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Message-ID: <20220524055341.eg2nsmgtyflwa3lj@kafai-mbp>
Date: Mon, 23 May 2022 22:53:41 -0700
From: Martin KaFai Lau <kafai@...com>
To: Stanislav Fomichev <sdf@...gle.com>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, ast@...nel.org,
daniel@...earbox.net, andrii@...nel.org
Subject: Re: [PATCH bpf-next v7 04/11] bpf: minimize number of allocated lsm
slots per program
On Mon, May 23, 2022 at 07:14:42PM -0700, Stanislav Fomichev wrote:
> On Fri, May 20, 2022 at 11:56 PM Martin KaFai Lau <kafai@...com> wrote:
> >
> > On Wed, May 18, 2022 at 03:55:24PM -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.
> > >
> > > struct cgroup_bpf {
> > > struct bpf_prog_array * effective[33]; /* 0 264 */
> > > /* --- cacheline 4 boundary (256 bytes) was 8 bytes ago --- */
> > > struct hlist_head progs[33]; /* 264 264 */
> > > /* --- cacheline 8 boundary (512 bytes) was 16 bytes ago --- */
> > > u8 flags[33]; /* 528 33 */
> > >
> > > /* XXX 7 bytes hole, try to pack */
> > >
> > > struct list_head storages; /* 568 16 */
> > > /* --- cacheline 9 boundary (576 bytes) was 8 bytes ago --- */
> > > struct bpf_prog_array * inactive; /* 584 8 */
> > > struct percpu_ref refcnt; /* 592 16 */
> > > struct work_struct release_work; /* 608 72 */
> > >
> > > /* size: 680, cachelines: 11, members: 7 */
> > > /* sum members: 673, holes: 1, sum holes: 7 */
> > > /* last cacheline: 40 bytes */
> > > };
> > >
> > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > ---
> > > include/linux/bpf-cgroup-defs.h | 3 +-
> > > include/linux/bpf_lsm.h | 6 --
> > > kernel/bpf/bpf_lsm.c | 5 --
> > > kernel/bpf/cgroup.c | 135 +++++++++++++++++++++++++++++---
> > > 4 files changed, 125 insertions(+), 24 deletions(-)
> > >
> > > diff --git a/include/linux/bpf-cgroup-defs.h b/include/linux/bpf-cgroup-defs.h
> > > index d5a70a35dace..359d3f16abea 100644
> > > --- a/include/linux/bpf-cgroup-defs.h
> > > +++ b/include/linux/bpf-cgroup-defs.h
> > > @@ -10,7 +10,8 @@
> > >
> > > 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
> > >
> > > enum cgroup_bpf_attach_type {
> > > CGROUP_BPF_ATTACH_TYPE_INVALID = -1,
> > > diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> > > index 7f0e59f5f9be..613de44aa429 100644
> > > --- a/include/linux/bpf_lsm.h
> > > +++ b/include/linux/bpf_lsm.h
> > > @@ -43,7 +43,6 @@ extern const struct bpf_func_proto bpf_inode_storage_delete_proto;
> > > void bpf_inode_storage_free(struct inode *inode);
> > >
> > > int bpf_lsm_find_cgroup_shim(const struct bpf_prog *prog, bpf_func_t *bpf_func);
> > > -int bpf_lsm_hook_idx(u32 btf_id);
> > >
> > > #else /* !CONFIG_BPF_LSM */
> > >
> > > @@ -74,11 +73,6 @@ static inline int bpf_lsm_find_cgroup_shim(const struct bpf_prog *prog,
> > > return -ENOENT;
> > > }
> > >
> > > -static inline int bpf_lsm_hook_idx(u32 btf_id)
> > > -{
> > > - return -EINVAL;
> > > -}
> > > -
> > > #endif /* CONFIG_BPF_LSM */
> > >
> > > #endif /* _LINUX_BPF_LSM_H */
> > > diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> > > index 654c23577ad3..96503c3e7a71 100644
> > > --- a/kernel/bpf/bpf_lsm.c
> > > +++ b/kernel/bpf/bpf_lsm.c
> > > @@ -71,11 +71,6 @@ int bpf_lsm_find_cgroup_shim(const struct bpf_prog *prog,
> > > return 0;
> > > }
> > >
> > > -int bpf_lsm_hook_idx(u32 btf_id)
> > > -{
> > > - return btf_id_set_index(&bpf_lsm_hooks, btf_id);
> > > -}
> > > -
> > > int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> > > const struct bpf_prog *prog)
> > > {
> > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > > index 2c356a38f4cf..a959cdd22870 100644
> > > --- a/kernel/bpf/cgroup.c
> > > +++ b/kernel/bpf/cgroup.c
> > > @@ -132,15 +132,110 @@ unsigned int __cgroup_bpf_run_lsm_current(const void *ctx,
> > > }
> > >
> > > #ifdef CONFIG_BPF_LSM
> > > +struct list_head unused_bpf_lsm_atypes;
> > > +struct list_head used_bpf_lsm_atypes;
> > > +
> > > +struct bpf_lsm_attach_type {
> > > + int index;
> > > + u32 btf_id;
> > > + int usecnt;
> > > + struct list_head atypes;
> > > + struct rcu_head rcu_head;
> > > +};
> > > +
> > > +static int __init bpf_lsm_attach_type_init(void)
> > > +{
> > > + struct bpf_lsm_attach_type *atype;
> > > + int i;
> > > +
> > > + INIT_LIST_HEAD_RCU(&unused_bpf_lsm_atypes);
> > > + INIT_LIST_HEAD_RCU(&used_bpf_lsm_atypes);
> > > +
> > > + for (i = 0; i < CGROUP_LSM_NUM; i++) {
> > > + atype = kzalloc(sizeof(*atype), GFP_KERNEL);
> > > + if (!atype)
> > > + continue;
> > > +
> > > + atype->index = i;
> > > + list_add_tail_rcu(&atype->atypes, &unused_bpf_lsm_atypes);
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +late_initcall(bpf_lsm_attach_type_init);
> > > +
> > > static enum cgroup_bpf_attach_type bpf_lsm_attach_type_get(u32 attach_btf_id)
> > > {
> > > - return CGROUP_LSM_START + bpf_lsm_hook_idx(attach_btf_id);
> > > + struct bpf_lsm_attach_type *atype;
> > > +
> > > + lockdep_assert_held(&cgroup_mutex);
> > > +
> > > + list_for_each_entry_rcu(atype, &used_bpf_lsm_atypes, atypes) {
> > > + if (atype->btf_id != attach_btf_id)
> > > + continue;
> > > +
> > > + atype->usecnt++;
> > > + return CGROUP_LSM_START + atype->index;
> > > + }
> > > +
> > > + atype = list_first_or_null_rcu(&unused_bpf_lsm_atypes, struct bpf_lsm_attach_type, atypes);
> > > + if (!atype)
> > > + return -E2BIG;
> > > +
> > > + list_del_rcu(&atype->atypes);
> > > + atype->btf_id = attach_btf_id;
> > > + atype->usecnt = 1;
> > > + list_add_tail_rcu(&atype->atypes, &used_bpf_lsm_atypes);
> > > +
> > > + return CGROUP_LSM_START + atype->index;
> > > +}
> > > +
> > > +static void bpf_lsm_attach_type_reclaim(struct rcu_head *head)
> > > +{
> > > + struct bpf_lsm_attach_type *atype =
> > > + container_of(head, struct bpf_lsm_attach_type, rcu_head);
> > > +
> > > + atype->btf_id = 0;
> > > + atype->usecnt = 0;
> > > + list_add_tail_rcu(&atype->atypes, &unused_bpf_lsm_atypes);
> > hmm...... no need to hold the cgroup_mutex when changing
> > the unused_bpf_lsm_atypes list ?
> > but it is a rcu callback, so spinlock is needed.
>
> Oh, good point.
>
> > > +}
> > > +
> > > +static void bpf_lsm_attach_type_put(u32 attach_btf_id)
> > > +{
> > > + struct bpf_lsm_attach_type *atype;
> > > +
> > > + lockdep_assert_held(&cgroup_mutex);
> > > +
> > > + list_for_each_entry_rcu(atype, &used_bpf_lsm_atypes, atypes) {
> > > + if (atype->btf_id != attach_btf_id)
> > > + continue;
> > > +
> > > + if (--atype->usecnt <= 0) {
> > > + list_del_rcu(&atype->atypes);
> > > + WARN_ON_ONCE(atype->usecnt < 0);
> > > +
> > > + /* call_rcu here prevents atype reuse within
> > > + * the same rcu grace period.
> > > + * shim programs use __bpf_prog_enter_lsm_cgroup
> > > + * which starts RCU read section.
> > It is a bit unclear for me to think through why
> > there is no need to assign 'shim_prog->aux->cgroup_atype = CGROUP_BPF_ATTACH_TYPE_INVALID'
> > here before reclaim and the shim_prog->bpf_func does not need to check
> > shim_prog->aux->cgroup_atype before using it.
> >
> > It will be very useful to have a few word comments here to explain this.
>
> My thinking is:
> - shim program starts rcu read section (via __bpf_prog_enter_lsm_cgroup)
> - on release (bpf_lsm_attach_type_put) we do
> list_del_rcu(&atype->atypes) to make sure that particular atype is
> "reserved" until grace period and not being reused
> - we won't reuse that particular atype for new attachments until grace period
> - existing shim programs will still use this atype until grace period,
> but we rely on cgroup effective array be empty by that point
> - after grace period, we reclaim that atype
>
> Does it clarify your concern? Am I missing something? Not sure how to
> put it into a small/concise comment :-)
Make sense. Thanks for the explanation.
>
> (maybe moot after your next comment)
>
> > > + */
> > > + call_rcu(&atype->rcu_head, bpf_lsm_attach_type_reclaim);
> > How about doing this bpf_lsm_attach_type_put() in bpf_prog_free_deferred().
> > And only do it for the shim_prog but not the cgroup-lsm prog.
> > The shim_prog is the only one needing cgroup_atype. Then the cgroup_atype
> > naturally can be reused when the shim_prog is being destroyed.
> >
> > bpf_prog_free_deferred has already gone through a rcu grace
> > period (__bpf_prog_put_rcu) and it can block, so cgroup_mutex
> > can be used.
> >
> > The need for the rcu_head here should go away also. The v6 array approach
> > could be reconsidered.
> >
> > The cgroup-lsm prog does not necessarily need to hold a usecnt to the cgroup_atype.
> > Their aux->cgroup_atype can be CGROUP_BPF_ATTACH_TYPE_INVALID.
> > My understanding is the replace_prog->aux->cgroup_atype during attach
> > is an optimization, it can always search again.
>
> I've considered using bpf_prog_free (I think Alexei also suggested
> it?), but not ended up using it because of the situation where the
> program can be attached, then detached but not actually freed (there
> is a link or an fd holding it); in this case we'll be blocking that
> atype reuse. But not sure if it's a real problem?
For cgroup-lsm prog loaded but not attached, it should not block the
atype to be reused. It probably will not as long as the cgroup-lsm
prog is not the one holding the atype's refcnt.
For shim_prog, it only does bpf_prog_free_deferred() when there is
no cgroup-lsm prog attached to it. That will happen after a rcu
grace period which should be fine.
> Let me try to see if it works again, your suggestions make sense.
> (especially the part about cgroup_atype for shim only, I don't like
> all these replace_prog->aux->cgroup_atype = atype in weird places)
Yep. Sounds good. It would be nice if some of the special handling for
BPF_LSM_CGROUP in cgroup.c can be simplified because of this.
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