linux-kernel - Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

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Message-ID: <a23fdd84-8c4b-0c1f-d877-24d946cff6d0@arm.com>
Date:   Tue, 12 Feb 2019 09:15:13 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        mingo@...hat.com, catalin.marinas@....com, will.deacon@....com,
        james.morse@....com, hpa@...or.com, valentin.schneider@....com
Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called
 in unsafe region



On 11/02/2019 13:51, Peter Zijlstra wrote:
> On Mon, Feb 11, 2019 at 02:45:27PM +0100, Ingo Molnar wrote:
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index a674c7db..b1bb7e9 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -3289,6 +3289,14 @@ static inline void schedule_debug(struct task_struct *prev)
>>>  		__schedule_bug(prev);
>>>  		preempt_count_set(PREEMPT_DISABLED);
>>>  	}
>>> +
>>> +	if (IS_ENABLED(CONFIG_DEBUG_UACCESS_SLEEP) &&
>>> +	    unlikely(unsafe_user_region_active())) {
>>> +		printk(KERN_ERR "BUG: scheduling while user_access enabled: %s/%d/0x%08x\n",
>>> +		       prev->comm, prev->pid, preempt_count());
>>> +		dump_stack();
>>> +	}
>>> +
>>>  	rcu_sleep_check();
>>>
>>>  	profile_hit(SCHED_PROFILING, __builtin_return_address(0));
>>> @@ -6151,6 +6159,20 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
>>>  EXPORT_SYMBOL(___might_sleep);
>>>  #endif
>>>
>>> +#ifdef CONFIG_DEBUG_UACCESS_SLEEP
>>> +void __might_resched(const char *file, int line)
>>> +{
>>> +	if (!unsafe_user_region_active())
>>> +		return;
>>
>> Could you please more clearly explain why you want/need an exception from 
>> the __might_resched() debug warning?

So, the scenarios I'm trying to avoid are of the following flavour:

	if (user_access_begin(ptr, size)) {

		[...]

		// Calling a function that might call schedule()

		[...]
		user_access_end();
	}


The thing is, as I understand, not all function that call schedule() are
annotated with might_resched(), and on the other hand, not every time we
call a function that might_resched() will it call schedule().

Now with Peter's remark I think I might have been overzealous.

> 
> In specific; how is the addition in schedule_debug() not triggering on
> PREEMPT=y kernels?
> 
> If code is preemptible, you can (get) schedule(d). If it is not
> preemptible; you do not need these additional tests.
> 

Yes that sounds right, might_resched() only potentially reschedules if
in a suitable context, so best case I issue two warnings, worst case I
actually be warn when the caller took care to disable preemption or
interrupts before calling a might_resched().

I guess I got a bit confused with might_sleep() which is "if you call
this in the wrong context I warn" whereas might_resched() is just "if
you call this in preemptible context, lets resched".

I guess I'll drop the might_resched() part of this patch if that sounds
alright.

Thanks,

-- 
Julien Thierry