Re: [PATCH bpf-next v7 05/11] bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, May 24, 2022 at 9:03 PM Stanislav Fomichev <sdf@google.com> wrote:
>
> On Tue, May 24, 2022 at 4:45 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Tue, May 24, 2022 at 10:50 AM Martin KaFai Lau <kafai@fb.com> wrote:
> > >
> > > On Tue, May 24, 2022 at 08:55:04AM -0700, Stanislav Fomichev wrote:
> > > > On Mon, May 23, 2022 at 8:49 PM Martin KaFai Lau <kafai@fb.com> wrote:
> > > > >
> > > > > On Wed, May 18, 2022 at 03:55:25PM -0700, Stanislav Fomichev wrote:
> > > > > > We have two options:
> > > > > > 1. Treat all BPF_LSM_CGROUP the same, regardless of attach_btf_id
> > > > > > 2. Treat BPF_LSM_CGROUP+attach_btf_id as a separate hook point
> > > > > >
> > > > > > I was doing (2) in the original patch, but switching to (1) here:
> > > > > >
> > > > > > * bpf_prog_query returns all attached BPF_LSM_CGROUP programs
> > > > > > regardless of attach_btf_id
> > > > > > * attach_btf_id is exported via bpf_prog_info
> > > > > >
> > > > > > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > > > > > ---
> > > > > >  include/uapi/linux/bpf.h |   5 ++
> > > > > >  kernel/bpf/cgroup.c      | 103 +++++++++++++++++++++++++++------------
> > > > > >  kernel/bpf/syscall.c     |   4 +-
> > > > > >  3 files changed, 81 insertions(+), 31 deletions(-)
> > > > > >
> > > > > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > > > > index b9d2d6de63a7..432fc5f49567 100644
> > > > > > --- a/include/uapi/linux/bpf.h
> > > > > > +++ b/include/uapi/linux/bpf.h
> > > > > > @@ -1432,6 +1432,7 @@ union bpf_attr {
> > > > > >               __u32           attach_flags;
> > > > > >               __aligned_u64   prog_ids;
> > > > > >               __u32           prog_cnt;
> > > > > > +             __aligned_u64   prog_attach_flags; /* output: per-program attach_flags */
> > > > > >       } query;
> > > > > >
> > > > > >       struct { /* anonymous struct used by BPF_RAW_TRACEPOINT_OPEN command */
> > > > > > @@ -5911,6 +5912,10 @@ struct bpf_prog_info {
> > > > > >       __u64 run_cnt;
> > > > > >       __u64 recursion_misses;
> > > > > >       __u32 verified_insns;
> > > > > > +     /* BTF ID of the function to attach to within BTF object identified
> > > > > > +      * by btf_id.
> > > > > > +      */
> > > > > > +     __u32 attach_btf_func_id;
> > > > > >  } __attribute__((aligned(8)));
> > > > > >
> > > > > >  struct bpf_map_info {
> > > > > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > > > > > index a959cdd22870..08a1015ee09e 100644
> > > > > > --- a/kernel/bpf/cgroup.c
> > > > > > +++ b/kernel/bpf/cgroup.c
> > > > > > @@ -1074,6 +1074,7 @@ static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
> > > > > >  static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
> > > > > >                             union bpf_attr __user *uattr)
> > > > > >  {
> > > > > > +     __u32 __user *prog_attach_flags = u64_to_user_ptr(attr->query.prog_attach_flags);
> > > > > >       __u32 __user *prog_ids = u64_to_user_ptr(attr->query.prog_ids);
> > > > > >       enum bpf_attach_type type = attr->query.attach_type;
> > > > > >       enum cgroup_bpf_attach_type atype;
> > > > > > @@ -1081,50 +1082,92 @@ static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
> > > > > >       struct hlist_head *progs;
> > > > > >       struct bpf_prog *prog;
> > > > > >       int cnt, ret = 0, i;
> > > > > > +     int total_cnt = 0;
> > > > > >       u32 flags;
> > > > > >
> > > > > > -     atype = to_cgroup_bpf_attach_type(type);
> > > > > > -     if (atype < 0)
> > > > > > -             return -EINVAL;
> > > > > > +     enum cgroup_bpf_attach_type from_atype, to_atype;
> > > > > >
> > > > > > -     progs = &cgrp->bpf.progs[atype];
> > > > > > -     flags = cgrp->bpf.flags[atype];
> > > > > > +     if (type == BPF_LSM_CGROUP) {
> > > > > > +             from_atype = CGROUP_LSM_START;
> > > > > > +             to_atype = CGROUP_LSM_END;
> > > > > > +     } else {
> > > > > > +             from_atype = to_cgroup_bpf_attach_type(type);
> > > > > > +             if (from_atype < 0)
> > > > > > +                     return -EINVAL;
> > > > > > +             to_atype = from_atype;
> > > > > > +     }
> > > > > >
> > > > > > -     effective = rcu_dereference_protected(cgrp->bpf.effective[atype],
> > > > > > -                                           lockdep_is_held(&cgroup_mutex));
> > > > > > +     for (atype = from_atype; atype <= to_atype; atype++) {
> > > > > > +             progs = &cgrp->bpf.progs[atype];
> > > > > > +             flags = cgrp->bpf.flags[atype];
> > > > > >
> > > > > > -     if (attr->query.query_flags & BPF_F_QUERY_EFFECTIVE)
> > > > > > -             cnt = bpf_prog_array_length(effective);
> > > > > > -     else
> > > > > > -             cnt = prog_list_length(progs);
> > > > > > +             effective = rcu_dereference_protected(cgrp->bpf.effective[atype],
> > > > > > +                                                   lockdep_is_held(&cgroup_mutex));
> > > > > >
> > > > > > -     if (copy_to_user(&uattr->query.attach_flags, &flags, sizeof(flags)))
> > > > > > -             return -EFAULT;
> > > > > > -     if (copy_to_user(&uattr->query.prog_cnt, &cnt, sizeof(cnt)))
> > > > > > +             if (attr->query.query_flags & BPF_F_QUERY_EFFECTIVE)
> > > > > > +                     total_cnt += bpf_prog_array_length(effective);
> > > > > > +             else
> > > > > > +                     total_cnt += prog_list_length(progs);
> > > > > > +     }
> > > > > > +
> > > > > > +     if (type != BPF_LSM_CGROUP)
> > > > > > +             if (copy_to_user(&uattr->query.attach_flags, &flags, sizeof(flags)))
> > > > > > +                     return -EFAULT;
> > > > > > +     if (copy_to_user(&uattr->query.prog_cnt, &total_cnt, sizeof(total_cnt)))
> > > > > >               return -EFAULT;
> > > > > > -     if (attr->query.prog_cnt == 0 || !prog_ids || !cnt)
> > > > > > +     if (attr->query.prog_cnt == 0 || !prog_ids || !total_cnt)
> > > > > >               /* return early if user requested only program count + flags */
> > > > > >               return 0;
> > > > > > -     if (attr->query.prog_cnt < cnt) {
> > > > > > -             cnt = attr->query.prog_cnt;
> > > > > > +
> > > > > > +     if (attr->query.prog_cnt < total_cnt) {
> > > > > > +             total_cnt = attr->query.prog_cnt;
> > > > > >               ret = -ENOSPC;
> > > > > >       }
> > > > > >
> > > > > > -     if (attr->query.query_flags & BPF_F_QUERY_EFFECTIVE) {
> > > > > > -             return bpf_prog_array_copy_to_user(effective, prog_ids, cnt);
> > > > > > -     } else {
> > > > > > -             struct bpf_prog_list *pl;
> > > > > > -             u32 id;
> > > > > > +     for (atype = from_atype; atype <= to_atype; atype++) {
> > > > > > +             if (total_cnt <= 0)
> > > > > > +                     break;
> > > > > >
> > > > > > -             i = 0;
> > > > > > -             hlist_for_each_entry(pl, progs, node) {
> > > > > > -                     prog = prog_list_prog(pl);
> > > > > > -                     id = prog->aux->id;
> > > > > > -                     if (copy_to_user(prog_ids + i, &id, sizeof(id)))
> > > > > > -                             return -EFAULT;
> > > > > > -                     if (++i == cnt)
> > > > > > -                             break;
> > > > > > +             progs = &cgrp->bpf.progs[atype];
> > > > > > +             flags = cgrp->bpf.flags[atype];
> > > > > > +
> > > > > > +             effective = rcu_dereference_protected(cgrp->bpf.effective[atype],
> > > > > > +                                                   lockdep_is_held(&cgroup_mutex));
> > > > > > +
> > > > > > +             if (attr->query.query_flags & BPF_F_QUERY_EFFECTIVE)
> > > > > > +                     cnt = bpf_prog_array_length(effective);
> > > > > > +             else
> > > > > > +                     cnt = prog_list_length(progs);
> > > > > > +
> > > > > > +             if (cnt >= total_cnt)
> > > > > > +                     cnt = total_cnt;
> > > > > > +
> > > > > > +             if (attr->query.query_flags & BPF_F_QUERY_EFFECTIVE) {
> > > > > > +                     ret = bpf_prog_array_copy_to_user(effective, prog_ids, cnt);
> > > > > > +             } else {
> > > > > > +                     struct bpf_prog_list *pl;
> > > > > > +                     u32 id;
> > > > > > +
> > > > > > +                     i = 0;
> > > > > > +                     hlist_for_each_entry(pl, progs, node) {
> > > > > > +                             prog = prog_list_prog(pl);
> > > > > > +                             id = prog->aux->id;
> > > > > > +                             if (copy_to_user(prog_ids + i, &id, sizeof(id)))
> > > > > > +                                     return -EFAULT;
> > > > > > +                             if (++i == cnt)
> > > > > > +                                     break;
> > > > > > +                     }
> > > > > >               }
> > > > > > +
> > > > > > +             if (prog_attach_flags)
> > > > > > +                     for (i = 0; i < cnt; i++)
> > > > > > +                             if (copy_to_user(prog_attach_flags + i, &flags, sizeof(flags)))
> > > > > > +                                     return -EFAULT;
> > > > > > +
> > > > > > +             prog_ids += cnt;
> > > > > > +             total_cnt -= cnt;
> > > > > > +             if (prog_attach_flags)
> > > > > > +                     prog_attach_flags += cnt;
> > > > > >       }
> > > > > >       return ret;
> > > > > >  }
> > > > > > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > > > > > index 5ed2093e51cc..4137583c04a2 100644
> > > > > > --- a/kernel/bpf/syscall.c
> > > > > > +++ b/kernel/bpf/syscall.c
> > > > > > @@ -3520,7 +3520,7 @@ static int bpf_prog_detach(const union bpf_attr *attr)
> > > > > >       }
> > > > > >  }
> > > > > >
> > > > > > -#define BPF_PROG_QUERY_LAST_FIELD query.prog_cnt
> > > > > > +#define BPF_PROG_QUERY_LAST_FIELD query.prog_attach_flags
> > > > > >
> > > > > >  static int bpf_prog_query(const union bpf_attr *attr,
> > > > > >                         union bpf_attr __user *uattr)
> > > > > > @@ -3556,6 +3556,7 @@ static int bpf_prog_query(const union bpf_attr *attr,
> > > > > >       case BPF_CGROUP_SYSCTL:
> > > > > >       case BPF_CGROUP_GETSOCKOPT:
> > > > > >       case BPF_CGROUP_SETSOCKOPT:
> > > > > > +     case BPF_LSM_CGROUP:
> > > > > >               return cgroup_bpf_prog_query(attr, uattr);
> > > > > >       case BPF_LIRC_MODE2:
> > > > > >               return lirc_prog_query(attr, uattr);
> > > > > > @@ -4066,6 +4067,7 @@ static int bpf_prog_get_info_by_fd(struct file *file,
> > > > > >
> > > > > >       if (prog->aux->btf)
> > > > > >               info.btf_id = btf_obj_id(prog->aux->btf);
> > > > > > +     info.attach_btf_func_id = prog->aux->attach_btf_id;
> > > > > Note that exposing prog->aux->attach_btf_id only may not be enough
> > > > > unless it can assume info.attach_btf_id is always referring to btf_vmlinux
> > > > > for all bpf prog types.
> > > >
> > > > We also export btf_id two lines above, right? Btw, I left a comment in
> > > > the bpftool about those btf_ids, I'm not sure how resolve them and
> > > > always assume vmlinux for now.
> > > yeah, that btf_id above is the cgroup-lsm prog's btf_id which has its
> > > func info, line info...etc.   It is not the one the attach_btf_id correspond
> > > to.  attach_btf_id refers to either aux->attach_btf or aux->dst_prog's btf (or
> > > target btf id here).
> > >
> > > It needs a consensus on where this attach_btf_id, target btf id, and
> > > prog_attach_flags should be.  If I read the patch 7 thread correctly,
> > > I think Andrii is suggesting to expose them to userspace through link, so
> > > potentially putting them in bpf_link_info.  The bpf_prog_query will
> > > output a list of link ids.  The same probably applies to
> >
> > Yep and I think it makes sense because link is representing one
> > specific attachment (and I presume flags can be stored inside the link
> > itself as well, right?).
> >
> > But if legacy non-link BPF_PROG_ATTACH is supported then using
> > bpf_link_info won't cover legacy prog-only attachments.
>
> I don't have any attachment to the legacy apis, I'm supporting them
> only because it takes two lines of code; we can go link-only if there
> is an agreement that it's inherently better.
>
> How about I keep sys_bpf(BPF_PROG_QUERY) as is and I do a loop in the
> userspace (for BPF_LSM_CGROUP only) over all links
> (BPF_LINK_GET_NEXT_ID) and will find the the ones with matching prog
> ids (BPF_LINK_GET_FD_BY_ID+BPF_OBJ_GET_INFO_BY_FD)?
>
> That way we keep new fields in bpf_link_info, but we don't have to
> extend sys_bpf(BPF_PROG_QUERY) because there doesn't seem to be a good
> way to do it. Exporting links via new link_fds would mean we'd have to
> support BPF_F_QUERY_EFFECTIVE, but getting an effective array of links
> seems to be messy. If, in the future, we figure out a better way to
> expose a list of attached/effective links per cgroup, we can
> convert/optimize bpftool.

Why not use iter/bpf_link program (see progs/bpf_iter_bpf_link.c for
an example) instead? Once you have struct bpf_link and you know it's
cgroup link, you can cast it to struct bpf_cgroup_link and get access
to prog and cgroup. From cgroup to cgroup_bpf you can even get access
to effective array. Basically whatever kernel has access to you can
have access to from bpftool without extending any UAPIs.

>
> WDYT?