On Wed, May 10, 2017 at 03:45:24AM +0100, Al Viro wrote:

FWIW, some parts of that queue are obviously sane; it's the conversions of
kernel_write() and friends to ->read_iter/->write_iter() that are non-starters.

Egads...  OK, I *have* misread what you are doing there.  Your vfs_iter_read()
works for files sans ->read_iter().  For strange values of "works" - you
hardwire "it's either iovec or kvec iterator" into its calling conventions,
which is a trouble waiting to happen.

What's the point?  What's wrong with having kernel_read()/kernel_readv()/etc.?
You still have set_fs() in there; doing that one level up in call chain would
be just fine...  IDGI.

Broken commit: "net: don't play with address limits in kernel_recvmsg".
It would be OK if it was only about data.  Unfortunately, that's not
true in one case: svc_udp_recvfrom() wants ->msg_control.

Another delicate place: you can't assume that write() always advances
file position by its (positive) return value.  btrfs stuff is sensitive
to that.

ashmem probably _is_ OK with demanding ->read_iter(), but I'm not sure
about blind asma->file->f_pos += ret.  That's?begging for races.  Actually,
scratch that - it *is* racy.