iio: use anon_inode_getfd() with O_CLOEXEC flag

IIO uses anon_inode_get() to allocate file descriptors as part
of its ioctls. But those ioctls are lacking a flag argument
allowing userspace to choose options for the newly opened file
descriptor.

In such case it's advised to use O_CLOEXEC by default so that
userspace is allowed to choose, without race, if the file descriptor
is going to be inherited across exec().

KVM usage of anon_inode_getfd() was fixed in a previous patchset [1],
so IIO is the only subsystem using anon_inode_getfd() with a fixed set
of flags not including O_CLOEXEC.

This patch set O_CLOEXEC flag on the event file descriptor created
with anon_inode_getfd() to not leak file descriptors across exec().

Links:

- Secure File Descriptor Handling (Ulrich Drepper, 2008)
  http://udrepper.livejournal.com/20407.html

- Excuse me son, but your code is leaking !!! (Dan Walsh, March 2012)
  http://danwalsh.livejournal.com/53603.html

- [1] kvm: use anon_inode_getfd() with O_CLOEXEC flag
  http://lkml.kernel.org/r/cover.1377372576.git.ydroneaud@opteya.com

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-event.c

@@ -159,7 +159,7 @@ int iio_event_getfd(struct iio_dev *indio_dev)
 	}
 	spin_unlock_irq(&ev_int->wait.lock);
 	fd = anon_inode_getfd("iio:event",
-				&iio_event_chrdev_fileops, ev_int, O_RDONLY);
+				&iio_event_chrdev_fileops, ev_int, O_RDONLY | O_CLOEXEC);
 	if (fd < 0) {
 		spin_lock_irq(&ev_int->wait.lock);
 		__clear_bit(IIO_BUSY_BIT_POS, &ev_int->flags);