Re: [PATCH bpf-next v1 1/2] bpf: Fix sleepable context checks and remove in_sleepable helper

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Thu Aug 6, 2026 at 11:20 PM CEST, Kumar Kartikeya Dwivedi wrote:
> On Thu Aug 6, 2026 at 11:10 PM CEST, Eduard Zingerman wrote:
>> On Thu, 2026-08-06 at 18:40 +0200, Kumar Kartikeya Dwivedi wrote:
>>
>> ...
>>
>>> Several call sites incorrectly used in_sleepable() where
>>> in_sleepable_context() is required. This allowed sleepable programs
>>> in non-sleepable contexts (e.g., inside bpf_rcu_read_lock()) to
>>> incorrectly use sleepable iterators and kfuncs:
>>>
>>>   - check_css_task_iter_allowlist() returned in_sleepable() and
>>>     therefore allowed css_task iterator in any sleepable program,
>>>     even inside RCU/preempt/lock/IRQ-disabled regions. Fix it to
>>>     use in_sleepable_context().
>>
>> ...
>>
>>> @@ -12083,7 +12078,7 @@ static bool check_css_task_iter_allowlist(struct bpf_verifier_env *env)
>>>  			return true;
>>>  		fallthrough;
>>>  	default:
>>> -		return in_sleepable(env);
>>> +		return in_sleepable_context(env);
>>>  	}
>>>  }
>>
>> I don't understand this change. The comment on top of the
>> check_css_task_iter_allowlist() says that it is about
>> cgroup.c:css_set_lock, not the current context per se.
>
> I haven't looked deeply into this on whether this should change, that said the
> usage of in_sleepable() is obviously wrong here, in that it is not enough.
>

Ok, I looked at it.

I guess I see what you mean, sleepable programs won't run inside css_set_lock
critical section, hence it should be ok to not check in_sleepable_context(). So
we probably only care about the program being sleepable, and not the context.

So the current thing should be ok except on PREEMPT_RT, when spin lock on
css_set_lock becomes a rt_mutex. It depends on whether we care enough to make it
context dependent on PREEMPT_RT and leave it open otherwise, or just let it be.

I can still move it to a filter outside the verifier though.

> [...]