[RFC bpf-next v2 1/3] bpf: Add PPPoE encap support to bpf_skb_adjust_room

[ThisSeanZhang <thisseanzhang@gmail.com>]

Add a new BPF_F_ADJ_ROOM_ENCAP_PPPOE flag to bpf_skb_adjust_room() so
that a TC BPF program can encapsulate an IPv4 or IPv6 packet into a
PPPoE session header:

    bpf_skb_adjust_room(skb, PPPOE_SES_HLEN, BPF_ADJ_ROOM_MAC,
                        BPF_F_ADJ_ROOM_ENCAP_PPPOE);

The flag inserts eight bytes between the MAC and network headers: the
six-byte PPPoE session header followed by the two-byte PPP protocol
field. It then updates the skb metadata, including skb->protocol and
skb->mac_len. The BPF program remains responsible for filling in the
PPPoE header and the Ethernet ethertype, for example with
bpf_skb_store_bytes().

Previously, a TC program could add the header bytes manually, but the
skb would continue to be treated as a plain IP packet because its
protocol metadata was unchanged. With the stale skb->protocol, software
GSO may select a non-PPPoE segmentation handler and process the packet
incorrectly.

This makes the skb metadata compatible with the PPPoE GRO/GSO handlers
registered for ETH_P_PPP_SES. The PPPoE GRO/GSO support must be built in
or loaded before receive-side GRO or software GSO uses those handlers.

Signed-off-by: ThisSeanZhang <thisseanzhang@gmail.com>
---
 include/uapi/linux/bpf.h       | 12 ++++++++++++
 net/core/filter.c              | 22 ++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h | 12 ++++++++++++
 3 files changed, 46 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 732b35cc0..30481d040 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3036,6 +3036,17 @@ union bpf_attr {
  *		  Use with BPF_F_ADJ_ROOM_ENCAP_L2 flag to further specify the
  *		  L2 type as Ethernet.
  *
+ *		* **BPF_F_ADJ_ROOM_ENCAP_PPPOE**:
+ *		  Encapsulate the packet in a PPPoE session header. Must be
+ *		  used with **BPF_ADJ_ROOM_MAC** mode and *len_diff* equal to
+ *		  the size of the PPPoE session header plus the PPP protocol
+ *		  field (8 bytes in total). The room is inserted between the
+ *		  MAC header and the network header, *skb->protocol* is set
+ *		  to **ETH_P_PPP_SES** and *skb->mac_len* is updated
+ *		  accordingly. The PPPoE header itself and the ethertype of
+ *		  the MAC header are filled in by the BPF program, e.g. via
+ *		  **bpf_skb_store_bytes**.
+ *
  *		* **BPF_F_ADJ_ROOM_DECAP_L3_IPV4**,
  *		  **BPF_F_ADJ_ROOM_DECAP_L3_IPV6**:
  *		  Indicate the new IP header version after decapsulating the
@@ -6323,6 +6334,7 @@ enum bpf_adj_room_flags {
 	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),
+	BPF_F_ADJ_ROOM_ENCAP_PPPOE	= (1ULL << 13),
 };
 
 enum {
diff --git a/net/core/filter.c b/net/core/filter.c
index 70dc62167..5b204e316 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -47,6 +47,7 @@
 #include <linux/ratelimit.h>
 #include <linux/seccomp.h>
 #include <linux/if_vlan.h>
+#include <linux/if_pppox.h>
 #include <linux/bpf.h>
 #include <linux/btf.h>
 #include <net/sch_generic.h>
@@ -3583,6 +3584,7 @@ static u32 bpf_skb_net_base_len(const struct sk_buff *skb)
 					 BPF_F_ADJ_ROOM_ENCAP_L4_GRE | \
 					 BPF_F_ADJ_ROOM_ENCAP_L4_UDP | \
 					 BPF_F_ADJ_ROOM_ENCAP_L2_ETH | \
+					 BPF_F_ADJ_ROOM_ENCAP_PPPOE | \
 					 BPF_F_ADJ_ROOM_ENCAP_L2( \
 					  BPF_ADJ_ROOM_ENCAP_L2_MASK))
 
@@ -3688,6 +3690,14 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
 			skb_dst_drop(skb);
 	}
 
+	if (flags & BPF_F_ADJ_ROOM_ENCAP_PPPOE) {
+		/* Network header points at the inserted PPPoE header. */
+		skb->protocol = htons(ETH_P_PPP_SES);
+		skb_reset_mac_len(skb);
+		if (skb_valid_dst(skb))
+			skb_dst_drop(skb);
+	}
+
 	if (skb_is_gso(skb)) {
 		struct skb_shared_info *shinfo = skb_shinfo(skb);
 
@@ -3874,6 +3884,18 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
 		return -ENOTSUPP;
 	}
 
+	if (flags & BPF_F_ADJ_ROOM_ENCAP_PPPOE) {
+		/* The PPPoE session header has a fixed size and is
+		 * inserted directly after the MAC header.
+		 */
+		if (shrink || mode != BPF_ADJ_ROOM_MAC ||
+		    len_diff != PPPOE_SES_HLEN ||
+		    flags & ((BPF_F_ADJ_ROOM_ENCAP_MASK |
+			      BPF_F_ADJ_ROOM_DECAP_MASK) &
+			     ~BPF_F_ADJ_ROOM_ENCAP_PPPOE))
+			return -EINVAL;
+	}
+
 	if (flags & BPF_F_ADJ_ROOM_DECAP_MASK) {
 		u32 len_decap_min = 0;
 
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 732b35cc0..30481d040 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3036,6 +3036,17 @@ union bpf_attr {
  *		  Use with BPF_F_ADJ_ROOM_ENCAP_L2 flag to further specify the
  *		  L2 type as Ethernet.
  *
+ *		* **BPF_F_ADJ_ROOM_ENCAP_PPPOE**:
+ *		  Encapsulate the packet in a PPPoE session header. Must be
+ *		  used with **BPF_ADJ_ROOM_MAC** mode and *len_diff* equal to
+ *		  the size of the PPPoE session header plus the PPP protocol
+ *		  field (8 bytes in total). The room is inserted between the
+ *		  MAC header and the network header, *skb->protocol* is set
+ *		  to **ETH_P_PPP_SES** and *skb->mac_len* is updated
+ *		  accordingly. The PPPoE header itself and the ethertype of
+ *		  the MAC header are filled in by the BPF program, e.g. via
+ *		  **bpf_skb_store_bytes**.
+ *
  *		* **BPF_F_ADJ_ROOM_DECAP_L3_IPV4**,
  *		  **BPF_F_ADJ_ROOM_DECAP_L3_IPV6**:
  *		  Indicate the new IP header version after decapsulating the
@@ -6323,6 +6334,7 @@ enum bpf_adj_room_flags {
 	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),
+	BPF_F_ADJ_ROOM_ENCAP_PPPOE	= (1ULL << 13),
 };
 
 enum {
-- 
2.47.3