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

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Message-ID: <e254a6db-66eb-8bfc-561f-464327a1005a@linux.dev>
Date:   Fri, 25 Aug 2023 15:52:02 -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 3/6] bpf: task_group_seq_get_next: fix the
 skip_if_dup_files check



On 8/25/23 10:04 AM, Oleg Nesterov wrote:
> 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.

It is okay. This won't be affecting correctness. We will end with
calling bpf program for 'next_task'.

> 
> 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
> 
>