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Date: Wed, 25 Oct 2017 06:00:21 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: "Tobin C. Harding" <me@...in.cc>
Cc: kernel-hardening@...ts.openwall.com, "Theodore Ts'o" <tytso@....edu>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Tycho Andersen <tycho@...ker.com>, 
	"Roberts, William C" <william.c.roberts@...el.com>, Tejun Heo <tj@...nel.org>, 
	Jordan Glover <Golden_Miller83@...tonmail.ch>, Greg KH <gregkh@...uxfoundation.org>, 
	Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, 
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <wilal.deacon@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com>, 
	Daniel Micay <danielmicay@...il.com>, Djalal Harouni <tixxdz@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7] printk: hash addresses printed with %p

On Wed, Oct 25, 2017 at 5:49 AM, Tobin C. Harding <me@...in.cc> wrote:
> static_branch_disable(&no_ptr_secret) : Doesn't sleep, just atomic read
> and set and maybe a WARN_ONCE.

Are you sure about that? I just looked myself, and though there is a
!HAVE_JUMP_LABEL ifdef that does what you described, there's also a
HAVE_JUMP_LABEL that takes a mutex, which sleeps:

static_branch_disable
  static_key_disable
    cpus_read_lock
      percpu_down_read
        percpu_down_read_preempt_disable
          might_sleep

> Now for the 'executes from process context' stuff.

Er, sorry, I meant to write non-process context in my original
message, which is generally where you're worried about sleeping.

> If the callback mechanism is utilized (i.e print before randomness is
> ready) then the call back will be executed the next time the randomness
> pool gets added to

So it sounds to me like this might be called in non-process context.
Disaster. I realize the static_key thing was my idea in the original
email, so sorry for leading you astray. But moving to do this in
early_initcall wound up fixing other issues too, so all and all a net
good in going this direction.

Two options: you stick with static_branch, because it's cool and speed
is fun, and work around all of the above with a call to queue_work so
that static_branch_enable is called only from process context.

Or, you give up on static_key, because it's not actually super
necessary, and instead just use an atomic, and reason that using `if
(unlikely(!atomic_read(&whatever)))` is probably good enough. In this
option, the code would be pretty much the same as v7, except you'd
s/static_branch/atomic_t/, and change the helpers, etc. This is
probably the more reasonable way.

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