Subject: Re: [PATCH bpf-next 1/3] bpf: introduce pinnable bpf_link abstraction

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 02 Mar 2020 22:40:36 +0100

Andrii Nakryiko <[email-masked]> writes:

> On Mon, Mar 2, 2020 at 2:13 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> Andrii Nakryiko <[email-masked]> writes:
>>
>> > Introduce bpf_link abstraction, representing an attachment of BPF program to
>> > a BPF hook point (e.g., tracepoint, perf event, etc). bpf_link encapsulates
>> > ownership of attached BPF program, reference counting of a link itself, when
>> > reference from multiple anonymous inodes, as well as ensures that release
>> > callback will be called from a process context, so that users can safely take
>> > mutex locks and sleep.
>> >
>> > Additionally, with a new abstraction it's now possible to generalize pinning
>> > of a link object in BPF FS, allowing to explicitly prevent BPF program
>> > detachment on process exit by pinning it in a BPF FS and let it open from
>> > independent other process to keep working with it.
>> >
>> > Convert two existing bpf_link-like objects (raw tracepoint and tracing BPF
>> > program attachments) into utilizing bpf_link framework, making them pinnable
>> > in BPF FS. More FD-based bpf_links will be added in follow up patches.
>> >
>> > Signed-off-by: Andrii Nakryiko <[email-masked]>
>> > ---
>> >  include/linux/bpf.h  |  13 +++
>> >  kernel/bpf/inode.c   |  42 ++++++++-
>> >  kernel/bpf/syscall.c | 209 ++++++++++++++++++++++++++++++++++++-------
>> >  3 files changed, 226 insertions(+), 38 deletions(-)
>> >
>> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> > index 6015a4daf118..f13c78c6f29d 100644
>> > --- a/include/linux/bpf.h
>> > +++ b/include/linux/bpf.h
>> > @@ -1056,6 +1056,19 @@ extern int sysctl_unprivileged_bpf_disabled;
>> >  int bpf_map_new_fd(struct bpf_map *map, int flags);
>> >  int bpf_prog_new_fd(struct bpf_prog *prog);
>> >
>> > +struct bpf_link;
>> > +
>> > +struct bpf_link_ops {
>> > +     void (*release)(struct bpf_link *link);
>> > +};
>> > +
>> > +void bpf_link_init(struct bpf_link *link, const struct bpf_link_ops *ops,
>> > +                struct bpf_prog *prog);
>> > +void bpf_link_inc(struct bpf_link *link);
>> > +void bpf_link_put(struct bpf_link *link);
>> > +int bpf_link_new_fd(struct bpf_link *link);
>> > +struct bpf_link *bpf_link_get_from_fd(u32 ufd);
>> > +
>> >  int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>> >  int bpf_obj_get_user(const char __user *pathname, int flags);
>> >
>> > diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
>> > index 5e40e7fccc21..95087d9f4ed3 100644
>> > --- a/kernel/bpf/inode.c
>> > +++ b/kernel/bpf/inode.c
>> > @@ -25,6 +25,7 @@ enum bpf_type {
>> >       BPF_TYPE_UNSPEC = 0,
>> >       BPF_TYPE_PROG,
>> >       BPF_TYPE_MAP,
>> > +     BPF_TYPE_LINK,
>> >  };
>> >
>> >  static void *bpf_any_get(void *raw, enum bpf_type type)
>> > @@ -36,6 +37,9 @@ static void *bpf_any_get(void *raw, enum bpf_type type)
>> >       case BPF_TYPE_MAP:
>> >               bpf_map_inc_with_uref(raw);
>> >               break;
>> > +     case BPF_TYPE_LINK:
>> > +             bpf_link_inc(raw);
>> > +             break;
>> >       default:
>> >               WARN_ON_ONCE(1);
>> >               break;
>> > @@ -53,6 +57,9 @@ static void bpf_any_put(void *raw, enum bpf_type type)
>> >       case BPF_TYPE_MAP:
>> >               bpf_map_put_with_uref(raw);
>> >               break;
>> > +     case BPF_TYPE_LINK:
>> > +             bpf_link_put(raw);
>> > +             break;
>> >       default:
>> >               WARN_ON_ONCE(1);
>> >               break;
>> > @@ -63,20 +70,32 @@ static void *bpf_fd_probe_obj(u32 ufd, enum bpf_type *type)
>> >  {
>> >       void *raw;
>> >
>> > -     *type = BPF_TYPE_MAP;
>> >       raw = bpf_map_get_with_uref(ufd);
>> > -     if (IS_ERR(raw)) {
>> > +     if (!IS_ERR(raw)) {
>> > +             *type = BPF_TYPE_MAP;
>> > +             return raw;
>> > +     }
>> > +
>> > +     raw = bpf_prog_get(ufd);
>> > +     if (!IS_ERR(raw)) {
>> >               *type = BPF_TYPE_PROG;
>> > -             raw = bpf_prog_get(ufd);
>> > +             return raw;
>> >       }
>> >
>> > -     return raw;
>> > +     raw = bpf_link_get_from_fd(ufd);
>> > +     if (!IS_ERR(raw)) {
>> > +             *type = BPF_TYPE_LINK;
>> > +             return raw;
>> > +     }
>> > +
>> > +     return ERR_PTR(-EINVAL);
>> >  }
>> >
>> >  static const struct inode_operations bpf_dir_iops;
>> >
>> >  static const struct inode_operations bpf_prog_iops = { };
>> >  static const struct inode_operations bpf_map_iops  = { };
>> > +static const struct inode_operations bpf_link_iops  = { };
>> >
>> >  static struct inode *bpf_get_inode(struct super_block *sb,
>> >                                  const struct inode *dir,
>> > @@ -114,6 +133,8 @@ static int bpf_inode_type(const struct inode *inode, enum bpf_type *type)
>> >               *type = BPF_TYPE_PROG;
>> >       else if (inode->i_op == &bpf_map_iops)
>> >               *type = BPF_TYPE_MAP;
>> > +     else if (inode->i_op == &bpf_link_iops)
>> > +             *type = BPF_TYPE_LINK;
>> >       else
>> >               return -EACCES;
>> >
>> > @@ -335,6 +356,12 @@ static int bpf_mkmap(struct dentry *dentry, umode_t mode, void *arg)
>> >                            &bpffs_map_fops : &bpffs_obj_fops);
>> >  }
>> >
>> > +static int bpf_mklink(struct dentry *dentry, umode_t mode, void *arg)
>> > +{
>> > +     return bpf_mkobj_ops(dentry, mode, arg, &bpf_link_iops,
>> > +                          &bpffs_obj_fops);
>> > +}
>> > +
>> >  static struct dentry *
>> >  bpf_lookup(struct inode *dir, struct dentry *dentry, unsigned flags)
>> >  {
>> > @@ -411,6 +438,9 @@ static int bpf_obj_do_pin(const char __user *pathname, void *raw,
>> >       case BPF_TYPE_MAP:
>> >               ret = vfs_mkobj(dentry, mode, bpf_mkmap, raw);
>> >               break;
>> > +     case BPF_TYPE_LINK:
>> > +             ret = vfs_mkobj(dentry, mode, bpf_mklink, raw);
>> > +             break;
>> >       default:
>> >               ret = -EPERM;
>> >       }
>> > @@ -487,6 +517,8 @@ int bpf_obj_get_user(const char __user *pathname, int flags)
>> >               ret = bpf_prog_new_fd(raw);
>> >       else if (type == BPF_TYPE_MAP)
>> >               ret = bpf_map_new_fd(raw, f_flags);
>> > +     else if (type == BPF_TYPE_LINK)
>> > +             ret = bpf_link_new_fd(raw);
>> >       else
>> >               return -ENOENT;
>> >
>> > @@ -504,6 +536,8 @@ static struct bpf_prog *__get_prog_inode(struct inode *inode, enum bpf_prog_type
>> >
>> >       if (inode->i_op == &bpf_map_iops)
>> >               return ERR_PTR(-EINVAL);
>> > +     if (inode->i_op == &bpf_link_iops)
>> > +             return ERR_PTR(-EINVAL);
>> >       if (inode->i_op != &bpf_prog_iops)
>> >               return ERR_PTR(-EACCES);
>> >
>> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> > index c536c65256ad..fca8de7e7872 100644
>> > --- a/kernel/bpf/syscall.c
>> > +++ b/kernel/bpf/syscall.c
>> > @@ -2173,23 +2173,153 @@ static int bpf_obj_get(const union bpf_attr *attr)
>> >                               attr->file_flags);
>> >  }
>> >
>> > -static int bpf_tracing_prog_release(struct inode *inode, struct file *filp)
>> > +struct bpf_link {
>> > +     atomic64_t refcnt;
>>
>> refcount_t ?
>
> Both bpf_map and bpf_prog stick to atomic64 for their refcounting, so
> I'd like to stay consistent and use refcount that can't possible leak
> resources (which refcount_t can, if it's overflown).

refcount_t is specifically supposed to turn a possible use-after-free on
under/overflow into a warning, isn't it? Not going to insist or anything
here, just found it odd that you'd prefer the other...

-Toke