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

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Message-ID: <e5e986a0-0bb9-6611-77f0-f8472346965e@bytedance.com>
Date:   Wed, 6 Sep 2023 20:37:53 +0800
From:   Chuyi Zhou <zhouchuyi@...edance.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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

Hello, Alexei.

在 2023/9/6 04:09, Alexei Starovoitov 写道:
> 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.

Thanks for your review!

I think bpf_iter_process_{new, next, destroy} should be protected by 
bpf_rcu_read_lock/unlock explicitly whether the prog is sleepable or 
not, right? I'm not very familiar with the BPF verifier, but I believe 
there is still a risk in directly calling these kfuns even if 
in_rcu_cs() is true.

Maby what we actually need here is to enforce BPF verifier to check 
env->cur_state->active_rcu_lock is true when we want to call these kfuncs.

Thanks.