netdev - [PATCH bpf-next 2/3] bpf: add kfunc for skb refcounting

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Message-Id: <20250220134503.835224-3-maciej.fijalkowski@intel.com>
Date: Thu, 20 Feb 2025 14:45:02 +0100
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: bpf@...r.kernel.org,
	ast@...nel.org,
	daniel@...earbox.net,
	andrii@...nel.org
Cc: netdev@...r.kernel.org,
	magnus.karlsson@...el.com,
	martin.lau@...ux.dev,
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Subject: [PATCH bpf-next 2/3] bpf: add kfunc for skb refcounting

These have been mostly taken from Amery Hung's work related to bpf qdisc
implementation. bpf_skb_{acquire,release}() are for increment/decrement
sk_buff::users whereas bpf_skb_destroy() is called for map entries that
have not been released and map is being wiped out from system.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
---
 net/core/filter.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 2ec162dd83c4..9bd2701be088 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12064,6 +12064,56 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
 
 __bpf_kfunc_end_defs();
 
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "Global functions as their definitions will be in vmlinux BTF");
+
+/* bpf_skb_acquire - Acquire a reference to an skb. An skb acquired by this
+ * kfunc which is not stored in a map as a kptr, must be released by calling
+ * bpf_skb_release().
+ * @skb: The skb on which a reference is being acquired.
+ */
+__bpf_kfunc struct sk_buff *bpf_skb_acquire(struct sk_buff *skb)
+{
+	if (refcount_inc_not_zero(&skb->users))
+		return skb;
+	return NULL;
+}
+
+/* bpf_skb_release - Release the reference acquired on an skb.
+ * @skb: The skb on which a reference is being released.
+ */
+__bpf_kfunc void bpf_skb_release(struct sk_buff *skb)
+{
+	skb_unref(skb);
+}
+
+/* bpf_skb_destroy - Release an skb reference acquired and exchanged into
+ * an allocated object or a map.
+ * @skb: The skb on which a reference is being released.
+ */
+__bpf_kfunc void bpf_skb_destroy(struct sk_buff *skb)
+{
+	(void)skb_unref(skb);
+	consume_skb(skb);
+}
+
+__diag_pop();
+
+BTF_KFUNCS_START(skb_kfunc_btf_ids)
+BTF_ID_FLAGS(func, bpf_skb_acquire, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_skb_release, KF_RELEASE)
+BTF_KFUNCS_END(skb_kfunc_btf_ids)
+
+static const struct btf_kfunc_id_set skb_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set   = &skb_kfunc_btf_ids,
+};
+
+BTF_ID_LIST(skb_kfunc_dtor_ids)
+BTF_ID(struct, sk_buff)
+BTF_ID_FLAGS(func, bpf_skb_destroy, KF_RELEASE)
+
 int bpf_dynptr_from_skb_rdonly(struct __sk_buff *skb, u64 flags,
 			       struct bpf_dynptr *ptr__uninit)
 {
@@ -12117,6 +12167,13 @@ static const struct btf_kfunc_id_set bpf_kfunc_set_tcp_reqsk = {
 
 static int __init bpf_kfunc_init(void)
 {
+	const struct btf_id_dtor_kfunc skb_kfunc_dtors[] = {
+		{
+			.btf_id       = skb_kfunc_dtor_ids[0],
+			.kfunc_btf_id = skb_kfunc_dtor_ids[1]
+		},
+	};
+
 	int ret;
 
 	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_kfunc_set_skb);
@@ -12133,6 +12190,11 @@ static int __init bpf_kfunc_init(void)
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &bpf_kfunc_set_xdp);
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
 					       &bpf_kfunc_set_sock_addr);
+	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &skb_kfunc_set);
+
+	ret = ret ?: register_btf_id_dtor_kfuncs(skb_kfunc_dtors,
+						 ARRAY_SIZE(skb_kfunc_dtors),
+						 THIS_MODULE);
 	return ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_kfunc_set_tcp_reqsk);
 }
 late_initcall(bpf_kfunc_init);
-- 
2.43.0