[PATCH] bpf: Fix preempt_enable_no_resched() abuse

[Peter Zijlstra]

On Tue, Apr 23, 2019 at 03:41:32PM -0400, Waiman Long wrote:
> On 4/23/19 3:34 PM, Peter Zijlstra wrote:
> > On Tue, Apr 23, 2019 at 03:12:16PM -0400, Waiman Long wrote:

> >> You are right on that. However, there is a variant called
> >> preempt_enable_no_resched() that doesn't have this side effect. So I am
> >> going to use that one instead.
> > Only if the very next line is schedule(). Otherwise you're very much not
> > going to use that function.
> 
> May I know the reason why. 

Because it can 'consume' a need_resched and introduces arbitrary delays
before the schedule() eventually happens, breaking the very notion of
PREEMPT=y (and the fundamentals RT relies on).

> I saw a number of instances of
> preempt_enable_no_resched() without right next a schedule().

Look more closely.. and let me know, if true, those are bugs that need
fixing.

Argghhh.. BPF...

Also, with the recent RCU rework, we can probably drop that
rcu_read_lock()/rcu_read_unlock() from there if we're disabling
preemption anyway.

---
Subject: bpf: Fix preempt_enable_no_resched() abuse

Unless the very next line is schedule(), or implies it, one must not use
preempt_enable_no_resched(). It can cause a preemption to go missing and
thereby cause arbitrary delays, breaking the PREEMPT=y invariant.

Cc: Roman Gushchin <guro@xxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index f02367faa58d..944ccc310201 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -510,7 +510,7 @@ int bpf_prog_array_copy(struct bpf_prog_array __rcu *old_array,
 		}					\
 _out:							\
 		rcu_read_unlock();			\
-		preempt_enable_no_resched();		\
+		preempt_enable();			\
 		_ret;					\
 	 })