On 08/30, David Rientjes wrote:

Using that function to both set oom_score_adj to OOM_SCORE_ADJ_MAX and
then reinstate the previous value is racy since it's possible that
userspace can set the value to something else itself before the old value
is reinstated.  That results in userspace setting current's oom_score_adj
to a different value and then the kernel immediately setting it back to
its previous value without notification.

Sure,

To fix this, a new compare_swap_oom_score_adj() function is introduced
with the same semantics as the compare and swap CAS instruction, or
CMPXCHG on x86.  It is used to reinstate the previous value of
oom_score_adj if and only if the present value is the same as the old
value.

But this can't fix the race completely ?

+void compare_swap_oom_score_adj(int old_val, int new_val)
+{
+	struct sighand_struct *sighand = current->sighand;
+
+	spin_lock_irq(&sighand->siglock);
+	if (current->signal->oom_score_adj == old_val)
+		current->signal->oom_score_adj = new_val;
+	spin_unlock_irq(&sighand->siglock);
+}

So. This is used this way:

	old_val = test_set_oom_score_adj(OOM_SCORE_ADJ_MAX);

	do_something();

	compare_swap_oom_score_adj(OOM_SCORE_ADJ_MAX, old_val);

What if userspace sets oom_score_adj = OOM_SCORE_ADJ_MAX in between?
May be the callers should use OOM_SCORE_ADJ_MAX + 1 instead, this way
we can't confuse old_val with the value from the userspace...




But in fact I am writing this email because I have the question.
Do we really need 2 helpers, and do we really need to allow to set
the arbitrary value?

I mean, perhaps we can do something like

	void set_oom_victim(bool on)
	{
		if (on) {
			oom_score_adj += ADJ_MAX - ADJ_MIN + 1;
		} else if (oom_score_adj > ADJ_MAX) {
			oom_score_adj -= ADJ_MAX - ADJ_MIN + 1;
		}
	}

Not sure this really makes sense, just curious.

Oleg.

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