Re: [PATCH v2 00/15] Make the user mode driver code a better citizen

[Eric W. Biederman]

Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:

> On Tue, Jun 30, 2020 at 07:29:34AM -0500, Eric W. Biederman wrote:
>> 
>> diff --git a/net/bpfilter/bpfilter_kern.c b/net/bpfilter/bpfilter_kern.c
>> index 91474884ddb7..3e1874030daa 100644
>> --- a/net/bpfilter/bpfilter_kern.c
>> +++ b/net/bpfilter/bpfilter_kern.c
>> @@ -19,8 +19,8 @@ static void shutdown_umh(void)
>>         struct pid *tgid = info->tgid;
>>  
>>         if (tgid) {
>> -               kill_pid_info(SIGKILL, SEND_SIG_PRIV, tgid);
>> -               wait_event(tgid->wait_pidfd, !pid_task(tgid, PIDTYPE_TGID));
>> +               kill_pid(tgid, SIGKILL, 1);
>> +               wait_event(tgid->wait_pidfd, !pid_has_task(tgid, PIDTYPE_TGID));
>>                 bpfilter_umh_cleanup(info);
>>         }
>>  }
>> 
>> > And then did:
>> > while true; do iptables -L;rmmod bpfilter; done
>> >  
>> > Unfortunately sometimes 'rmmod bpfilter' hangs in wait_event().
>> 
>> Hmm.  The wake up happens just of tgid->wait_pidfd happens just before
>> release_task is called so there is a race.  As it is possible to wake
>> up and then go back to sleep before pid_has_task becomes false.
>> 
>> So I think I need a friendly helper that does:
>> 
>> bool task_has_exited(struct pid *tgid)
>> {
>> 	bool exited = false;
>> 
>> 	rcu_read_lock();
>>         tsk = pid_task(tgid, PIDTYPE_TGID);
>>         exited = !!tsk;
>>         if (tsk) {
>>         	exited = !!tsk->exit_state;
>> out:
>> 	rcu_unlock();
>> 	return exited;
>> }
>
> All makes sense to me.
> If I understood the race condition such helper should indeed solve it.
> Are you going to add such patch to your series?
> I'll proceed with my work on top of your series and will ignore this
> race for now, but I think it should be fixed before we land this set
> into multiple trees.

Yes. I am just finishing it up now.

Eric