[PATCH v5 3/6] bpf: add BPF_F_ADJ_ROOM_DECAP_* flags for tunnel decapsulation

Nick Hudson
2026-04-20T10:41:26Z

Add new bpf_skb_adjust_room() decapsulation flags:

- BPF_F_ADJ_ROOM_DECAP_L4_GRE
- BPF_F_ADJ_ROOM_DECAP_L4_UDP
- BPF_F_ADJ_ROOM_DECAP_IPXIP4
- BPF_F_ADJ_ROOM_DECAP_IPXIP6

These flags let BPF programs describe which tunnel layer is being
removed, so later changes can update tunnel-related GSO state
accordingly during decapsulation.

This patch only introduces the UAPI flag definitions and helper
documentation.

Co-developed-by: Max Tottenham <mtottenh@akamai.com>
Signed-off-by: Max Tottenham <mtottenh@akamai.com>
Co-developed-by: Anna Glasgall <aglasgal@akamai.com>
Signed-off-by: Anna Glasgall <aglasgal@akamai.com>
Signed-off-by: Nick Hudson <nhudson@akamai.com>
---
include/uapi/linux/bpf.h | 34 ++++++++++++++++++++++++++++++++--
tools/include/uapi/linux/bpf.h | 34 ++++++++++++++++++++++++++++++++--
2 files changed, 64 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index c021ed8d7b44..4a53e731c554 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3010,8 +3010,34 @@ union bpf_attr {
*
* * **BPF_F_ADJ_ROOM_DECAP_L3_IPV4**,
* **BPF_F_ADJ_ROOM_DECAP_L3_IPV6**:
- * Indicate the new IP header version after decapsulating the outer
- * IP header. Used when the inner and outer IP versions are different.
+ * Indicate the new IP header version after decapsulating the
+ * outer IP header. Used when the inner and outer IP versions
+ * are different. These flags only trigger a protocol change
+ * without clearing any tunnel-specific GSO flags.
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_L4_GRE**:
+ * Clear GRE tunnel GSO flags (SKB_GSO_GRE and SKB_GSO_GRE_CSUM)
+ * when decapsulating a GRE tunnel.
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_L4_UDP**:
+ * Clear UDP tunnel GSO flags (SKB_GSO_UDP_TUNNEL and
+ * SKB_GSO_UDP_TUNNEL_CSUM) when decapsulating a UDP tunnel.
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_IPXIP4**:
+ * Clear IPIP/SIT tunnel GSO flag (SKB_GSO_IPXIP4) when decapsulating
+ * a tunnel with an outer IPv4 header (IPv4-in-IPv4 or IPv6-in-IPv4).
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_IPXIP6**:
+ * Clear IPv6 encapsulation tunnel GSO flag (SKB_GSO_IPXIP6) when
+ * decapsulating a tunnel with an outer IPv6 header (IPv6-in-IPv6
+ * or IPv4-in-IPv6).
+ *
+ * When using the decapsulation flags above, the skb->encapsulation
+ * flag is automatically cleared if all tunnel-specific GSO flags
+ * (SKB_GSO_UDP_TUNNEL, SKB_GSO_UDP_TUNNEL_CSUM, SKB_GSO_GRE,
+ * SKB_GSO_GRE_CSUM, SKB_GSO_IPXIP4, SKB_GSO_IPXIP6) have been
+ * removed from the packet. This handles cases where all tunnel
+ * layers have been decapsulated.
*
* A call to this helper is susceptible to change the underlying
* packet buffer. Therefore, at load time, all checks on pointers
@@ -6221,6 +6247,10 @@ enum bpf_adj_room_flags {
BPF_F_ADJ_ROOM_ENCAP_L2_ETH = (1ULL << 6),
BPF_F_ADJ_ROOM_DECAP_L3_IPV4 = (1ULL << 7),
BPF_F_ADJ_ROOM_DECAP_L3_IPV6 = (1ULL << 8),
+ BPF_F_ADJ_ROOM_DECAP_L4_GRE = (1ULL << 9),
+ BPF_F_ADJ_ROOM_DECAP_L4_UDP = (1ULL << 10),
+ BPF_F_ADJ_ROOM_DECAP_IPXIP4 = (1ULL << 11),
+ BPF_F_ADJ_ROOM_DECAP_IPXIP6 = (1ULL << 12),
};

enum {
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index ca35ed622ed5..f4c2fbd8fe68 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3010,8 +3010,34 @@ union bpf_attr {
*
* * **BPF_F_ADJ_ROOM_DECAP_L3_IPV4**,
* **BPF_F_ADJ_ROOM_DECAP_L3_IPV6**:
- * Indicate the new IP header version after decapsulating the outer
- * IP header. Used when the inner and outer IP versions are different.
+ * Indicate the new IP header version after decapsulating the
+ * outer IP header. Used when the inner and outer IP versions
+ * are different. These flags only trigger a protocol change
+ * without clearing any tunnel-specific GSO flags.
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_L4_GRE**:
+ * Clear GRE tunnel GSO flags (SKB_GSO_GRE and SKB_GSO_GRE_CSUM)
+ * when decapsulating a GRE tunnel.
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_L4_UDP**:
+ * Clear UDP tunnel GSO flags (SKB_GSO_UDP_TUNNEL and
+ * SKB_GSO_UDP_TUNNEL_CSUM) when decapsulating a UDP tunnel.
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_IPXIP4**:
+ * Clear IPIP/SIT tunnel GSO flag (SKB_GSO_IPXIP4) when decapsulating
+ * a tunnel with an outer IPv4 header (IPv4-in-IPv4 or IPv6-in-IPv4).
+ *
+ * * **BPF_F_ADJ_ROOM_DECAP_IPXIP6**:
+ * Clear IPv6 encapsulation tunnel GSO flag (SKB_GSO_IPXIP6) when
+ * decapsulating a tunnel with an outer IPv6 header (IPv6-in-IPv6
+ * or IPv4-in-IPv6).
+ *
+ * When using the decapsulation flags above, the skb->encapsulation
+ * flag is automatically cleared if all tunnel-specific GSO flags
+ * (SKB_GSO_UDP_TUNNEL, SKB_GSO_UDP_TUNNEL_CSUM, SKB_GSO_GRE,
+ * SKB_GSO_GRE_CSUM, SKB_GSO_IPXIP4, SKB_GSO_IPXIP6) have been
+ * removed from the packet. This handles cases where all tunnel
+ * layers have been decapsulated.
*
* A call to this helper is susceptible to change the underlying
* packet buffer. Therefore, at load time, all checks on pointers
@@ -6221,6 +6247,10 @@ enum bpf_adj_room_flags {
BPF_F_ADJ_ROOM_ENCAP_L2_ETH = (1ULL << 6),
BPF_F_ADJ_ROOM_DECAP_L3_IPV4 = (1ULL << 7),
BPF_F_ADJ_ROOM_DECAP_L3_IPV6 = (1ULL << 8),
+ BPF_F_ADJ_ROOM_DECAP_L4_GRE = (1ULL << 9),
+ BPF_F_ADJ_ROOM_DECAP_L4_UDP = (1ULL << 10),
+ BPF_F_ADJ_ROOM_DECAP_IPXIP4 = (1ULL << 11),
+ BPF_F_ADJ_ROOM_DECAP_IPXIP6 = (1ULL << 12),
};

enum {
--
2.34.1