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

From: Oleg Nesterov
Date: Tue Nov 14 2023 - 11:33:52 EST


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@xxxxxxxxxx>
---
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;

if (skip_if_dup_files && task->files == task->group_leader->files)
goto retry;
--
2.25.1.362.g51ebf55