dir: directory to check

fs/ext2/namei.c

···
 	return ext2_add_nondir(dentry, inode);
 }
 
+static int ext2_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
+{
+	struct inode *inode = ext2_new_inode(dir, mode, NULL);
+	if (IS_ERR(inode))
+		return PTR_ERR(inode);
+
+	inode->i_op = &ext2_file_inode_operations;
+	if (ext2_use_xip(inode->i_sb)) {
+		inode->i_mapping->a_ops = &ext2_aops_xip;
+		inode->i_fop = &ext2_xip_file_operations;
+	} else if (test_opt(inode->i_sb, NOBH)) {
+		inode->i_mapping->a_ops = &ext2_nobh_aops;
+		inode->i_fop = &ext2_file_operations;
+	} else {
+		inode->i_mapping->a_ops = &ext2_aops;
+		inode->i_fop = &ext2_file_operations;
+	}
+	mark_inode_dirty(inode);
+	d_tmpfile(dentry, inode);
+	unlock_new_inode(inode);
+	return 0;
+}
+
 static int ext2_mknod (struct inode * dir, struct dentry *dentry, umode_t mode, dev_t rdev)
 {
 	struct inode * inode;
···
 #endif
 	.setattr	= ext2_setattr,
 	.get_acl	= ext2_get_acl,
+	.tmpfile	= ext2_tmpfile,
 };
 
 const struct inode_operations ext2_special_inode_operations = {

fs/minix/namei.c

···
 	return error;
 }
 
+static int minix_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
+{
+	int error;
+	struct inode *inode = minix_new_inode(dir, mode, &error);
+	if (inode) {
+		minix_set_inode(inode, 0);
+		mark_inode_dirty(inode);
+		d_tmpfile(dentry, inode);
+	}
+	return error;
+}
+
 static int minix_create(struct inode *dir, struct dentry *dentry, umode_t mode,
 		bool excl)
 {
···
 	.mknod		= minix_mknod,
 	.rename		= minix_rename,
 	.getattr	= minix_getattr,
+	.tmpfile	= minix_tmpfile,
 };

fs/namei.c

···
 	goto retry_lookup;
 }
 
+static int do_tmpfile(int dfd, struct filename *pathname,
+		struct nameidata *nd, int flags,
+		const struct open_flags *op,
+		struct file *file, int *opened)
+{
+	static const struct qstr name = QSTR_INIT("/", 1);
+	struct dentry *dentry, *child;
+	struct inode *dir;
+	int error = path_lookupat(dfd, pathname->name,
+				  flags | LOOKUP_DIRECTORY, nd);
+	if (unlikely(error))
+		return error;
+	error = mnt_want_write(nd->path.mnt);
+	if (unlikely(error))
+		goto out;
+	/* we want directory to be writable */
+	error = inode_permission(nd->inode, MAY_WRITE | MAY_EXEC);
+	if (error)
+		goto out2;
+	dentry = nd->path.dentry;
+	dir = dentry->d_inode;
+	if (!dir->i_op->tmpfile) {
+		error = -EOPNOTSUPP;
+		goto out2;
+	}
+	child = d_alloc(dentry, &name);
+	if (unlikely(!child)) {
+		error = -ENOMEM;
+		goto out2;
+	}
+	nd->flags &= ~LOOKUP_DIRECTORY;
+	nd->flags |= op->intent;
+	dput(nd->path.dentry);
+	nd->path.dentry = child;
+	error = dir->i_op->tmpfile(dir, nd->path.dentry, op->mode);
+	if (error)
+		goto out2;
+	audit_inode(pathname, nd->path.dentry, 0);
+	error = may_open(&nd->path, op->acc_mode, op->open_flag);
+	if (error)
+		goto out2;
+	file->f_path.mnt = nd->path.mnt;
+	error = finish_open(file, nd->path.dentry, NULL, opened);
+	if (error)
+		goto out2;
+	error = open_check_o_direct(file);
+	if (error)
+		fput(file);
+out2:
+	mnt_drop_write(nd->path.mnt);
+out:
+	path_put(&nd->path);
+	return error;
+}
+
 static struct file *path_openat(int dfd, struct filename *pathname,
 		struct nameidata *nd, const struct open_flags *op, int flags)
 {
···
 		return file;
 
 	file->f_flags = op->open_flag;
+
+	if (unlikely(file->f_flags & O_TMPFILE)) {
+		error = do_tmpfile(dfd, pathname, nd, flags, op, file, &opened);
+		goto out;
+	}
 
 	error = path_init(dfd, pathname->name, flags | LOOKUP_PARENT, nd, &base);
 	if (unlikely(error))

fs/open.c

···
 	if (flags & __O_SYNC)
 		flags |= O_DSYNC;
 
-	/*
-	 * If we have O_PATH in the open flag. Then we
-	 * cannot have anything other than the below set of flags
-	 */
-	if (flags & O_PATH) {
+	if (flags & O_TMPFILE) {
+		if (!(flags & O_CREAT))
+			return -EINVAL;
+		acc_mode = MAY_OPEN | ACC_MODE(flags);
+	} else if (flags & O_PATH) {
+		/*
+		 * If we have O_PATH in the open flag. Then we
+		 * cannot have anything other than the below set of flags
+		 */
 		flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH;
 		acc_mode = 0;
 	} else {

include/linux/dcache.h

···
 /* <clickety>-<click> the ramfs-type tree */
 extern void d_genocide(struct dentry *);
 
+extern void d_tmpfile(struct dentry *, struct inode *);
+
 extern struct dentry *d_find_alias(struct inode *);
 extern void d_prune_aliases(struct inode *);
 

include/linux/fs.h

···
 	int (*atomic_open)(struct inode *, struct dentry *,
 			   struct file *, unsigned open_flag,
 			   umode_t create_mode, int *opened);
+	int (*tmpfile) (struct inode *, struct dentry *, umode_t);
 } ____cacheline_aligned;
 
 ssize_t rw_copy_check_uvector(int type, const struct iovec __user * uvector,

include/uapi/asm-generic/fcntl.h

···
 #define O_PATH		010000000
 #endif
 
+#ifndef O_TMPFILE
+#define O_TMPFILE	020000000
+#endif
+
 #ifndef O_NDELAY
 #define O_NDELAY	O_NONBLOCK
 #endif

mm/shmem.c

···
 	return error;
 }
 
+static int
+shmem_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
+{
+	struct inode *inode;
+	int error = -ENOSPC;
+
+	inode = shmem_get_inode(dir->i_sb, dir, mode, 0, VM_NORESERVE);
+	if (inode) {
+		error = security_inode_init_security(inode, dir,
+						     NULL,
+						     shmem_initxattrs, NULL);
+		if (error) {
+			if (error != -EOPNOTSUPP) {
+				iput(inode);
+				return error;
+			}
+		}
+#ifdef CONFIG_TMPFS_POSIX_ACL
+		error = generic_acl_init(inode, dir);
+		if (error) {
+			iput(inode);
+			return error;
+		}
+#else
+		error = 0;
+#endif
+		d_tmpfile(dentry, inode);
+	}
+	return error;
+}
+
 static int shmem_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
 {
 	int error;
···
 	.rmdir		= shmem_rmdir,
 	.mknod		= shmem_mknod,
 	.rename		= shmem_rename,
+	.tmpfile	= shmem_tmpfile,
 #endif
 #ifdef CONFIG_TMPFS_XATTR
 	.setxattr	= shmem_setxattr,