[RFC PATCH net-next] net: gro: coalesce padded small IPv4 TCP segments

[Glenn Judd <gmj@meta.com>]

Software GRO fails to coalesce small IPv4/TCP segment that was
padded up to the 60-byte minimum Ethernet frame.

The selftest tools/testing/selftests/drivers/net/hw/gro.py subtest
sw_ipv4_data_lrg_1byte sends {100, 1} expecting to receive {101}.
In current code, it receives {100, 1} (no coalescing) instead.

Cause: inet_gro_receive() computes its flush term from
tot_len ^ skb_gro_len() before skb_gro_pull(), while skb_gro_len()
still includes trailing Ethernet padding. A small IPv4/TCP segment
padded up to the 60-byte minimum frame has tot_len != skb_gro_len(),
so flush is set and the runt never coalesces.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5.6
Assisted-by: Meta:internal-AI-tooling
Signed-off-by: Glenn Judd <gmj@meta.com>
---

Notes:
    RFC notes
    ---------
    Per Jakub Kicinski, the open question is fast-path cost: this adds two
    operations to the common IPv4 GRO path for every packet -- reading
    iph->tot_len and the skb_gro_len() comparison.  Everything expensive
    (linear check, trim, pointer refresh, csum recompute) is behind unlikely()
    on the slow path.  Is that per-packet cost worth the coalescing win for
    padded runts?
    
    Testing: netdevsim cannot reproduce this -- it never pads short frames to
    ETH_ZLEN -- so sw_ipv4_data_lrg_1byte passes trivially there.  Reproduced
    and fixed on a real NIC (cx7): baseline FAIL -> patched PASS.  Also
    validated locally under KASAN + CONFIG_FAIL_SKB_REALLOC (no UAF).

 net/ipv4/af_inet.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 32d006c1a8ee..998ff77fd7b9 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1470,6 +1470,7 @@ struct sk_buff *inet_gro_receive(struct list_head *head, struct sk_buff *skb)
 	const struct net_offload *ops;
 	struct sk_buff *pp = NULL;
 	const struct iphdr *iph;
+	unsigned int tot_len;
 	struct sk_buff *p;
 	unsigned int hlen;
 	unsigned int off;
@@ -1498,6 +1499,25 @@ struct sk_buff *inet_gro_receive(struct list_head *head, struct sk_buff *skb)
 		goto out;
 
 	NAPI_GRO_CB(skb)->proto = proto;
+
+	tot_len = ntohs(iph->tot_len);
+	if (unlikely(skb_gro_len(skb) > tot_len)) {
+		if (!skb_is_nonlinear(skb)) {
+			if (tot_len < sizeof(*iph) ||
+			    pskb_trim_rcsum(skb, off + tot_len))
+				goto out;
+
+			NAPI_GRO_CB(skb)->frag0 = skb->data;
+			NAPI_GRO_CB(skb)->frag0_len = skb->len;
+			iph = skb_gro_header(skb, hlen, off);
+			if (unlikely(!iph))
+				goto out;
+			if (skb->ip_summed == CHECKSUM_COMPLETE)
+				NAPI_GRO_CB(skb)->csum =
+					skb_checksum(skb, off, tot_len, 0);
+		}
+	}
+
 	flush = (u16)((ntohl(*(__be32 *)iph) ^ skb_gro_len(skb)) | (ntohl(*(__be32 *)&iph->id) & ~IP_DF));
 
 	list_for_each_entry(p, head, list) {

base-commit: 2fbade66245059c78daeaccfce13ecf499fffb51
-- 
2.53.0-Meta