netdev - Re: [PATCH bpf-next v3 3/7] bpf: minimize number of allocated lsm slots per program

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Date:   Mon, 11 Apr 2022 11:44:54 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     Martin KaFai Lau <kafai@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org
Subject: Re: [PATCH bpf-next v3 3/7] bpf: minimize number of allocated lsm
 slots per program

On Sat, Apr 9, 2022 at 11:10 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Fri, Apr 08, 2022 at 03:56 PM -07, Martin KaFai Lau wrote:
> > On Thu, Apr 07, 2022 at 03:31:08PM -0700, Stanislav Fomichev wrote:
> >> Previous patch adds 1:1 mapping between all 211 LSM hooks
> >> and bpf_cgroup program array. Instead of reserving a slot per
> >> possible hook, reserve 10 slots per cgroup for lsm programs.
> >> Those slots are dynamically allocated on demand and reclaimed.
> >> This still adds some bloat to the cgroup and brings us back to
> >> roughly pre-cgroup_bpf_attach_type times.
> >>
> >> It should be possible to eventually extend this idea to all hooks if
> >> the memory consumption is unacceptable and shrink overall effective
> >> programs array.
> >>
> >> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> >> ---
> >>  include/linux/bpf-cgroup-defs.h |  4 +-
> >>  include/linux/bpf_lsm.h         |  6 ---
> >>  kernel/bpf/bpf_lsm.c            |  9 ++--
> >>  kernel/bpf/cgroup.c             | 96 ++++++++++++++++++++++++++++-----
> >>  4 files changed, 90 insertions(+), 25 deletions(-)
> >>
> >> diff --git a/include/linux/bpf-cgroup-defs.h b/include/linux/bpf-cgroup-defs.h
> >> index 6c661b4df9fa..d42516e86b3a 100644
> >> --- a/include/linux/bpf-cgroup-defs.h
> >> +++ b/include/linux/bpf-cgroup-defs.h
> >> @@ -10,7 +10,9 @@
> >>
> >>  struct bpf_prog_array;
> >>
> >> -#define CGROUP_LSM_NUM 211 /* will be addressed in the next patch */
> >> +/* Maximum number of concurrently attachable per-cgroup LSM hooks.
> >> + */
> >> +#define CGROUP_LSM_NUM 10
> > hmm...only 10 different lsm hooks (or 10 different attach_btf_ids) can
> > have BPF_LSM_CGROUP programs attached.  This feels quite limited but having
> > a static 211 (and potentially growing in the future) is not good either.
> > I currently do not have a better idea also. :/
> >
> > Have you thought about other dynamic schemes or they would be too slow ?
>
> As long as we're talking ideas - how about a 2-level lookup?
>
> L1: 0..255 -> { 0..31, -1 }, where -1 is inactive cgroup_bp_attach_type
> L2: 0..31 -> struct bpf_prog_array * for cgroup->bpf.effective[],
>              struct hlist_head [^1]  for cgroup->bpf.progs[],
>              u32                     for cgroup->bpf.flags[],
>
> This way we could have 32 distinct _active_ attachment types for each
> cgroup instance, to be shared among regular cgroup attach types and BPF
> LSM attach types.
>
> It is 9 extra slots in comparison to today, so if anyone has cgroups
> that make use of all available attach types at the same time, we don't
> break their setup.
>
> The L1 lookup table would still a few slots for new cgroup [^2] or LSM
> hooks:
>
>   256 - 23 (cgroup attach types) - 211 (LSM hooks) = 22
>
> Memory bloat:
>
>  +256 B - L1 lookup table
>  + 72 B - extra effective[] slots
>  + 72 B - extra progs[] slots
>  + 36 B - extra flags[] slots
>  -184 B - savings from switching to hlist_head
>  ------
>  +252 B per cgroup instance
>
> Total cgroup_bpf{} size change - 720 B -> 968 B.
>
> WDYT?

Sounds workable, thanks! Let me try and see how it goes. I guess we
don't even have to increase the size of the effective array with this
mode,;having 23 unique slots per cgroup seems like a good start? So
the cgroup_bpf{} growth would be +256B L1 (technically, we only need 5
bits per entry, so can shrink to 160B) -185B for hlist_head

> [^1] It looks like we can easily switch from cgroup->bpf.progs[] from
>      list_head to hlist_head and save some bytes!
>
>      We only access the list tail in __cgroup_bpf_attach(). We can
>      either iterate over the list and eat the cost there or push the new
>      prog onto the front.
>
>      I think we treat cgroup->bpf.progs[] everywhere like an unordered
>      set. Except for __cgroup_bpf_query, where the user might notice the
>      order change in the BPF_PROG_QUERY dump.


[...]

> [^2] Unrelated, but we would like to propose a
>      CGROUP_INET[46]_POST_CONNECT hook in the near future to make it
>      easier to bind UDP sockets to 4-tuple without creating conflicts:
>
>      https://github.com/cloudflare/cloudflare-blog/tree/master/2022-02-connectx/ebpf_connect4

Do you think those new lsm hooks can be used instead? If not, what's missing?