[PATCH bpf-next] bpf: fix bpf_iter's task iterator logic

[Andrii Nakryiko <andriin@fb.com>]

task_seq_get_next might stop prematurely if get_pid_task() fails to get
task_struct. Failure to do so doesn't mean that there are no more tasks with
higher pids. Procfs's iteration algorithm (see next_tgid in fs/proc/base.c)
does a retry in such case. After this fix, instead of stopping prematurely
after about 300 tasks on my server, bpf_iter program now returns >4000, which
sounds much closer to reality.

Cc: Yonghong Song <yhs@fb.com>
Fixes: eaaacd23910f ("bpf: Add task and task/file iterator targets")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 kernel/bpf/task_iter.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index a9b7264dda08..e1836def6738 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -27,9 +27,15 @@ static struct task_struct *task_seq_get_next(struct pid_namespace *ns,
 	struct pid *pid;
 
 	rcu_read_lock();
+retry:
 	pid = idr_get_next(&ns->idr, tid);
-	if (pid)
+	if (pid) {
 		task = get_pid_task(pid, PIDTYPE_PID);
+		if (!task) {
+			*tid++;
+			goto retry;
+		}
+	}
 	rcu_read_unlock();
 
 	return task;
-- 
2.24.1