linux-kernel - Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert

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Date:	Thu, 18 Jun 2015 08:52:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert_state

On Thu, Jun 18, 2015 at 4:07 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Jun 18, 2015 at 2:57 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>
>> * Andy Lutomirski <luto@...capital.net> wrote:
>>
>>> On Wed, Jun 17, 2015 at 2:41 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>> >
>>> > * Andy Lutomirski <luto@...nel.org> wrote:
>>> >
>>> >> This will let us sprinkle sanity checks around the kernel without
>>> >> making too much of a mess.
>>> >>
>>> >> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>>> >> ---
>>> >>  include/linux/context_tracking.h | 8 ++++++++
>>> >>  1 file changed, 8 insertions(+)
>>> >>
>>> >> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
>>> >> index 2821838256b4..0fbea4b152e1 100644
>>> >> --- a/include/linux/context_tracking.h
>>> >> +++ b/include/linux/context_tracking.h
>>> >> @@ -57,6 +57,13 @@ static inline void context_tracking_task_switch(struct task_struct *prev,
>>> >>       if (context_tracking_is_enabled())
>>> >>               __context_tracking_task_switch(prev, next);
>>> >>  }
>>> >> +
>>> >> +static inline void context_tracking_assert_state(enum ctx_state state)
>>> >> +{
>>> >> +     rcu_lockdep_assert(!context_tracking_is_enabled() ||
>>> >> +                        this_cpu_read(context_tracking.state) == state,
>>> >> +                        "context tracking state was wrong");
>>> >> +}
>>> >
>>> > Please don't introduce assert() style debug check interfaces!
>>> >
>>> > (And RCU should be fixed too I suspect.)
>>> >
>>> > They are absolutely horrible on the brain when mixed with WARN_ON() interfaces,
>>> > which are the dominant runtime check interface in the kernel.
>>> >
>>> > Instead make it something like:
>>> >
>>> >   #define ct_state() (this_cpu_read(context_tracking.state))
>>> >
>>> >   #define CT_WARN_ON(cond) \
>>> >         WARN_ON(context_tracking_is_enabled() && (cond))
>>> >
>>> > and then the debug checks can be written as:
>>> >
>>> >         CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
>>> >
>>> > This is IMHO _far_ more readable than:
>>> >
>>> >         context_tracking_assert_state(CONTEXT_KERNEL);
>>> >
>>> > ok?
>>> >
>>> > (Assuming people will accept 'ct/CT' as an abbreviation for context tracking.)
>>>
>>> Hmm, ok I guess.  The part I don't like is having ct_state() at all on
>>> non-context-tracking kernels -- it seems like it's asking for trouble.
>>
>> Well:
>>
>>  - if # CONFIG_CONTEXT_TRACKING is not se, then CT_WARN_ON() does nothing.
>>
>>  - if CONFIG_CONTEXT_TRACKING=y, but !context_tracking_is_enabled(), then
>>    CT_WARN_ON() will evaluate 'cond', but won't calculate it.
>>
>>  - only if CONFIG_CONTEXT_TRACKING=y && context_tracking_is_enabled() should we
>>    get as far as ct_state() evaluation.
>>
>> so I'm not sure I see the problem you are seeing.
>>
>>> We could make CT_WARN_ON not even evaluate its argument if
>>> !CONFIG_CONTEXT_TRACKING, but then we still have ct_state() returning garbage if
>>> !context_tracking_is_enabled().
>>
>> My understanding is that if !context_tracking_is_enabled() then the compiler
>> should not even try to evaluate the rest. This is why doing a NULL pointer check
>> like this is safe:
>
> I'm fine with everything you just covered.  My only objection is that,
> if ct_state() exists, then someone might call it outside CT_WARN_ON,
> in which case it will break on non-context-tracking setups.

The more I think about it, the more I dislike ct_state().  We have
in_atomic(), which is already problematic because the return value
isn't reliable.  ct_state(), if callable on non context-tracking
kernels, will also be unreliable.  I prefer things like
lockdep_assert_held because they can't be misused.

It would be far too easy for someone to read:

CT_WARN_ON(ct_state() != CONTEXT_KERNEL);

and add:

if (ct_state() == CONTEXT_KERNEL)
  do_something();

and that would be bad.

--Andy

>
> --Andy



-- 
Andy Lutomirski
AMA Capital Management, LLC
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