netdev - [PATCH bpf-next v5 2/9] bpf: Enable read/write access to skb metadata through a dynptr

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Message-Id: <20250731-skb-metadata-thru-dynptr-v5-2-f02f6b5688dc@cloudflare.com>
Date: Thu, 31 Jul 2025 12:28:16 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: bpf@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>, 
 Andrii Nakryiko <andrii@...nel.org>, Arthur Fabre <arthur@...hurfabre.com>, 
 Daniel Borkmann <daniel@...earbox.net>, 
 Eduard Zingerman <eddyz87@...il.com>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>, 
 Jesse Brandeburg <jbrandeburg@...udflare.com>, 
 Joanne Koong <joannelkoong@...il.com>, 
 Lorenzo Bianconi <lorenzo@...nel.org>, 
 Martin KaFai Lau <martin.lau@...ux.dev>, 
 Toke Høiland-Jørgensen <thoiland@...hat.com>, 
 Yan Zhai <yan@...udflare.com>, kernel-team@...udflare.com, 
 netdev@...r.kernel.org, Stanislav Fomichev <sdf@...ichev.me>
Subject: [PATCH bpf-next v5 2/9] bpf: Enable read/write access to skb
 metadata through a dynptr

Now that we can create a dynptr to skb metadata, make reads to the metadata
area possible with bpf_dynptr_read() or through a bpf_dynptr_slice(), and
make writes to the metadata area possible with bpf_dynptr_write() or
through a bpf_dynptr_slice_rdwr().

Reviewed-by: Jesse Brandeburg <jbrandeburg@...udflare.com>
Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 include/linux/filter.h |  6 ++++++
 kernel/bpf/helpers.c   | 10 +++++++---
 net/core/filter.c      | 10 ++++++++++
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 1e7fd3ee759e..9ed21b65e2e9 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -1784,6 +1784,7 @@ int __bpf_xdp_store_bytes(struct xdp_buff *xdp, u32 offset, void *buf, u32 len);
 void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len);
 void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
 		      void *buf, unsigned long len, bool flush);
+void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset);
 #else /* CONFIG_NET */
 static inline int __bpf_skb_load_bytes(const struct sk_buff *skb, u32 offset,
 				       void *to, u32 len)
@@ -1818,6 +1819,11 @@ static inline void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off, voi
 				    unsigned long len, bool flush)
 {
 }
+
+static inline void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset)
+{
+	return NULL;
+}
 #endif /* CONFIG_NET */
 
 #endif /* __LINUX_FILTER_H__ */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 9552b32208c5..cdffd74ddbe6 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1781,7 +1781,8 @@ static int __bpf_dynptr_read(void *dst, u32 len, const struct bpf_dynptr_kern *s
 	case BPF_DYNPTR_TYPE_XDP:
 		return __bpf_xdp_load_bytes(src->data, src->offset + offset, dst, len);
 	case BPF_DYNPTR_TYPE_SKB_META:
-		return -EOPNOTSUPP; /* not implemented */
+		memmove(dst, bpf_skb_meta_pointer(src->data, src->offset + offset), len);
+		return 0;
 	default:
 		WARN_ONCE(true, "bpf_dynptr_read: unknown dynptr type %d\n", type);
 		return -EFAULT;
@@ -1839,7 +1840,10 @@ int __bpf_dynptr_write(const struct bpf_dynptr_kern *dst, u32 offset, void *src,
 			return -EINVAL;
 		return __bpf_xdp_store_bytes(dst->data, dst->offset + offset, src, len);
 	case BPF_DYNPTR_TYPE_SKB_META:
-		return -EOPNOTSUPP; /* not implemented */
+		if (flags)
+			return -EINVAL;
+		memmove(bpf_skb_meta_pointer(dst->data, dst->offset + offset), src, len);
+		return 0;
 	default:
 		WARN_ONCE(true, "bpf_dynptr_write: unknown dynptr type %d\n", type);
 		return -EFAULT;
@@ -2716,7 +2720,7 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr *p, u32 offset,
 		return buffer__opt;
 	}
 	case BPF_DYNPTR_TYPE_SKB_META:
-		return NULL; /* not implemented */
+		return bpf_skb_meta_pointer(ptr->data, ptr->offset + offset);
 	default:
 		WARN_ONCE(true, "unknown dynptr type %d\n", type);
 		return NULL;
diff --git a/net/core/filter.c b/net/core/filter.c
index 83df346b474e..6cce89bef456 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -11987,6 +11987,16 @@ bpf_sk_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	return func;
 }
 
+/**
+ * bpf_skb_meta_pointer() - Gets a mutable pointer within the skb metadata area.
+ * @skb: socket buffer carrying the metadata
+ * @offset: offset into the metadata area, must be <= skb_metadata_len()
+ */
+void *bpf_skb_meta_pointer(struct sk_buff *skb, u32 offset)
+{
+	return skb_metadata_end(skb) - skb_metadata_len(skb) + offset;
+}
+
 __bpf_kfunc_start_defs();
 __bpf_kfunc int bpf_dynptr_from_skb(struct __sk_buff *s, u64 flags,
 				    struct bpf_dynptr *ptr__uninit)

-- 
2.43.0