Merge 'xfrm: Add compat layer'

Merge 'xfrm: Add compat layer'

Dmitry Safonov says:

====================
Changes since v2:
- added struct xfrm_translator as API to register xfrm_compat.ko with
  xfrm_state.ko. This allows compilation of translator as a loadable
  module
- fixed indention and collected reviewed-by (Johannes Berg)
- moved boilerplate from commit messages into cover-letter (Steffen
  Klassert)
- found on KASAN build and fixed non-initialised stack variable usage
  in the translator

The resulting v2/v3 diff can be found here:
https://gist.github.com/0x7f454c46/8f68311dfa1f240959fdbe7c77ed2259

Patches as a .git branch:
https://github.com/0x7f454c46/linux/tree/xfrm-compat-v3

Changes since v1:
- reworked patches set to use translator
- separated the compat layer into xfrm_compat.c,
  compiled under XFRM_USER_COMPAT config
- 32-bit messages now being sent in frag_list (like wext-core does)
- instead of __packed add compat_u64 members in compat structures
- selftest reworked to kselftest lib API
- added netlink dump testing to the selftest

XFRM is disabled for compatible users because of the UABI difference.
The difference is in structures paddings and in the result the size
of netlink messages differ.

Possibility for compatible application to manage xfrm tunnels was
disabled by: the commmit 791be403ea9a ("xfrm: Refuse to insert 32 bit
userspace socket policies on 64 bit systems") and the commit 040d8533f778
("xfrm: Do not parse 32bits compiled xfrm netlink msg on 64bits host").

This is my second attempt to resolve the xfrm/compat problem by adding
the 64=>32 and 32=>64 bit translators those non-visibly to a user
provide translation between compatible user and kernel.
Previous attempt was to interrupt the message ABI according to a syscall
by xfrm_user, which resulted in over-complicated code [1].

Florian Westphal provided the idea of translator and some draft patches
in the discussion. In these patches, his idea is reused and some of his
initial code is also present.

There were a couple of attempts to solve xfrm compat problem:
https://lkml.org/lkml/2017/1/20/733
https://patchwork.ozlabs.org/patch/44600/
http://netdev.vger.kernel.narkive.com/2Gesykj6/patch-net-next-xfrm-correctly-parse-netlink-msg-from-32bits-ip-command-on-64bits-host

All the discussions end in the conclusion that xfrm should have a full
compatible layer to correctly work with 32-bit applications on 64-bit
kernels:
https://lkml.org/lkml/2017/1/23/413
https://patchwork.ozlabs.org/patch/433279/

In some recent lkml discussion, Linus said that it's worth to fix this
problem and not giving people an excuse to stay on 32-bit kernel:
https://lkml.org/lkml/2018/2/13/752

There is also an selftest for ipsec tunnels.
It doesn't depend on any library and compat version can be easy
build with: make CFLAGS=-m32 net/ipsec

[1]: https://lkml.kernel.org/r/20180726023144.31066-1-dima@arista.com
====================

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>