linux-kernel - Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of next

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Date:   Mon, 21 Aug 2023 10:55:58 -0700
From:   Kui-Feng Lee <sinquersw@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>, Yonghong Song <yhs@...com>,
        Kui-Feng Lee <kuifeng@...com>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: task_group_seq_get_next: cleanup the usage of
 next_thread()



On 8/21/23 08:09, Oleg Nesterov wrote:
> 1. find_pid_ns() + get_pid_task() under rcu_read_lock() guarantees that we
>     can safely iterate the task->thread_group list. Even if this task exits
>     right after get_pid_task() (or goto retry) and pid_alive() returns 0 >
>     Kill the unnecessary pid_alive() check.

This function will return next_task holding a refcount, and release the
refcount until the next time calling the same function. Meanwhile,
the returned task A may be killed, and its next task B may be
killed after A as well, before calling this function again.
However, even task B is destroyed (free), A's next is still pointing to
task B. When this function is called again for the same iterator,
it doesn't promise that B is still there.

Does that make sense to you?

> 
> 2. next_thread() simply can't return NULL, kill the bogus "if (!next_task)"
>     check.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>   kernel/bpf/task_iter.c | 7 -------
>   1 file changed, 7 deletions(-)
> 
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index c4ab9d6cdbe9..4d1125108014 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -75,15 +75,8 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>   		return NULL;
>   
>   retry:
> -	if (!pid_alive(task)) {
> -		put_task_struct(task);
> -		return NULL;
> -	}
> -
>   	next_task = next_thread(task);
>   	put_task_struct(task);
> -	if (!next_task)
> -		return NULL;
>   
>   	saved_tid = *tid;
>   	*tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);