Re: [RFC nf-next v3 1/2] netfilter: bpf: support prog update

[Alexei Starovoitov]

On Wed, Dec 20, 2023 at 6:09 AM D. Wythe <alibuda@xxxxxxxxxxxxxxxxx> wrote:
>
> From: "D. Wythe" <alibuda@xxxxxxxxxxxxxxxxx>
>
> To support the prog update, we need to ensure that the prog seen
> within the hook is always valid. Considering that hooks are always
> protected by rcu_read_lock(), which provide us the ability to
> access the prog under rcu.
>
> Signed-off-by: D. Wythe <alibuda@xxxxxxxxxxxxxxxxx>
> ---
>  net/netfilter/nf_bpf_link.c | 63 ++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 48 insertions(+), 15 deletions(-)
>
> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
> index e502ec0..9bc91d1 100644
> --- a/net/netfilter/nf_bpf_link.c
> +++ b/net/netfilter/nf_bpf_link.c
> @@ -8,17 +8,8 @@
>  #include <net/netfilter/nf_bpf_link.h>
>  #include <uapi/linux/netfilter_ipv4.h>
>
> -static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb,
> -                                   const struct nf_hook_state *s)
> -{
> -       const struct bpf_prog *prog = bpf_prog;
> -       struct bpf_nf_ctx ctx = {
> -               .state = s,
> -               .skb = skb,
> -       };
> -
> -       return bpf_prog_run(prog, &ctx);
> -}
> +/* protect link update in parallel */
> +static DEFINE_MUTEX(bpf_nf_mutex);
>
>  struct bpf_nf_link {
>         struct bpf_link link;
> @@ -26,8 +17,20 @@ struct bpf_nf_link {
>         struct net *net;
>         u32 dead;
>         const struct nf_defrag_hook *defrag_hook;
> +       struct rcu_head head;

I have to point out the same issues as before, but
will ask them differently...

Why do you think above rcu_head is necessary?

>  };
>
> +static unsigned int nf_hook_run_bpf(void *bpf_link, struct sk_buff *skb,
> +                                   const struct nf_hook_state *s)
> +{
> +       const struct bpf_nf_link *nf_link = bpf_link;
> +       struct bpf_nf_ctx ctx = {
> +               .state = s,
> +               .skb = skb,
> +       };
> +       return bpf_prog_run(rcu_dereference_raw(nf_link->link.prog), &ctx);
> +}
> +
>  #if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4) || IS_ENABLED(CONFIG_NF_DEFRAG_IPV6)
>  static const struct nf_defrag_hook *
>  get_proto_defrag_hook(struct bpf_nf_link *link,
> @@ -126,8 +129,7 @@ static void bpf_nf_link_release(struct bpf_link *link)
>  static void bpf_nf_link_dealloc(struct bpf_link *link)
>  {
>         struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link);
> -
> -       kfree(nf_link);
> +       kfree_rcu(nf_link, head);

Why is this needed ?
Have you looked at tcx_link_lops ?

>  }
>
>  static int bpf_nf_link_detach(struct bpf_link *link)
> @@ -162,7 +164,34 @@ static int bpf_nf_link_fill_link_info(const struct bpf_link *link,
>  static int bpf_nf_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
>                               struct bpf_prog *old_prog)
>  {
> -       return -EOPNOTSUPP;
> +       struct bpf_nf_link *nf_link = container_of(link, struct bpf_nf_link, link);
> +       int err = 0;
> +
> +       mutex_lock(&bpf_nf_mutex);

Why do you need this mutex?
What race does it solve?

> +
> +       if (nf_link->dead) {
> +               err = -EPERM;
> +               goto out;
> +       }
> +
> +       /* target old_prog mismatch */
> +       if (old_prog && link->prog != old_prog) {
> +               err = -EPERM;
> +               goto out;
> +       }
> +
> +       old_prog = link->prog;
> +       if (old_prog == new_prog) {
> +               /* don't need update */
> +               bpf_prog_put(new_prog);
> +               goto out;
> +       }
> +
> +       old_prog = xchg(&link->prog, new_prog);
> +       bpf_prog_put(old_prog);
> +out:
> +       mutex_unlock(&bpf_nf_mutex);
> +       return err;
>  }
>
>  static const struct bpf_link_ops bpf_nf_link_lops = {
> @@ -226,7 +255,11 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>
>         link->hook_ops.hook = nf_hook_run_bpf;
>         link->hook_ops.hook_ops_type = NF_HOOK_OP_BPF;
> -       link->hook_ops.priv = prog;
> +
> +       /* bpf_nf_link_release & bpf_nf_link_dealloc() can ensures that link remains
> +        * valid at all times within nf_hook_run_bpf().
> +        */
> +       link->hook_ops.priv = link;
>
>         link->hook_ops.pf = attr->link_create.netfilter.pf;
>         link->hook_ops.priority = attr->link_create.netfilter.priority;
> --
> 1.8.3.1
>