livepatch: Basic API to track system state changes

livepatch: Basic API to track system state changes

This is another step how to help maintaining more livepatches.

One big help was the atomic replace and cumulative livepatches. These
livepatches replace the already installed ones. Therefore it should
be enough when each cumulative livepatch is consistent.

The problems might come with shadow variables and callbacks. They might
change the system behavior or state so that it is no longer safe to
go back and use an older livepatch or the original kernel code. Also,
a new livepatch must be able to detect changes which were made by
the already installed livepatches.

This is where the livepatch system state tracking gets useful. It
allows to:

  - find whether a system state has already been modified by
    previous livepatches

  - store data needed to manipulate and restore the system state

The information about the manipulated system states is stored in an
array of struct klp_state. It can be searched by two new functions
klp_get_state() and klp_get_prev_state().

The dependencies are going to be solved by a version field added later.
The only important information is that it will be allowed to modify
the same state by more non-cumulative livepatches. It is similar
to allowing to modify the same function several times. The livepatch
author is responsible for preventing incompatible changes.

Link: http://lkml.kernel.org/r/20191030154313.13263-3-pmladek@suse.com
To: Jiri Kosina <jikos@kernel.org>
Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Cc: Nicolai Stange <nstange@suse.de>
Cc: live-patching@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>