On Wed, May 13, 2020 at 2:23 PM Andrii Nakryiko <andr...@fb.com> wrote:
>
> 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 <y...@fb.com>
> Fixes: eaaacd23910f ("bpf: Add task and task/file iterator targets")
> Signed-off-by: Andrii Nakryiko <andr...@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++;
../kernel/bpf/task_iter.c: In function ‘task_seq_get_next’:
../kernel/bpf/task_iter.c:35:4: warning: value computed is not used
[-Wunused-value]
35 | *tid++;
| ^~~~~~
