bpf: Fix bpf_iter's task iterator logic

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.

Fixes: eaaacd23 ("bpf: Add task and task/file iterator targets")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200514055137.1564581-1-andriin@fb.com

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;