linux-kernel - [PATCH 6/6] bpf: task_group_seq_get_next: use __next_thread() rather than next

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Message-ID: <20230825161959.GA16893@redhat.com>
Date:   Fri, 25 Aug 2023 18:19:59 +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: [PATCH 6/6] bpf: task_group_seq_get_next: use __next_thread() rather
 than next_thread()

Lockless use of next_thread() should be avoided, task_group_seq_get_next()
is the last user, it too 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, so we can also change this code to retry
if next_tid == 0.

Signed-off-by: Oleg Nesterov <oleg@...hat.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 7473068ed313..8c847d91cdd9 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -68,15 +68,13 @@ 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;
 
 	if (skip_if_dup_files && task->files == task->group_leader->files)
 		goto retry;
-- 
2.25.1.362.g51ebf55