linux-kernel - Re: [RFC PATCH bpf-next 2/4] bpf: Introduce process open coded iterator kfuncs

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Date:   Tue, 5 Sep 2023 13:09:26 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Chuyi Zhou <zhouchuyi@...edance.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH bpf-next 2/4] bpf: Introduce process open coded
 iterator kfuncs

On Sun, Aug 27, 2023 at 12:21 AM Chuyi Zhou <zhouchuyi@...edance.com> wrote:
>
> This patch adds kfuncs bpf_iter_process_{new,next,destroy} which allow
> creation and manipulation of struct bpf_iter_process in open-coded iterator
> style. BPF programs can use these kfuncs or through bpf_for_each macro to
> iterate all processes in the system.
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@...edance.com>
> ---
>  include/uapi/linux/bpf.h       |  4 ++++
>  kernel/bpf/helpers.c           |  3 +++
>  kernel/bpf/task_iter.c         | 31 +++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  4 ++++
>  tools/lib/bpf/bpf_helpers.h    |  5 +++++
>  5 files changed, 47 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 2a6e9b99564b..cfbd527e3733 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7199,4 +7199,8 @@ struct bpf_iter_css_task {
>         __u64 __opaque[1];
>  } __attribute__((aligned(8)));
>
> +struct bpf_iter_process {
> +       __u64 __opaque[1];
> +} __attribute__((aligned(8)));
> +
>  #endif /* _UAPI__LINUX_BPF_H__ */
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index cf113ad24837..81a2005edc26 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2458,6 +2458,9 @@ BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_iter_css_task_new, KF_ITER_NEW)
>  BTF_ID_FLAGS(func, bpf_iter_css_task_next, KF_ITER_NEXT | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
> +BTF_ID_FLAGS(func, bpf_iter_process_new, KF_ITER_NEW)
> +BTF_ID_FLAGS(func, bpf_iter_process_next, KF_ITER_NEXT | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_iter_process_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_dynptr_adjust)
>  BTF_ID_FLAGS(func, bpf_dynptr_is_null)
>  BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index b1bdba40b684..a6717a76c1e0 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -862,6 +862,37 @@ __bpf_kfunc void bpf_iter_css_task_destroy(struct bpf_iter_css_task *it)
>         kfree(kit->css_it);
>  }
>
> +struct bpf_iter_process_kern {
> +       struct task_struct *tsk;
> +} __attribute__((aligned(8)));
> +
> +__bpf_kfunc int bpf_iter_process_new(struct bpf_iter_process *it)
> +{
> +       struct bpf_iter_process_kern *kit = (void *)it;
> +
> +       BUILD_BUG_ON(sizeof(struct bpf_iter_process_kern) != sizeof(struct bpf_iter_process));
> +       BUILD_BUG_ON(__alignof__(struct bpf_iter_process_kern) !=
> +                                       __alignof__(struct bpf_iter_process));
> +
> +       rcu_read_lock();
> +       kit->tsk = &init_task;
> +       return 0;
> +}
> +
> +__bpf_kfunc struct task_struct *bpf_iter_process_next(struct bpf_iter_process *it)
> +{
> +       struct bpf_iter_process_kern *kit = (void *)it;
> +
> +       kit->tsk = next_task(kit->tsk);
> +
> +       return kit->tsk == &init_task ? NULL : kit->tsk;
> +}
> +
> +__bpf_kfunc void bpf_iter_process_destroy(struct bpf_iter_process *it)
> +{
> +       rcu_read_unlock();
> +}

This iter can be used in all ctx-s which is nice, but let's
make the verifier enforce rcu_read_lock/unlock done by bpf prog
instead of doing in the ctor/dtor of iter, since
in sleepable progs the verifier won't recognize that body is RCU CS.
We'd need to teach the verifier to allow bpf_iter_process_new()
inside in_rcu_cs() and make sure there is no rcu_read_unlock
while BPF_ITER_STATE_ACTIVE.
bpf_iter_process_destroy() would become a nop.