linux-kernel - [PATCH 2/3] bpf: bpf_iter_task_next: use __next_thread() rather than next

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Message-ID: <20231114163237.GA897@redhat.com>
Date:   Tue, 14 Nov 2023 17:32:37 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>
Cc:     Chuyi Zhou <zhouchuyi@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kui-Feng Lee <kuifeng@...com>, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: [PATCH 2/3] bpf: bpf_iter_task_next: use __next_thread() rather than
 next_thread()

Lockless use of next_thread() should be avoided, kernel/bpf/task_iter.c
is the last user and the usage is wrong.

bpf_iter_task_next() can loop forever, "kit->pos == kit->task" can never
happen if kit->pos execs. Change this code to use __next_thread().

With or without this change the usage of kit->pos/task and next_task()
doesn't look nice, see the next patch.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/bpf/task_iter.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 51ae15e2b290..d42e08d0d0b7 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -1015,12 +1015,11 @@ __bpf_kfunc struct task_struct *bpf_iter_task_next(struct bpf_iter_task *it)
 	if (flags == BPF_TASK_ITER_ALL_PROCS)
 		goto get_next_task;
 
-	kit->pos = next_thread(kit->pos);
-	if (kit->pos == kit->task) {
-		if (flags == BPF_TASK_ITER_PROC_THREADS) {
-			kit->pos = NULL;
+	kit->pos = __next_thread(kit->pos);
+	if (!kit->pos) {
+		if (flags == BPF_TASK_ITER_PROC_THREADS)
 			return pos;
-		}
+		kit->pos = kit->task;
 	} else
 		return pos;
 
-- 
2.25.1.362.g51ebf55