Date:   Mon, 23 May 2022 22:40:07 -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 03/11] bpf: per-cgroup lsm flavor

On Mon, May 23, 2022 at 07:15:03PM -0700, Stanislav Fomichev wrote:
> ,
> 
> On Fri, May 20, 2022 at 5:53 PM Martin KaFai Lau <kafai@...com> wrote:
> >
> > On Wed, May 18, 2022 at 03:55:23PM -0700, Stanislav Fomichev wrote:
> >
> > [ ... ]
> >
> > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > index ea3674a415f9..70cf1dad91df 100644
> > > --- a/include/linux/bpf.h
> > > +++ b/include/linux/bpf.h
> > > @@ -768,6 +768,10 @@ void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start, struct bpf_tramp_
> > >  u64 notrace __bpf_prog_enter_sleepable(struct bpf_prog *prog, struct bpf_tramp_run_ctx *run_ctx);
> > >  void notrace __bpf_prog_exit_sleepable(struct bpf_prog *prog, u64 start,
> > >                                      struct bpf_tramp_run_ctx *run_ctx);
> > > +u64 notrace __bpf_prog_enter_lsm_cgroup(struct bpf_prog *prog,
> > > +                                     struct bpf_tramp_run_ctx *run_ctx);
> > > +void notrace __bpf_prog_exit_lsm_cgroup(struct bpf_prog *prog, u64 start,
> > > +                                     struct bpf_tramp_run_ctx *run_ctx);
> > >  void notrace __bpf_tramp_enter(struct bpf_tramp_image *tr);
> > >  void notrace __bpf_tramp_exit(struct bpf_tramp_image *tr);
> > >
> > > @@ -1035,6 +1039,7 @@ struct bpf_prog_aux {
> > >       u64 load_time; /* ns since boottime */
> > >       u32 verified_insns;
> > >       struct bpf_map *cgroup_storage[MAX_BPF_CGROUP_STORAGE_TYPE];
> > > +     int cgroup_atype; /* enum cgroup_bpf_attach_type */
> > >       char name[BPF_OBJ_NAME_LEN];
> > >  #ifdef CONFIG_SECURITY
> > >       void *security;
> > > @@ -1107,6 +1112,12 @@ struct bpf_tramp_link {
> > >       u64 cookie;
> > >  };
> > >
> > > +struct bpf_shim_tramp_link {
> > > +     struct bpf_tramp_link tramp_link;
> > > +     struct bpf_trampoline *tr;
> > > +     atomic64_t refcnt;
> > There is already a refcnt in 'struct bpf_link'.
> > Reuse that one if possible.
> 
> I was assuming that having a per-bpf_shim_tramp_link recfnt might be
> more readable. I'll switch to the one from bpf_link per comments
> below.
> 
> > [ ... ]
> >
> > > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > > index 01ce78c1df80..c424056f0b35 100644
> > > --- a/kernel/bpf/trampoline.c
> > > +++ b/kernel/bpf/trampoline.c
> > > @@ -11,6 +11,8 @@
> > >  #include <linux/rcupdate_wait.h>
> > >  #include <linux/module.h>
> > >  #include <linux/static_call.h>
> > > +#include <linux/bpf_verifier.h>
> > > +#include <linux/bpf_lsm.h>
> > >
> > >  /* dummy _ops. The verifier will operate on target program's ops. */
> > >  const struct bpf_verifier_ops bpf_extension_verifier_ops = {
> > > @@ -497,6 +499,163 @@ int bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_trampolin
> > >       return err;
> > >  }
> > >
> > > +#if defined(CONFIG_BPF_JIT) && defined(CONFIG_BPF_SYSCALL)
> > > +static struct bpf_shim_tramp_link *cgroup_shim_alloc(const struct bpf_prog *prog,
> > > +                                                  bpf_func_t bpf_func)
> > > +{
> > > +     struct bpf_shim_tramp_link *shim_link = NULL;
> > > +     struct bpf_prog *p;
> > > +
> > > +     shim_link = kzalloc(sizeof(*shim_link), GFP_USER);
> > > +     if (!shim_link)
> > > +             return NULL;
> > > +
> > > +     p = bpf_prog_alloc(1, 0);
> > > +     if (!p) {
> > > +             kfree(shim_link);
> > > +             return NULL;
> > > +     }
> > > +
> > > +     p->jited = false;
> > > +     p->bpf_func = bpf_func;
> > > +
> > > +     p->aux->cgroup_atype = prog->aux->cgroup_atype;
> > > +     p->aux->attach_func_proto = prog->aux->attach_func_proto;
> > > +     p->aux->attach_btf_id = prog->aux->attach_btf_id;
> > > +     p->aux->attach_btf = prog->aux->attach_btf;
> > > +     btf_get(p->aux->attach_btf);
> > > +     p->type = BPF_PROG_TYPE_LSM;
> > > +     p->expected_attach_type = BPF_LSM_MAC;
> > > +     bpf_prog_inc(p);
> > > +     bpf_link_init(&shim_link->tramp_link.link, BPF_LINK_TYPE_TRACING, NULL, p);
> > > +     atomic64_set(&shim_link->refcnt, 1);
> > > +
> > > +     return shim_link;
> > > +}
> > > +
> > > +static struct bpf_shim_tramp_link *cgroup_shim_find(struct bpf_trampoline *tr,
> > > +                                                 bpf_func_t bpf_func)
> > > +{
> > > +     struct bpf_tramp_link *link;
> > > +     int kind;
> > > +
> > > +     for (kind = 0; kind < BPF_TRAMP_MAX; kind++) {
> > > +             hlist_for_each_entry(link, &tr->progs_hlist[kind], tramp_hlist) {
> > > +                     struct bpf_prog *p = link->link.prog;
> > > +
> > > +                     if (p->bpf_func == bpf_func)
> > > +                             return container_of(link, struct bpf_shim_tramp_link, tramp_link);
> > > +             }
> > > +     }
> > > +
> > > +     return NULL;
> > > +}
> > > +
> > > +static void cgroup_shim_put(struct bpf_shim_tramp_link *shim_link)
> > > +{
> > > +     if (shim_link->tr)
> > I have been spinning back and forth with this "shim_link->tr" test and
> > the "!shim_link->tr" test below with an atomic64_dec_and_test() test
> > in between  :)
> 
> I did this dance so I can call cgroup_shim_put from
> bpf_trampoline_link_cgroup_shim, I guess that's confusing.
> bpf_trampoline_link_cgroup_shim can call cgroup_shim_put when
> __bpf_trampoline_link_prog fails (shim_prog->tr==NULL);
> cgroup_shim_put can be also called to unlink the prog from the
> trampoline (shim_prog->tr!=NULL).
> 
> > > +             bpf_trampoline_put(shim_link->tr);
> > Why put(tr) here?
> >
> > Intuitive thinking is that should be done after __bpf_trampoline_unlink_prog(.., tr)
> > which is still using the tr.
> > or I missed something inside __bpf_trampoline_unlink_prog(..., tr) ?
> >
> > > +
> > > +     if (!atomic64_dec_and_test(&shim_link->refcnt))
> > > +             return;
> > > +
> > > +     if (!shim_link->tr)
> > And this is only for the error case in bpf_trampoline_link_cgroup_shim()?
> > Can it be handled locally in bpf_trampoline_link_cgroup_shim()
> > where it could actually happen ?
> 
> Yeah, agreed, I'll move the cleanup path to
> bpf_trampoline_link_cgroup_shim to make it less confusing here.
> 
> > > +             return;
> > > +
> > > +     WARN_ON_ONCE(__bpf_trampoline_unlink_prog(&shim_link->tramp_link, shim_link->tr));
> > > +     kfree(shim_link);
> > How about shim_link->tramp_link.link.prog, is the prog freed ?
> >
> > Considering the bpf_link_put() does bpf_prog_put(link->prog).
> > Is there a reason the bpf_link_put() not used and needs to
> > manage its own shim_link->refcnt here ?
> 
> Good catch, I've missed the bpf_prog_put(link->prog) part. Let me see
> if I can use the link's refcnt, it seems like I can define my own
> link->ops->dealloc to call __bpf_trampoline_unlink_prog and the rest
> will be taken care of.
> 
> > > +}
> > > +
> > > +int bpf_trampoline_link_cgroup_shim(struct bpf_prog *prog,
> > > +                                 struct bpf_attach_target_info *tgt_info)
> > > +{
> > > +     struct bpf_shim_tramp_link *shim_link = NULL;
> > > +     struct bpf_trampoline *tr;
> > > +     bpf_func_t bpf_func;
> > > +     u64 key;
> > > +     int err;
> > > +
> > > +     key = bpf_trampoline_compute_key(NULL, prog->aux->attach_btf,
> > > +                                      prog->aux->attach_btf_id);
> > > +
> > > +     err = bpf_lsm_find_cgroup_shim(prog, &bpf_func);
> > > +     if (err)
> > > +             return err;
> > > +
> > > +     tr = bpf_trampoline_get(key, tgt_info);
> > > +     if (!tr)
> > > +             return  -ENOMEM;
> > > +
> > > +     mutex_lock(&tr->mutex);
> > > +
> > > +     shim_link = cgroup_shim_find(tr, bpf_func);
> > > +     if (shim_link) {
> > > +             /* Reusing existing shim attached by the other program. */
> > > +             atomic64_inc(&shim_link->refcnt);
> > > +             /* note, we're still holding tr refcnt from above */
> > hmm... why it still needs to hold the tr refcnt ?
> 
> I'm assuming we need to hold the trampoline for as long as shim_prog
> is attached to it, right? Otherwise it gets kfreed.
Each 'attached' cgroup-lsm prog holds the shim_link's refcnt.
shim_link holds both the trampoline's and the shim_prog's refcnt.

As long as there is attached cgroup-lsm prog(s).  shim_link's refcnt
should not be zero.  The shim_link will stay and so does the
shim_link's trampoline and shim_prog.

When the last cgroup-lsm prog is detached, bpf_link_put() should
unlink itself (and its shim_prog) from the trampoline first and
then do a bpf_trampoline_put(tr) and bpf_prog_put(shim_prog).
I think bpf_tracing_link_release() is doing something similar also.

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