CUSE: implement CUSE - Character device in Userspace

CUSE enables implementing character devices in userspace.  With recent
additions of ioctl and poll support, FUSE already has most of what's
necessary to implement character devices.  All CUSE has to do is
bonding all those components - FUSE, chardev and the driver model -
nicely.

When client opens /dev/cuse, kernel starts conversation with
CUSE_INIT.  The client tells CUSE which device it wants to create.  As
the previous patch made fuse_file usable without associated
fuse_inode, CUSE doesn't create super block or inodes.  It attaches
fuse_file to cdev file->private_data during open and set ff->fi to
NULL.  The rest of the operation is almost identical to FUSE direct IO
case.

Each CUSE device has a corresponding directory /sys/class/cuse/DEVNAME
(which is symlink to /sys/devices/virtual/class/DEVNAME if
SYSFS_DEPRECATED is turned off) which hosts "waiting" and "abort"
among other things.  Those two files have the same meaning as the FUSE
control files.

The only notable lacking feature compared to in-kernel implementation
is mmap support.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/Kconfig

@@ -62,6 +62,16 @@ source "fs/autofs/Kconfig"
 source "fs/autofs4/Kconfig"
 source "fs/fuse/Kconfig"
 
+config CUSE
+	tristate "Character device in Userpace support"
+	depends on FUSE_FS
+	help
+	  This FUSE extension allows character devices to be
+	  implemented in userspace.
+
+	  If you want to develop or use userspace character device
+	  based on CUSE, answer Y or M.
+
 config GENERIC_ACL
 	bool
 	select FS_POSIX_ACL

fs/fuse/Makefile

@@ -3,5 +3,6 @@
 #
 
 obj-$(CONFIG_FUSE_FS) += fuse.o
+obj-$(CONFIG_CUSE) += cuse.o
 
 fuse-objs := dev.o dir.o file.o inode.o control.o

include/linux/fuse.h

@@ -120,6 +120,13 @@ struct fuse_file_lock {
 #define FUSE_EXPORT_SUPPORT	(1 << 4)
 #define FUSE_BIG_WRITES		(1 << 5)
 
+/**
+ * CUSE INIT request/reply flags
+ *
+ * CUSE_UNRESTRICTED_IOCTL:  use unrestricted ioctl
+ */
+#define CUSE_UNRESTRICTED_IOCTL	(1 << 0)
+
 /**
  * Release flags
  */
@@ -210,6 +217,9 @@ enum fuse_opcode {
 	FUSE_DESTROY       = 38,
 	FUSE_IOCTL         = 39,
 	FUSE_POLL          = 40,
+
+	/* CUSE specific operations */
+	CUSE_INIT          = 4096,
 };
 
 enum fuse_notify_code {
@@ -401,6 +411,27 @@ struct fuse_init_out {
 	__u32	max_write;
 };
 
+#define CUSE_INIT_INFO_MAX 4096
+
+struct cuse_init_in {
+	__u32	major;
+	__u32	minor;
+	__u32	unused;
+	__u32	flags;
+};
+
+struct cuse_init_out {
+	__u32	major;
+	__u32	minor;
+	__u32	unused;
+	__u32	flags;
+	__u32	max_read;
+	__u32	max_write;
+	__u32	dev_major;		/* chardev major */
+	__u32	dev_minor;		/* chardev minor */
+	__u32	spare[10];
+};
+
 struct fuse_interrupt_in {
 	__u64	unique;
 };