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

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Message-ID: <20190211135159.GC32511@hirez.programming.kicks-ass.net>
Date:   Mon, 11 Feb 2019 14:51:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Julien Thierry <julien.thierry@....com>,
        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 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?

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.