linux-kernel - Re: [PATCH 1/6] bpf: task_group_seq_get_next: cleanup the usage of next

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Message-ID: <7ed83b2c-fc96-1d6e-d442-b4261190a9de@linux.dev>
Date:   Fri, 25 Aug 2023 15:45:21 -0700
From:   Yonghong Song <yonghong.song@...ux.dev>
To:     Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yonghong Song <yhs@...com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kui-Feng Lee <kuifeng@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] bpf: task_group_seq_get_next: cleanup the usage of
 next_thread()



On 8/25/23 9:19 AM, 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.
> 
> 2. next_thread() simply can't return NULL, kill the bogus "if (!next_task)"
>     check.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

__unhash_process() tries to detach_pid() (pid_alive() may become false)
and ist_del_rcu(&p->thread_group). So yes, next_thread() should be safe
to grab next_task since &task->thread_group is rcu protected. So

Acked-by: Yonghong Song <yonghong.song@...ux.dev>

> ---
>   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);