netdev - [PATCH RFC bpf-next 08/20] trait: Propagate presence of traits to sk

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Message-Id: <20250305-afabre-traits-010-rfc2-v1-8-d0ecfb869797@cloudflare.com>
Date: Wed, 05 Mar 2025 15:32:05 +0100
From: arthur@...hurfabre.com
To: netdev@...r.kernel.org, bpf@...r.kernel.org
Cc: jakub@...udflare.com, hawk@...nel.org, yan@...udflare.com, 
 jbrandeburg@...udflare.com, thoiland@...hat.com, lbiancon@...hat.com, 
 Arthur Fabre <afabre@...udflare.com>
Subject: [PATCH RFC bpf-next 08/20] trait: Propagate presence of traits to
 sk_buff

From: Arthur Fabre <afabre@...udflare.com>

Use a bit in sk_buff to track whether or not an skb stores traits in
it's headroom.

It's tempting to store it in skb_shared_info like the XDP metadata,
but storing it in the skb allows us to more easily handle cases such as
skb clones in a few patches.

(I couldn't find an existing hole to use in sk_buff, so this makes a 4
byte hole... any pointers to a free bit?)

The following patches add support for handful of drivers, in the final
form we'd like to cover all drivers that currently support XDP metadata.

Signed-off-by: Arthur Fabre <afabre@...udflare.com>
---
 include/linux/skbuff.h |  7 +++++++
 include/net/xdp.h      | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 03553c2200ab1c3750b799edb94fa3b94e11a5f1..d7dfee152ebd26ce87a230222e94076aca793adc 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -725,6 +725,12 @@ enum skb_tstamp_type {
 	__SKB_CLOCK_MAX = SKB_CLOCK_TAI,
 };
 
+enum skb_traits_type {
+	SKB_TRAITS_NONE,
+	/* Trait store in headroom, offset by sizeof(struct xdp_frame) */
+	SKB_TRAITS_AFTER_XDP,
+};
+
 /**
  * DOC: Basic sk_buff geometry
  *
@@ -1023,6 +1029,7 @@ struct sk_buff {
 	__u8			csum_not_inet:1;
 #endif
 	__u8			unreadable:1;
+	__u8			traits_type:1;	/* See enum skb_traits_type */
 #if defined(CONFIG_NET_SCHED) || defined(CONFIG_NET_XGRESS)
 	__u16			tc_index;	/* traffic control index */
 #endif
diff --git a/include/net/xdp.h b/include/net/xdp.h
index 84afe07d09efdb2ab0cb78b904f02cb74f9a56b6..e2e7c270efa3cbd11bddd234010b91daee5009a2 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -119,6 +119,12 @@ static __always_inline void xdp_buff_set_frag_pfmemalloc(struct xdp_buff *xdp)
 static bool xdp_data_meta_unsupported(const struct xdp_buff *xdp);
 static void xdp_set_data_meta_invalid(struct xdp_buff *xdp);
 
+static __always_inline void xdp_buff_update_skb(struct xdp_buff *xdp, struct sk_buff *skb)
+{
+	if (!xdp_data_meta_unsupported(xdp))
+		skb->traits_type = SKB_TRAITS_AFTER_XDP;
+}
+
 static __always_inline void *xdp_buff_traits(const struct xdp_buff *xdp)
 {
 	return xdp->data_hard_start + _XDP_FRAME_SIZE;
@@ -298,6 +304,12 @@ xdp_frame_is_frag_pfmemalloc(const struct xdp_frame *frame)
 	return !!(frame->flags & XDP_FLAGS_FRAGS_PF_MEMALLOC);
 }
 
+static __always_inline void xdp_frame_update_skb(struct xdp_frame *frame, struct sk_buff *skb)
+{
+	if (!frame->meta_unsupported)
+		skb->traits_type = SKB_TRAITS_AFTER_XDP;
+}
+
 #define XDP_BULK_QUEUE_SIZE	16
 struct xdp_frame_bulk {
 	int count;

-- 
2.43.0