sysctl: add a new register_sysctl_init() interface

Patch series "sysctl: first set of kernel/sysctl cleanups", v2.

Finally had time to respin the series of the work we had started last
year on cleaning up the kernel/sysct.c kitchen sink.  People keeps
stuffing their sysctls in that file and this creates a maintenance
burden.  So this effort is aimed at placing sysctls where they actually
belong.

I'm going to split patches up into series as there is quite a bit of
work.

This first set adds register_sysctl_init() for uses of registerting a
sysctl on the init path, adds const where missing to a few places,
generalizes common values so to be more easy to share, and starts the
move of a few kernel/sysctl.c out where they belong.

The majority of rework on v2 in this first patch set is 0-day fixes.
Eric Biederman's feedback is later addressed in subsequent patch sets.

I'll only post the first two patch sets for now.  We can address the
rest once the first two patch sets get completely reviewed / Acked.

This patch (of 9):

The kernel/sysctl.c is a kitchen sink where everyone leaves their dirty
dishes, this makes it very difficult to maintain.

To help with this maintenance let's start by moving sysctls to places
where they actually belong.  The proc sysctl maintainers do not want to
know what sysctl knobs you wish to add for your own piece of code, we
just care about the core logic.

Today though folks heavily rely on tables on kernel/sysctl.c so they can
easily just extend this table with their needed sysctls.  In order to
help users move their sysctls out we need to provide a helper which can
be used during code initialization.

We special-case the initialization use of register_sysctl() since it
*is* safe to fail, given all that sysctls do is provide a dynamic
interface to query or modify at runtime an existing variable.  So the
use case of register_sysctl() on init should *not* stop if the sysctls
don't end up getting registered.  It would be counter productive to stop
boot if a simple sysctl registration failed.

Provide a helper for init then, and document the recommended init levels
to use for callers of this routine.  We will later use this in
subsequent patches to start slimming down kernel/sysctl.c tables and
moving sysctl registration to the code which actually needs these
sysctls.

[[email protected]: major commit log and documentation rephrasing also moved to fs/proc/proc_sysctl.c                  ]

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Xiaoming Ni <[email protected]>
Signed-off-by: Luis Chamberlain <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Iurii Zaikin <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Sebastian Reichel <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Qing Wang <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Amir Goldstein <[email protected]>
Cc: Stephen Kitt <[email protected]>
Cc: Antti Palosaari <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Clemens Ladisch <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: Julia Lawall <[email protected]>
Cc: Lukas Middendorf <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Phillip Potter <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Douglas Gilbert <[email protected]>
Cc: James E.J. Bottomley <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: John Ogness <[email protected]>
Cc: Martin K. Petersen <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Steven Rostedt (VMware) <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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