linux-kernel - Re: [External] Re: [PATCH bpf-next] bpf: add bpf_map_lookup_percpu

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Message-ID: <d20aef2a-273a-3183-0923-bde9657d4418@bytedance.com>
Date:   Tue, 10 May 2022 10:41:44 +0800
From:   Feng Zhou <zhoufeng.zf@...edance.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...nel.org>,
        Dave Marchevsky <davemarchevsky@...com>,
        Joanne Koong <joannekoong@...com>,
        Geliang Tang <geliang.tang@...e.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        duanxiongchun@...edance.com,
        Muchun Song <songmuchun@...edance.com>,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        zhouchengming@...edance.com
Subject: Re: [External] Re: [PATCH bpf-next] bpf: add
 bpf_map_lookup_percpu_elem for percpu map

在 2022/5/10 上午9:04, Yosry Ahmed 写道:
> On Mon, May 9, 2022 at 5:34 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
>> On Fri, May 6, 2022 at 7:49 PM Feng zhou <zhoufeng.zf@...edance.com> wrote:
>>> From: Feng Zhou <zhoufeng.zf@...edance.com>
>>>
>>> Trace some functions, such as enqueue_task_fair, need to access the
>>> corresponding cpu, not the current cpu, and bpf_map_lookup_elem percpu map
>>> cannot do it. So add bpf_map_lookup_percpu_elem to accomplish it for
>>> percpu_array_map, percpu_hash_map, lru_percpu_hash_map.
>>>
>>> The implementation method is relatively simple, refer to the implementation
>>> method of map_lookup_elem of percpu map, increase the parameters of cpu, and
>>> obtain it according to the specified cpu.
>>>
>> I don't think it's safe in general to access per-cpu data from another
>> CPU. I'd suggest just having either a ARRAY_OF_MAPS or adding CPU ID
>> as part of the key, if you need such a custom access pattern.
> I actually just sent an RFC patch series containing a similar patch
> for the exact same purpose. There are instances in the kernel where
> per-cpu data is accessed from other cpus (e.g.
> mem_cgroup_css_rstat_flush()). I believe, like any other variable,
> percpu data can be safe or not safe to access, based on the access
> pattern. It is up to the user to coordinate accesses to the variable.
>
> For example, in my use case, one of the accessors only reads percpu
> values of different cpus, so it should be safe. If a user accesses
> percpu data of another cpu without guaranteeing safety, they corrupt
> their own data. I understand that the main purpose of percpu data is
> lockless (and therefore fast) access, but in some use cases the user
> may be able to safely (and locklessly) access the data concurrently.
>

Regarding data security, I think users need to consider before using it, 
such
as hook enqueue_task_fair, the function itself takes the rq lock of the
corresponding cpu, there is no problem, and the kernel only provides a 
method,
like bpf_per_cpu_ptr and bpf_this_cpu_ptr, data security needs to be 
guaranteed
by users in different scenarios, such as using bpf_spin_lock.


>>> Signed-off-by: Feng Zhou <zhoufeng.zf@...edance.com>
>>> ---
>>>   include/linux/bpf.h            |  2 ++
>>>   include/uapi/linux/bpf.h       |  9 +++++++++
>>>   kernel/bpf/arraymap.c          | 15 +++++++++++++++
>>>   kernel/bpf/core.c              |  1 +
>>>   kernel/bpf/hashtab.c           | 32 ++++++++++++++++++++++++++++++++
>>>   kernel/bpf/helpers.c           | 18 ++++++++++++++++++
>>>   kernel/bpf/verifier.c          | 17 +++++++++++++++--
>>>   kernel/trace/bpf_trace.c       |  2 ++
>>>   tools/include/uapi/linux/bpf.h |  9 +++++++++
>>>   9 files changed, 103 insertions(+), 2 deletions(-)
>>>
>> [...]