fuse: device fd clone

Allow an open fuse device to be "cloned".  Userspace can create a clone by:

      newfd = open("/dev/fuse", O_RDWR)
      ioctl(newfd, FUSE_DEV_IOC_CLONE, &oldfd);

At this point newfd will refer to the same fuse connection as oldfd.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Reviewed-by: Ashish Samant <ashish.samant@oracle.com>

Documentation/ioctl/ioctl-number.txt

@@ -321,6 +321,7 @@ Code  Seq#(hex)	Include File		Comments
 0xDB	00-0F	drivers/char/mwave/mwavepub.h
 0xDD	00-3F	ZFCP device driver	see drivers/s390/scsi/
 					<mailto:aherrman@de.ibm.com>
+0xE5	00-3F	linux/fuse.h
 0xEC	00-01	drivers/platform/chrome/cros_ec_dev.h	ChromeOS EC driver
 0xF3	00-3F	drivers/usb/misc/sisusbvga/sisusb.h	sisfb (in development)
 					<mailto:thomas@winischhofer.net>

fs/fuse/dev.c

@@ -2197,6 +2197,44 @@ static int fuse_dev_fasync(int fd, struct file *file, int on)
 	return fasync_helper(fd, file, on, &fc->iq.fasync);
 }
 
+static int fuse_device_clone(struct fuse_conn *fc, struct file *new)
+{
+	if (new->private_data)
+		return -EINVAL;
+
+	new->private_data = fuse_conn_get(fc);
+
+	return 0;
+}
+
+static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
+			   unsigned long arg)
+{
+	int err = -ENOTTY;
+
+	if (cmd == FUSE_DEV_IOC_CLONE) {
+		int oldfd;
+
+		err = -EFAULT;
+		if (!get_user(oldfd, (__u32 __user *) arg)) {
+			struct file *old = fget(oldfd);
+
+			err = -EINVAL;
+			if (old) {
+				struct fuse_conn *fc = fuse_get_conn(old);
+
+				if (fc) {
+					mutex_lock(&fuse_mutex);
+					err = fuse_device_clone(fc, file);
+					mutex_unlock(&fuse_mutex);
+				}
+				fput(old);
+			}
+		}
+	}
+	return err;
+}
+
 const struct file_operations fuse_dev_operations = {
 	.owner		= THIS_MODULE,
 	.open		= fuse_dev_open,
@@ -2208,6 +2246,8 @@ const struct file_operations fuse_dev_operations = {
 	.poll		= fuse_dev_poll,
 	.release	= fuse_dev_release,
 	.fasync		= fuse_dev_fasync,
+	.unlocked_ioctl = fuse_dev_ioctl,
+	.compat_ioctl   = fuse_dev_ioctl,
 };
 EXPORT_SYMBOL_GPL(fuse_dev_operations);
 

include/uapi/linux/fuse.h

@@ -755,4 +755,7 @@ struct fuse_notify_retrieve_in {
 	uint64_t	dummy4;
 };
 
+/* Device ioctls: */
+#define FUSE_DEV_IOC_CLONE	_IOR(229, 0, uint32_t)
+
 #endif /* _LINUX_FUSE_H */