This patch enhances GSO segment handling by properly checking
the SKB_GSO_DODGY flag for frag_list GSO packets, addressing
low throughput issues observed when a station accesses IPv4
servers via hotspots with an IPv6-only upstream interface.

Specifically, it fixes a bug in GSO segmentation when forwarding
GRO packets containing a frag_list. The function skb_segment_list
cannot correctly process GRO skbs that have been converted by XLAT,
since XLAT only translates the header of the head skb. Consequently,
skbs in the frag_list may remain untranslated, resulting in protocol
inconsistencies and reduced throughput.

To address this, the patch explicitly sets the SKB_GSO_DODGY flag
for GSO packets in XLAT's IPv4/IPv6 protocol translation helpers
(bpf_skb_proto_4_to_6 and bpf_skb_proto_6_to_4). This marks GSO
packets as potentially modified after protocol translation. As a
result, GSO segmentation will avoid using skb_segment_list and
instead falls back to skb_segment for packets with the SKB_GSO_DODGY
flag. This ensures that only safe and fully translated frag_list
packets are processed by skb_segment_list, resolving protocol
inconsistencies and improving throughput when forwarding GRO packets
converted by XLAT.

Signed-off-by: Jibin Zhang <redacted>
---
v4: Change according to Willem de Bruijn's suggestions
    1. Set SKB_GSO_DODGY when XLAT modifies headers for GSO packets.
    2. GSO segmentation is downgraded to use skb_segment instead of
       skb_segment_list to ensure robust handling of potentially
       modified packets when the SKB_GSO_DODGY flag is set on a
       GSO packet.

v3: Apply the same fix to tcp6_gso_segment(), as suggested.

v2: To apply the added condition to a narrower scop

  In this version, the condition (skb_has_frag_list(gso_skb) &&
(gso_skb->protocol == skb_shinfo(gso_skb)->frag_list->protocol))
is moved into inner 'if' statement to a narrower scope.

  Send out the patch again for further discussion because:

1. This issue has a significant impact and has occurred in many
countries and regions.
2. Currently, modifying BPF is not a good option, because BPF code
cannot access the header of skb on the fraglist, and the required
changes would affect a wide range of code.
3. Directly disabling GRO aggregation for XLAT flows is also not a
good solution, as this change would disable GRO even when forwarding
is not needed, and it would also require cooperation from all device
drivers.

[3]: https://patchwork.kernel.org/patch/14397344

[2]: https://patchwork.kernel.org/patch/14375646

[1]: https://patchwork.kernel.org/patch/14350844

---
 net/core/filter.c        | 2 ++
 net/ipv4/tcp_offload.c   | 3 ++-
 net/ipv4/udp_offload.c   | 3 ++-
 net/ipv6/tcpv6_offload.c | 3 ++-
 4 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 616e0520a0bb..bcd73d9bd764 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -3353,6 +3353,7 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
 			shinfo->gso_type &= ~SKB_GSO_TCPV4;
 			shinfo->gso_type |=  SKB_GSO_TCPV6;
 		}
+		shinfo->gso_type |=  SKB_GSO_DODGY;
 	}
 
 	bpf_skb_change_protocol(skb, ETH_P_IPV6);

@@ -3383,6 +3384,7 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
 			shinfo->gso_type &= ~SKB_GSO_TCPV6;
 			shinfo->gso_type |=  SKB_GSO_TCPV4;
 		}
+		shinfo->gso_type |=  SKB_GSO_DODGY;
 	}
 
 	bpf_skb_change_protocol(skb, ETH_P_IP);

diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
index fdda18b1abda..942a948f1a31 100644
--- a/net/ipv4/tcp_offload.c
+++ b/net/ipv4/tcp_offload.c

@@ -107,7 +107,8 @@ static struct sk_buff *tcp4_gso_segment(struct sk_buff *skb,
 	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
 		struct tcphdr *th = tcp_hdr(skb);
 
-		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
+		if ((skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size) &&
+		    !(skb_shinfo(skb)->gso_type & SKB_GSO_DODGY))
 			return __tcp4_gso_segment_list(skb, features);
 
 		skb->ip_summed = CHECKSUM_NONE;

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 19d0b5b09ffa..589456bd8b5f 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c

@@ -514,7 +514,8 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 
 	if (skb_shinfo(gso_skb)->gso_type & SKB_GSO_FRAGLIST) {
 		 /* Detect modified geometry and pass those to skb_segment. */
-		if (skb_pagelen(gso_skb) - sizeof(*uh) == skb_shinfo(gso_skb)->gso_size)
+		if ((skb_pagelen(gso_skb) - sizeof(*uh) == skb_shinfo(gso_skb)->gso_size) &&
+		    !(skb_shinfo(gso_skb)->gso_type & SKB_GSO_DODGY))
 			return __udp_gso_segment_list(gso_skb, features, is_ipv6);
 
 		ret = __skb_linearize(gso_skb);

diff --git a/net/ipv6/tcpv6_offload.c b/net/ipv6/tcpv6_offload.c
index effeba58630b..5670d32c27f8 100644
--- a/net/ipv6/tcpv6_offload.c
+++ b/net/ipv6/tcpv6_offload.c

@@ -170,7 +170,8 @@ static struct sk_buff *tcp6_gso_segment(struct sk_buff *skb,
 	if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST) {
 		struct tcphdr *th = tcp_hdr(skb);
 
-		if (skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size)
+		if ((skb_pagelen(skb) - th->doff * 4 == skb_shinfo(skb)->gso_size) &&
+		    !(skb_shinfo(skb)->gso_type & SKB_GSO_DODGY))
 			return __tcp6_gso_segment_list(skb, features);
 
 		skb->ip_summed = CHECKSUM_NONE;

-- 
2.45.2