On Thu, Jan 18, 2018 at 04:43:02AM +0000, Al Viro wrote:

	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.

	That's the only caller we need it for, but that thing looks cheap
enough.  Obviously needs to pass testing, including "is it too ugly to
live as far as Davem is concerned" test, though...

	BTW, there's another series of set_fs-removal patches in
net ioctls; still needs review, though.  With that one we would be down
to 11 instances in the entire net/*:

* SO_RCVTIMEO/SO_SNDTIMEO handling in compat [sg]etsockopt()
* passing SIOC{ADD,DEL}TUNNEL down (ipmr_del_tunnel(),ipmr_new_tunnel(),
  addrconf_set_dstaddr())
* SIOCGSTAMP/SIOCGSTAMPNS in compat ioctls
* SIOCADDRT/SIOCDELRT in compat ioctls
* kernel_[gs]etsockopt()
* ipv6_renew_options_kern()

I don't know if all of that stuff can be realistically done without set_fs().
kernel_setsockopt(), in particular, is unpleasant...

The patches need review and testing, obviously; I'll post them in followups,
the entire series (on top of net/master) is in
git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #work.net-ioctl