From fb8d8d8fcaf39fc5e7e38526b5c95e3d923ad4b8 Mon Sep 17 00:00:00 2001
From: Leadaxe <247031499+Leadaxe@users.noreply.github.com>
Date: Sun, 21 Jun 2026 15:49:57 +0300
Subject: [PATCH] fix(010): never enable UDP_GRO on android (GRO split-brain
 killed download)
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runtime.GOOS=="android" (not "linux"), so StdNetBind enabled UDP_GRO and read
rxOffload=true, but the GRO receive dispatcher in bind_std.go is gated on
GOOS=="linux" → dead on android. A coalesced GRO super-packet was read as one
datagram, corrupting the WG transport stream → download died on no-detour
WG-endpoint (detour path uses ClientBind, no offload, so it worked).

Gate both the UDP_GRO setsockopt (controlfns_linux.go) and the rxOffload
readback (features_linux.go) behind !android. TX/GSO untouched, so non-android
linux performance is unchanged. Confirmed on device (CPH2411/Android-15:
probe gave rxOffload=true + dispatch=single).

See SPECS/010-B-O-WG_ENDPOINT_GRO_SPLIT_BRAIN.
---
 conn/controlfns_linux.go |  9 +++++++++
 conn/features_linux.go   | 10 ++++++++++
 2 files changed, 19 insertions(+)

diff --git a/conn/controlfns_linux.go b/conn/controlfns_linux.go
index 752fbca..7602665 100644
--- a/conn/controlfns_linux.go
+++ b/conn/controlfns_linux.go
@@ -60,6 +60,15 @@ func init() {
 
 		// Attempt to enable UDP_GRO
 		func(network, address string, c syscall.RawConn) error {
+			// lx(010): skip UDP_GRO on android. The GRO receive path in bind_std.go
+			// is gated on runtime.GOOS=="linux", which is false on android — so a
+			// coalesced super-packet is never split and corrupts the WG stream
+			// (download dies). Belt-and-suspenders with the rxOffload guard in
+			// features_linux.go. TX/GSO untouched.
+			// See SPECS/010-B-O-WG_ENDPOINT_GRO_SPLIT_BRAIN.
+			if runtime.GOOS == "android" {
+				return nil
+			}
 			c.Control(func(fd uintptr) {
 				_ = unix.SetsockoptInt(int(fd), unix.IPPROTO_UDP, socketOptionUDPGRO, 1)
 			})
diff --git a/conn/features_linux.go b/conn/features_linux.go
index 513202e..b74c398 100644
--- a/conn/features_linux.go
+++ b/conn/features_linux.go
@@ -7,6 +7,7 @@ package conn
 
 import (
 	"net"
+	"runtime"
 
 	"golang.org/x/sys/unix"
 )
@@ -29,6 +30,15 @@ func supportsUDPOffload(conn *net.UDPConn) (txOffload, rxOffload bool) {
 			return
 		}
 		txOffload = true
+		// lx(010): never advertise RX offload on android. runtime.GOOS=="android"
+		// (not "linux"), so the GRO receive dispatcher in bind_std.go (gated on
+		// GOOS=="linux") is dead there — a coalesced GRO super-packet would be read
+		// as one datagram and corrupt the WG transport stream, killing download.
+		// Confirmed on device (CPH2411/Android-15: rxOffload=true, dispatch=single).
+		// TX is left untouched. See SPECS/010-B-O-WG_ENDPOINT_GRO_SPLIT_BRAIN.
+		if runtime.GOOS == "android" {
+			return
+		}
 		opt, errSyscall := unix.GetsockoptInt(int(fd), unix.IPPROTO_UDP, socketOptionUDPGRO)
 		if errSyscall != nil {
 			return