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

Yosry Ahmed
2022-10-17T20:19:08Z

On Mon, Oct 17, 2022 at 1:15 PM Yonghong Song <yhs@meta.com> wrote:
>
>
>
> 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.
>

This might not make any sense, but is this doable without a flag?
Basically extend the new map type so that it has some special
behaviors for cgroup attached programs (allocate memory on program
attach, bpf_get_local_storage() automatically gets entry for the
attached cgroup, etc).

> >
> >>
> >> 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
> …