netdev - [PATCH bpf-next 05/13] bpf: Enable write access to skb metadata with bpf_dynptr

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Message-Id: <20250630-skb-metadata-thru-dynptr-v1-5-f17da13625d8@cloudflare.com>
Date: Mon, 30 Jun 2025 16:55:38 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: bpf@...r.kernel.org
Cc: Alexei Starovoitov <ast@...nel.org>, 
 Arthur Fabre <arthur@...hurfabre.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>, 
 Toke Høiland-Jørgensen <thoiland@...hat.com>, 
 Yan Zhai <yan@...udflare.com>, netdev@...r.kernel.org, 
 kernel-team@...udflare.com, Stanislav Fomichev <sdf@...ichev.me>
Subject: [PATCH bpf-next 05/13] bpf: Enable write access to skb metadata
 with bpf_dynptr_write

Make it possible to write to skb metadata area using the
bpf_dynptr_write() BPF helper.

This prepares ground for access to skb metadata from all BPF hooks
which operate on __sk_buff context.

Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 net/core/filter.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index f71b4b6b09fb..ab6599f42bb7 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12005,8 +12005,15 @@ int bpf_dynptr_skb_write(const struct bpf_dynptr_kern *dst, u32 offset,
 	case SKB_DYNPTR_PAYLOAD:
 		return ____bpf_skb_store_bytes(skb, offset, src, len, flags);
 
-	case SKB_DYNPTR_METADATA:
-		return -EOPNOTSUPP; /* not implemented */
+	case SKB_DYNPTR_METADATA: {
+		u32 meta_len = skb_metadata_len(skb);
+
+		if (len > meta_len || offset > meta_len - len)
+			return -E2BIG; /* out of bounds */
+
+		memmove(skb_metadata_end(skb) - meta_len + offset, src, len);
+		return 0;
+	}
 
 	default:
 		WARN_ONCE(true, "%s: unknown skb dynptr offset %d\n", __func__, dst->offset);

-- 
2.43.0