netdev - [PATCH bpf-next 01/13] bpf: Ignore dynptr offset in skb data access

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Message-Id: <20250630-skb-metadata-thru-dynptr-v1-1-f17da13625d8@cloudflare.com>
Date: Mon, 30 Jun 2025 16:55:34 +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 01/13] bpf: Ignore dynptr offset in skb data
 access

Prepare to use (struct bpf_dynptr)->offset to distinguish between an skb
dynptr for the payload vs the metadata area.

ptr->offset is always set to zero by bpf_dynptr_from_skb(). We don't need
to account for it on access.

Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 kernel/bpf/helpers.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index f48fa3fe8dec..40c18b37ab05 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1776,7 +1776,7 @@ static int __bpf_dynptr_read(void *dst, u32 len, const struct bpf_dynptr_kern *s
 		memmove(dst, src->data + src->offset + offset, len);
 		return 0;
 	case BPF_DYNPTR_TYPE_SKB:
-		return __bpf_skb_load_bytes(src->data, src->offset + offset, dst, len);
+		return __bpf_skb_load_bytes(src->data, offset, dst, len);
 	case BPF_DYNPTR_TYPE_XDP:
 		return __bpf_xdp_load_bytes(src->data, src->offset + offset, dst, len);
 	default:
@@ -1829,8 +1829,7 @@ int __bpf_dynptr_write(const struct bpf_dynptr_kern *dst, u32 offset, void *src,
 		memmove(dst->data + dst->offset + offset, src, len);
 		return 0;
 	case BPF_DYNPTR_TYPE_SKB:
-		return __bpf_skb_store_bytes(dst->data, dst->offset + offset, src, len,
-					     flags);
+		return __bpf_skb_store_bytes(dst->data, offset, src, len, flags);
 	case BPF_DYNPTR_TYPE_XDP:
 		if (flags)
 			return -EINVAL;
@@ -2695,9 +2694,9 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr *p, u32 offset,
 		return ptr->data + ptr->offset + offset;
 	case BPF_DYNPTR_TYPE_SKB:
 		if (buffer__opt)
-			return skb_header_pointer(ptr->data, ptr->offset + offset, len, buffer__opt);
+			return skb_header_pointer(ptr->data, offset, len, buffer__opt);
 		else
-			return skb_pointer_if_linear(ptr->data, ptr->offset + offset, len);
+			return skb_pointer_if_linear(ptr->data, offset, len);
 	case BPF_DYNPTR_TYPE_XDP:
 	{
 		void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset + offset, len);

-- 
2.43.0