On Thu, Jan 18, 2018 at 08:29:57AM -0800, Christoph Hellwig wrote:

quoted

	We could turn ->msg_control/->msg_controllen into another
iov_iter, but seeing that we never do scatter-gather for those
IMO that would be a massive overkill.  A flag controlling whether
->msg_control is kernel or userland pointer would do, especially
since we already have a flag for "do we want a native or compat
layout for cmsg" in there.

While your current hack seems like a nice short term improvement
I think we need an iov_iter or iov_iter-light there in the long run.

For one caller in the entire history of the kernel?

Same for ioctl so that we can pass properly typed kernel or user
buffers through without all these set_fs hacks.

Umm...  Most of the PITA with ioctls is due to compat ones being
reformatted for native and fed under set_fs().  I actually have
a series dealing with most of such places for net ioctls.  Sure,
there's also ioctl_by_bdev(), but for those we might be better
off exposing the things like ->get_last_session() and its ilk to
filesystems that want to deal with cdroms...

It's kernel_setsockopt() that is the real PITA...