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

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Date:   Wed, 01 Jul 2020 12:12:35 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>, bpf <bpf@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Gary Lin <GLin@...e.com>, Bruno Meneguele <bmeneg@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 00/15] Make the user mode driver code a better citizen

Alexei Starovoitov <alexei.starovoitov@...il.com> 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