Re: [PATCH bpf-next v5 1/2] bpf: support BPF_PROG_QUERY for progs attached to sockmap

[Andrii Nakryiko]

On Thu, Jan 13, 2022 at 8:15 AM Jakub Sitnicki <jakub@xxxxxxxxxxxxxx> wrote:
>
> On Thu, Jan 13, 2022 at 10:00 AM CET, Di Zhu wrote:
> > Right now there is no way to query whether BPF programs are
> > attached to a sockmap or not.
> >
> > we can use the standard interface in libbpf to query, such as:
> > bpf_prog_query(mapFd, BPF_SK_SKB_STREAM_PARSER, 0, NULL, ...);
> > the mapFd is the fd of sockmap.
> >
> > Signed-off-by: Di Zhu <zhudi2@xxxxxxxxxx>
> > Acked-by: Yonghong Song <yhs@xxxxxx>
> > ---
> >  include/linux/bpf.h  |  9 +++++
> >  kernel/bpf/syscall.c |  5 +++
> >  net/core/sock_map.c  | 78 ++++++++++++++++++++++++++++++++++++++++----
> >  3 files changed, 85 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 6e947cd91152..c4ca14c9f838 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -2071,6 +2071,9 @@ int bpf_prog_test_run_syscall(struct bpf_prog *prog,
> >  int sock_map_get_from_fd(const union bpf_attr *attr, struct bpf_prog *prog);
> >  int sock_map_prog_detach(const union bpf_attr *attr, enum bpf_prog_type ptype);
> >  int sock_map_update_elem_sys(struct bpf_map *map, void *key, void *value, u64 flags);
> > +int sock_map_bpf_prog_query(const union bpf_attr *attr,
> > +                         union bpf_attr __user *uattr);
> > +
> >  void sock_map_unhash(struct sock *sk);
> >  void sock_map_close(struct sock *sk, long timeout);
> >  #else
> > @@ -2124,6 +2127,12 @@ static inline int sock_map_update_elem_sys(struct bpf_map *map, void *key, void
> >  {
> >       return -EOPNOTSUPP;
> >  }
> > +
> > +static inline int sock_map_bpf_prog_query(const union bpf_attr *attr,
> > +                                       union bpf_attr __user *uattr)
> > +{
> > +     return -EINVAL;
> > +}
> >  #endif /* CONFIG_BPF_SYSCALL */
> >  #endif /* CONFIG_NET && CONFIG_BPF_SYSCALL */
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index fa4505f9b611..9e0631f091a6 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -3318,6 +3318,11 @@ static int bpf_prog_query(const union bpf_attr *attr,
> >       case BPF_FLOW_DISSECTOR:
> >       case BPF_SK_LOOKUP:
> >               return netns_bpf_prog_query(attr, uattr);
> > +     case BPF_SK_SKB_STREAM_PARSER:
> > +     case BPF_SK_SKB_STREAM_VERDICT:
> > +     case BPF_SK_MSG_VERDICT:
> > +     case BPF_SK_SKB_VERDICT:
> > +             return sock_map_bpf_prog_query(attr, uattr);
> >       default:
> >               return -EINVAL;
> >       }
> > diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> > index 1827669eedd6..a424f51041ca 100644
> > --- a/net/core/sock_map.c
> > +++ b/net/core/sock_map.c
> > @@ -1416,38 +1416,50 @@ static struct sk_psock_progs *sock_map_progs(struct bpf_map *map)
> >       return NULL;
> >  }
> >
> > -static int sock_map_prog_update(struct bpf_map *map, struct bpf_prog *prog,
> > -                             struct bpf_prog *old, u32 which)
> > +static int sock_map_prog_lookup(struct bpf_map *map, struct bpf_prog ***pprog,
> > +                             u32 which)
> >  {
> >       struct sk_psock_progs *progs = sock_map_progs(map);
> > -     struct bpf_prog **pprog;
> >
> >       if (!progs)
> >               return -EOPNOTSUPP;
> >
> >       switch (which) {
> >       case BPF_SK_MSG_VERDICT:
> > -             pprog = &progs->msg_parser;
> > +             *pprog = &progs->msg_parser;
> >               break;
> >  #if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
> >       case BPF_SK_SKB_STREAM_PARSER:
> > -             pprog = &progs->stream_parser;
> > +             *pprog = &progs->stream_parser;
> >               break;
> >  #endif
> >       case BPF_SK_SKB_STREAM_VERDICT:
> >               if (progs->skb_verdict)
> >                       return -EBUSY;
> > -             pprog = &progs->stream_verdict;
> > +             *pprog = &progs->stream_verdict;
> >               break;
> >       case BPF_SK_SKB_VERDICT:
> >               if (progs->stream_verdict)
> >                       return -EBUSY;
> > -             pprog = &progs->skb_verdict;
> > +             *pprog = &progs->skb_verdict;
> >               break;
> >       default:
> >               return -EOPNOTSUPP;
> >       }
> >
> > +     return 0;
> > +}
> > +
> > +static int sock_map_prog_update(struct bpf_map *map, struct bpf_prog *prog,
> > +                             struct bpf_prog *old, u32 which)
> > +{
> > +     struct bpf_prog **pprog;
> > +     int ret;
> > +
> > +     ret = sock_map_prog_lookup(map, &pprog, which);
> > +     if (ret)
> > +             return ret;
> > +
> >       if (old)
> >               return psock_replace_prog(pprog, prog, old);
> >
> > @@ -1455,6 +1467,58 @@ static int sock_map_prog_update(struct bpf_map *map, struct bpf_prog *prog,
> >       return 0;
> >  }
> >
> > +int sock_map_bpf_prog_query(const union bpf_attr *attr,
> > +                         union bpf_attr __user *uattr)
> > +{
> > +     __u32 __user *prog_ids = u64_to_user_ptr(attr->query.prog_ids);
> > +     u32 prog_cnt = 0, flags = 0, ufd = attr->target_fd;
> > +     struct bpf_prog **pprog;
> > +     struct bpf_prog *prog;
> > +     struct bpf_map *map;
> > +     struct fd f;
> > +     u32 id = 0;
> > +     int ret;
> > +
> > +     if (attr->query.query_flags)
> > +             return -EINVAL;
> > +
> > +     f = fdget(ufd);
> > +     map = __bpf_map_get(f);
> > +     if (IS_ERR(map))
> > +             return PTR_ERR(map);
> > +
> > +     rcu_read_lock();
> > +
> > +     ret = sock_map_prog_lookup(map, &pprog, attr->query.attach_type);
> > +     if (ret)
> > +             goto end;
> > +
> > +     prog = *pprog;
> > +     prog_cnt = !prog ? 0 : 1;
> > +
> > +     if (!attr->query.prog_cnt || !prog_ids || !prog_cnt)
> > +             goto end;
> > +
> > +     id = prog->aux->id;
>
> ^ This looks like a concurrent read/write.

You mean that bpf_prog_load() might be setting it in a different
thread? I think ID is allocated and fixed before prog FD is available
to the user-space, so prog->aux->id is set in stone and immutable in
that regard.

>
> Would wrap with READ_ONCE() and corresponding WRITE_ONCE() in
> bpf_prog_free_id(). See [1] for rationale.
>
> [1] https://github.com/google/kernel-sanitizers/blob/master/other/READ_WRITE_ONCE.md
>
> > +
> > +     /* we do not hold the refcnt, the bpf prog may be released
> > +      * asynchronously and the id would be set to 0.
> > +      */
> > +     if (id == 0)
> > +             prog_cnt = 0;
> > +
> > +end:
> > +     rcu_read_unlock();
> > +
> > +     if (copy_to_user(&uattr->query.attach_flags, &flags, sizeof(flags)) ||
> > +         (id != 0 && copy_to_user(prog_ids, &id, sizeof(u32))) ||
> > +         copy_to_user(&uattr->query.prog_cnt, &prog_cnt, sizeof(prog_cnt)))
> > +             ret = -EFAULT;
> > +
> > +     fdput(f);
> > +     return ret;
> > +}
> > +
> >  static void sock_map_unlink(struct sock *sk, struct sk_psock_link *link)
> >  {
> >       switch (link->map->map_type) {