Re: [PATCH v7] printk: hash addresses printed with %p

[Jason A. Donenfeld]

On Wed, Oct 25, 2017 at 5:49 AM, Tobin C. Harding <me@xxxxxxxx> 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.