Re: [External] Re: [PATCH bpf-next] bpf: add bpf_map_lookup_percpu_elem for percpu map

[Feng Zhou <zhoufeng.zf@bytedance.com>]

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@gmail.com> wrote:
>> On Fri, May 6, 2022 at 7:49 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote:
>>> From: Feng Zhou <zhoufeng.zf@bytedance.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@bytedance.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(-)
>>>
>> [...]