bpf: task_group_seq_get_next: cleanup the usage of next_thread()

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@redhat.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230905154646.GA24928@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index c4ab9d6cdbe9c81fd13c9cbabc99790b656ea125..4d112510801410759b19e0afe2cf58b7d2a91cfd 100644 (file)
--- 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);