linux-kernel - Re: [PATCH 3/6] bpf: task_group_seq_get_next: fix the skip_if_dup

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Message-ID: <20230825170406.GA16800@redhat.com>
Date:   Fri, 25 Aug 2023 19:04:06 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     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 3/6] bpf: task_group_seq_get_next: fix the
 skip_if_dup_files check

Forgot to mention in the changelog...

In any case this doesn't look right. ->group_leader can exit before other
threads, call exit_files(), and in this case task_group_seq_get_next() will
check task->files == NULL.

On 08/25, Oleg Nesterov wrote:
>
> Unless I am notally confused it is wrong. We are going to return or
> skip next_task so we need to check next_task-files, not task->files.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  kernel/bpf/task_iter.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index 1589ec3faded..2264870ae3fc 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -82,7 +82,7 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
>
>  	common->pid_visiting = *tid;
>
> -	if (skip_if_dup_files && task->files == task->group_leader->files) {
> +	if (skip_if_dup_files && next_task->files == next_task->group_leader->files) {
>  		task = next_task;
>  		goto retry;
>  	}
> --
> 2.25.1.362.g51ebf55