Re: [PATCH 1/3] bpf: task_group_seq_get_next: use __next_thread() rather than next_thread()

[Yonghong Song <yonghong.song@linux.dev>]

On 11/14/23 11:32 AM, Oleg Nesterov wrote:
> Lockless use of next_thread() should be avoided, kernel/bpf/task_iter.c
> is the last user and the usage is wrong.
>
> task_group_seq_get_next() can return the group leader twice if it races
> with mt-thread exec which changes the group->leader's pid.
>
> Change the main loop to use __next_thread(), kill "next_tid == common->pid"
> check.
>
> __next_thread() can't loop forever, we can also change this code to retry
> if next_tid == 0.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>   kernel/bpf/task_iter.c | 12 +++++-------
>   1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index 26082b97894d..51ae15e2b290 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -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 unless some
task is migrated to other namespace which I think is not possible.

common->ns is assigned as below:
   common->ns = get_pid_ns(task_active_pid_ns(current))
so we are searching tasks in the *current* namespace.

Look at:
pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
{
         struct upid *upid;
         pid_t nr = 0;

         if (pid && ns->level <= pid->level) {
                 upid = &pid->numbers[ns->level];
                 if (upid->ns == ns)
                         nr = upid->nr;
         }
         return nr;
}

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);
         rcu_read_unlock();
         
         return nr;
}

In func pid_nr_ns(), ns->level should be equal to pid->level if pid is
in input parameter 'ns'. and in this case the return value 'nr'
should be none zero.

If this is the case, could you remove
	if (!next_tid)
		goto retry;

Other than above, the change looks good to me.

>   
>   	if (skip_if_dup_files && task->files == task->group_leader->files)
>   		goto retry;