Re: [PATCH bpf-next 2/5] bpf: Implement cgroup storage available to non-cgroup-attached bpf progs

[Yonghong Song <yhs@meta.com>]

On 10/17/22 11:47 AM, Yosry Ahmed wrote:
> On Mon, Oct 17, 2022 at 11:43 AM Stanislav Fomichev <sdf@google.com> wrote:
>>
>> On Mon, Oct 17, 2022 at 11:26 AM Yosry Ahmed <yosryahmed@google.com> wrote:
>>>
>>> On Mon, Oct 17, 2022 at 11:02 AM <sdf@google.com> wrote:
>>>>
>>>> On 10/13, Yonghong Song wrote:
>>>>> Similar to sk/inode/task storage, implement similar cgroup local storage.
>>>>
>>>>> There already exists a local storage implementation for cgroup-attached
>>>>> bpf programs.  See map type BPF_MAP_TYPE_CGROUP_STORAGE and helper
>>>>> bpf_get_local_storage(). But there are use cases such that non-cgroup
>>>>> attached bpf progs wants to access cgroup local storage data. For example,
>>>>> tc egress prog has access to sk and cgroup. It is possible to use
>>>>> sk local storage to emulate cgroup local storage by storing data in
>>>>> socket.
>>>>> But this is a waste as it could be lots of sockets belonging to a
>>>>> particular
>>>>> cgroup. Alternatively, a separate map can be created with cgroup id as
>>>>> the key.
>>>>> But this will introduce additional overhead to manipulate the new map.
>>>>> A cgroup local storage, similar to existing sk/inode/task storage,
>>>>> should help for this use case.
>>>>
>>>>> The life-cycle of storage is managed with the life-cycle of the
>>>>> cgroup struct.  i.e. the storage is destroyed along with the owning cgroup
>>>>> with a callback to the bpf_cgroup_storage_free when cgroup itself
>>>>> is deleted.
>>>>
>>>>> The userspace map operations can be done by using a cgroup fd as a key
>>>>> passed to the lookup, update and delete operations.
>>>>
>>>>
>>>> [..]
>>>>
>>>>> Since map name BPF_MAP_TYPE_CGROUP_STORAGE has been used for old cgroup
>>>>> local
>>>>> storage support, the new map name BPF_MAP_TYPE_CGROUP_LOCAL_STORAGE is
>>>>> used
>>>>> for cgroup storage available to non-cgroup-attached bpf programs. The two
>>>>> helpers are named as bpf_cgroup_local_storage_get() and
>>>>> bpf_cgroup_local_storage_delete().
>>>>
>>>> Have you considered doing something similar to 7d9c3427894f ("bpf: Make
>>>> cgroup storages shared between programs on the same cgroup") where
>>>> the map changes its behavior depending on the key size (see key_size checks
>>>> in cgroup_storage_map_alloc)? Looks like sizeof(int) for fd still
>>>> can be used so we can, in theory, reuse the name..
>>>>
>>>> Pros:
>>>> - no need for a new map name
>>>>
>>>> Cons:
>>>> - existing BPF_MAP_TYPE_CGROUP_STORAGE is already messy; might be not a
>>>>     good idea to add more stuff to it?
>>>>
>>>> But, for the very least, should we also extend
>>>> Documentation/bpf/map_cgroup_storage.rst to cover the new map? We've
>>>> tried to keep some of the important details in there..
>>>
>>> This might be a long shot, but is it possible to switch completely to
>>> this new generic cgroup storage, and for programs that attach to
>>> cgroups we can still do lookups/allocations during attachment like we
>>> do today? IOW, maintain the current API for cgroup progs but switch it
>>> to use this new map type instead.
>>>
>>> It feels like this map type is more generic and can be a superset of
>>> the existing cgroup storage, but I feel like I am missing something.
>>
>> I feel like the biggest issue is that the existing
>> bpf_get_local_storage helper is guaranteed to always return non-null
>> and the verifier doesn't require the programs to do null checks on it;
>> the new helper might return NULL making all existing programs fail the
>> verifier.
> 
> What I meant is, keep the old bpf_get_local_storage helper only for
> cgroup-attached programs like we have today, and add a new generic
> bpf_cgroup_local_storage_get() helper.
> 
> For cgroup-attached programs, make sure a cgroup storage entry is
> allocated and hooked to the helper on program attach time, to keep
> today's behavior constant.
> 
> For other programs, the bpf_cgroup_local_storage_get() will do the
> normal lookup and allocate if necessary.
> 
> Does this make any sense to you?

Right. This is what I plan to do. The map will add a flag to
distinguish the old and new behavior.

> 
>>
>> There might be something else I don't remember at this point (besides
>> that weird per-prog_type that we'd have to emulate as well)..
> 
> Yeah there are things that will need to be emulated, but I feel like
> we may end up with less confusing code (and less code in general).
> 
>>
>>>>
>>>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>>>> ---
>>>>>    include/linux/bpf.h             |   3 +
>>>>>    include/linux/bpf_types.h       |   1 +
>>>>>    include/linux/cgroup-defs.h     |   4 +
>>>>>    include/uapi/linux/bpf.h        |  39 +++++
>>>>>    kernel/bpf/Makefile             |   2 +-
>>>>>    kernel/bpf/bpf_cgroup_storage.c | 280 ++++++++++++++++++++++++++++++++
>>>>>    kernel/bpf/helpers.c            |   6 +
>>>>>    kernel/bpf/syscall.c            |   3 +-
>>>>>    kernel/bpf/verifier.c           |  14 +-
>>>>>    kernel/cgroup/cgroup.c          |   4 +
>>>>>    kernel/trace/bpf_trace.c        |   4 +
>>>>>    scripts/bpf_doc.py              |   2 +
>>>>>    tools/include/uapi/linux/bpf.h  |  39 +++++
>>>>>    13 files changed, 398 insertions(+), 3 deletions(-)
>>>>>    create mode 100644 kernel/bpf/bpf_cgroup_storage.c
[...]