netdev - [PATCH RFC bpf-next 8/9] bpf: Make bpf_skb_change

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Message-Id: <20250929-skb-meta-rx-path-v1-8-de700a7ab1cb@cloudflare.com>
Date: Mon, 29 Sep 2025 16:09:13 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: bpf@...r.kernel.org
Cc: netdev@...r.kernel.org, kernel-team@...udflare.com
Subject: [PATCH RFC bpf-next 8/9] bpf: Make bpf_skb_change_head helper
 metadata-safe

Although bpf_skb_change_head() doesn't move packet data after skb_push(),
skb metadata still needs to be relocated. Use the dedicated helper to
handle it.

Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 net/core/filter.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index c4b18b7fa95e..7a5ee02098dc 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3873,6 +3873,7 @@ static const struct bpf_func_proto sk_skb_change_tail_proto = {
 static inline int __bpf_skb_change_head(struct sk_buff *skb, u32 head_room,
 					u64 flags)
 {
+	const u8 meta_len = skb_metadata_len(skb);
 	u32 max_len = BPF_SKB_MAX_LEN;
 	u32 new_len = skb->len + head_room;
 	int ret;
@@ -3881,7 +3882,7 @@ static inline int __bpf_skb_change_head(struct sk_buff *skb, u32 head_room,
 		     new_len < skb->len))
 		return -EINVAL;
 
-	ret = skb_cow(skb, head_room);
+	ret = skb_cow(skb, meta_len + head_room);
 	if (likely(!ret)) {
 		/* Idea for this helper is that we currently only
 		 * allow to expand on mac header. This means that
@@ -3893,6 +3894,7 @@ static inline int __bpf_skb_change_head(struct sk_buff *skb, u32 head_room,
 		 * for redirection into L2 device.
 		 */
 		__skb_push(skb, head_room);
+		skb_postpush_data_move(skb, head_room, 0);
 		memset(skb->data, 0, head_room);
 		skb_reset_mac_header(skb);
 		skb_reset_mac_len(skb);

-- 
2.43.0