Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Date:   Thu, 16 Nov 2023 10:34:28 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Yonghong Song <yonghong.song@linux.dev>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Chuyi Zhou <zhouchuyi@bytedance.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Kui-Feng Lee <kuifeng@fb.com>, linux-kernel@vger.kernel.org,
        bpf@vger.kernel.org
Subject: Re: [PATCH 1/3] bpf: task_group_seq_get_next: use __next_thread()
 rather than next_thread()
Message-ID: <20231116093428.GA18748@redhat.com>
References: <20231114163234.GA890@redhat.com>
 <34440ea4-3780-45e4-9e7c-1b36b535171b@linux.dev>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <34440ea4-3780-45e4-9e7c-1b36b535171b@linux.dev>
User-Agent: Mutt/1.5.24 (2015-08-30)
Precedence: bulk

On 11/15, Yonghong Song wrote:
>
> On 11/14/23 11:32 AM, Oleg Nesterov wrote:
> >@@ -70,15 +70,13 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
> >  		return NULL;
> >  retry:
> >-	task = next_thread(task);
> >+	task = __next_thread(task);
> >+	if (!task)
> >+		return NULL;
> >  	next_tid = __task_pid_nr_ns(task, PIDTYPE_PID, common->ns);
> >-	if (!next_tid || next_tid == common->pid) {
> >-		/* Run out of tasks of a process.  The tasks of a
> >-		 * thread_group are linked as circular linked list.
> >-		 */
> >-		return NULL;
> >-	}
> >+	if (!next_tid)
> >+		goto retry;
>
> Look at the code. Looks like next_tid should never be 0

...

> pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
>                         struct pid_namespace *ns)
> {
>         pid_t nr = 0;
>
>         rcu_read_lock();
>         if (!ns)
>                 ns = task_active_pid_ns(current);
>         nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^

Please note that task_pid_ptr(task, type)) can return NULL if this
task has already exited and called detach_pid().

detach_pid() does __change_pid(task, type, NULL), please note the

	*pid_ptr = new; // NULL in this case

assignment in __change_pid().

IOW. The problem is not that ns can change, the problem is that
task->thread_pid (and other pid links) can be NULL, and in this
case pid_nr_ns() returns zero.


This code should be rewritten from the very beginning, it should
not rely on pid_nr. If nothing else common->pid and/or pid_visiting
can be reused. But currently my only concern is next_thread().

> Other than above, the change looks good to me.

Thanks for review!

Oleg.