netdev - [PATCH bpf-next v3 02/16] net: Preserve metadata on pskb_expand

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Message-Id: <20251026-skb-meta-rx-path-v3-2-37cceebb95d3@cloudflare.com>
Date: Sun, 26 Oct 2025 15:18:22 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: bpf@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
 Martin KaFai Lau <martin.lau@...ux.dev>, 
 Daniel Borkmann <daniel@...earbox.net>, 
 John Fastabend <john.fastabend@...il.com>, 
 Stanislav Fomichev <sdf@...ichev.me>, Alexei Starovoitov <ast@...nel.org>, 
 Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman <eddyz87@...il.com>, 
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, 
 KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>, 
 Jiri Olsa <jolsa@...nel.org>, Arthur Fabre <arthur@...hurfabre.com>, 
 Jesper Dangaard Brouer <hawk@...nel.org>, netdev@...r.kernel.org, 
 kernel-team@...udflare.com
Subject: [PATCH bpf-next v3 02/16] net: Preserve metadata on
 pskb_expand_head

pskb_expand_head() copies headroom, including skb metadata, into the newly
allocated head, but then clears the metadata. As a result, metadata is lost
when BPF helpers trigger an skb head reallocation.

Let the skb metadata remain in the newly created copy of head.

Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 net/core/skbuff.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 6be01454f262..b4fa9aa2df22 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2218,6 +2218,10 @@ EXPORT_SYMBOL(__pskb_copy_fclone);
  *
  *	All the pointers pointing into skb header may change and must be
  *	reloaded after call to this function.
+ *
+ *	Note: If you skb_push() the start of the buffer after reallocating the
+ *	header, call skb_postpush_data_move() first to move the metadata out of
+ *	the way before writing to &sk_buff->data.
  */

 int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
@@ -2289,8 +2293,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	skb->nohdr    = 0;
 	atomic_set(&skb_shinfo(skb)->dataref, 1);

-	skb_metadata_clear(skb);
-
 	/* It is not generally safe to change skb->truesize.
 	 * For the moment, we really care of rx path, or
 	 * when skb is orphaned (not attached to a socket).

-- 
2.43.0