Re: [RFC nf-next v4 1/2] netfilter: bpf: support prog update

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On 12/29/23 3:54 PM, D. Wythe 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 | 50 ++++++++++++++++++++++++++++++---------------
  1 file changed, 34 insertions(+), 16 deletions(-)

diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
index e502ec0..7c32ccb 100644
--- a/net/netfilter/nf_bpf_link.c
+++ b/net/netfilter/nf_bpf_link.c
@@ -8,26 +8,26 @@
  #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);
-}
-
  struct bpf_nf_link {
  	struct bpf_link link;
  	struct nf_hook_ops hook_ops;
  	struct net *net;
  	u32 dead;
  	const struct nf_defrag_hook *defrag_hook;
+	struct rcu_head head;
  };
+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 +126,7 @@ 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);
  }
static int bpf_nf_link_detach(struct bpf_link *link)
@@ -162,7 +161,22 @@ 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;
+
+	if (nf_link->dead)
+		return -EPERM;
+
+	if (old_prog) {
+		/* target old_prog mismatch */
+		if (!cmpxchg(&link->prog, old_prog, new_prog))
+			return -EPERM;
+	} else {
+		old_prog = xchg(&link->prog, new_prog);
+	}
+
+	bpf_prog_put(old_prog);
+	return err;
  }

I made a mistake here, and I will fix it in the next version.
Sorry for that.

D. Wythe

static const struct bpf_link_ops bpf_nf_link_lops = {
@@ -226,7 +240,11 @@ 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;





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