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 11:10 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.

>
> The flags on bpf_iter_css_task_new() and bpf_iter_css_task_next()
> is what should govern the decision regarding whether the function is
> allowed within some kind of a critical section.
>
>   BTF_ID_FLAGS(func, bpf_iter_css_task_new, KF_ITER_NEW)
>   BTF_ID_FLAGS(func, bpf_iter_css_task_next, KF_ITER_NEXT | KF_RET_NULL)
>
> Should these wield a KF_SLEEPABLE?
>
> Looking at the bpf_iter_css_task_new() body, it uses bpf_mem_alloc()
> which does not sleep and css_task_iter_start() which takes and
> releases a spin lock.
>
> Please elaborate.
>

Going back to
https://lore.kernel.org/all/272de0e9-539c-4d89-9b9c-0652b0826cdd@bytedance.com,
the reasoning certainly seems a bit dubious, esp. with changes since.  In any
case I will take a look at it tomorrow, but I think the conversion is ok. We
might also want to use a filter instead of hardcoding this into the verifier.

> ...