Commit 5f1a6ef3 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Avoid signed integer overflow in rcu_preempt_deferred_qs() 



Subtracting INT_MIN can be interpreted as unconditional signed integer
overflow, which according to the C standard is undefined behavior.
Therefore, kernel build arguments notwithstanding, it would be good to
future-proof the code.  This commit therefore substitutes INT_MAX for
INT_MIN in order to avoid undefined behavior.

While in the neighborhood, this commit also creates some meaningful names
for INT_MAX and friends in order to improve readability, as suggested
by Joel Fernandes.

Reported-by: default avatarRan Rozenstein <ranro@mellanox.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.ibm.com>
parent 117f683c

kernel/rcu/tree_plugin.h

+13 −8
Original line number Diff line number Diff line
@@ -397,6 +397,11 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp) 
	return rnp->gp_tasks != NULL;

}



/* Bias and limit values for ->rcu_read_lock_nesting. */

#define RCU_NEST_BIAS INT_MAX

#define RCU_NEST_NMAX (-INT_MAX / 2)

#define RCU_NEST_PMAX (INT_MAX / 2)



/*

 * Preemptible RCU implementation for rcu_read_lock().

 * Just increment ->rcu_read_lock_nesting, shared state will be updated
@@ -405,6 +410,8 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp) 
void __rcu_read_lock(void)

{

	current->rcu_read_lock_nesting++;

	if (IS_ENABLED(CONFIG_PROVE_LOCKING))

		WARN_ON_ONCE(current->rcu_read_lock_nesting > RCU_NEST_PMAX);

	barrier();  /* critical section after entry code. */

}

EXPORT_SYMBOL_GPL(__rcu_read_lock);
@@ -424,20 +431,18 @@ void __rcu_read_unlock(void) 
		--t->rcu_read_lock_nesting;

	} else {

		barrier();  /* critical section before exit code. */

		t->rcu_read_lock_nesting = INT_MIN;

		t->rcu_read_lock_nesting = -RCU_NEST_BIAS;

		barrier();  /* assign before ->rcu_read_unlock_special load */

		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))

			rcu_read_unlock_special(t);

		barrier();  /* ->rcu_read_unlock_special load before assign */

		t->rcu_read_lock_nesting = 0;

	}

#ifdef CONFIG_PROVE_LOCKING

	{

		int rrln = READ_ONCE(t->rcu_read_lock_nesting);

	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {

		int rrln = t->rcu_read_lock_nesting;



		WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);

		WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);

	}

#endif /* #ifdef CONFIG_PROVE_LOCKING */

}

EXPORT_SYMBOL_GPL(__rcu_read_unlock);
@@ -617,11 +622,11 @@ static void rcu_preempt_deferred_qs(struct task_struct *t) 
	if (!rcu_preempt_need_deferred_qs(t))

		return;

	if (couldrecurse)

		t->rcu_read_lock_nesting -= INT_MIN;

		t->rcu_read_lock_nesting -= RCU_NEST_BIAS;

	local_irq_save(flags);

	rcu_preempt_deferred_qs_irqrestore(t, flags);

	if (couldrecurse)

		t->rcu_read_lock_nesting += INT_MIN;

		t->rcu_read_lock_nesting += RCU_NEST_BIAS;

}



/*

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