From 6210399e65ef6eaa64fbd20c3351f0c7235c2bd4 Mon Sep 17 00:00:00 2001
From: Viktor Liu <17948409+lixmal@users.noreply.github.com>
Date: Mon, 17 Aug 2026 20:10:12 +0900
Subject: [PATCH] [client] Declare multi-buffer support for the loopback XDP
 program (#7230)

---
 client/internal/ebpf/ebpf/manager_linux.go | 47 ++++++++++++++++++----
 1 file changed, 40 insertions(+), 7 deletions(-)

diff --git a/client/internal/ebpf/ebpf/manager_linux.go b/client/internal/ebpf/ebpf/manager_linux.go
index 7520a6387..64a3e5b54 100644
--- a/client/internal/ebpf/ebpf/manager_linux.go
+++ b/client/internal/ebpf/ebpf/manager_linux.go
@@ -2,17 +2,21 @@ package ebpf
 
 import (
 	_ "embed"
+	"fmt"
 	"net"
 	"sync"
 
 	"github.com/cilium/ebpf/link"
 	"github.com/cilium/ebpf/rlimit"
 	log "github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
 
 	"github.com/netbirdio/netbird/client/internal/ebpf/manager"
 )
 
 const (
+	xdpProgName = "nb_xdp_prog"
+
 	mapKeyFeatures uint32 = 0
 
 	featureFlagWGProxy      = 0b00000001
@@ -68,21 +72,50 @@ func (tf *GeneralManager) loadXdp() error {
 		return err
 	}
 
-	// load pre-compiled programs into the kernel.
-	err = loadBpfObjects(&tf.bpfObjs, nil)
+	// lo has no native XDP, so the program runs in generic mode. Unless it
+	// declares multi-buffer support the kernel must linearize every non-linear
+	// skb before running it. Loopback packets are up to 64 KB, so that is a
+	// contiguous GFP_ATOMIC allocation per packet, and when it fails the packet
+	// is dropped before the program runs, stalling local TCP connections.
+	// Multi-buffer XDP in generic mode requires kernel 6.3, so fall back to a
+	// plain attach when the kernel rejects it.
+	err = tf.attachXdp(iFace.Index, true)
+	if err == nil {
+		return nil
+	}
+	log.Debugf("failed to attach multi-buffer xdp program, retrying without it: %s", err)
+
+	return tf.attachXdp(iFace.Index, false)
+}
+
+func (tf *GeneralManager) attachXdp(iFaceIndex int, multiBuffer bool) error {
+	spec, err := loadBpf()
 	if err != nil {
-		return err
+		return fmt.Errorf("load bpf spec: %w", err)
+	}
+
+	if multiBuffer {
+		prog, ok := spec.Programs[xdpProgName]
+		if !ok {
+			return fmt.Errorf("program %s not found in bpf spec", xdpProgName)
+		}
+		prog.Flags |= unix.BPF_F_XDP_HAS_FRAGS
+	}
+
+	if err := spec.LoadAndAssign(&tf.bpfObjs, nil); err != nil {
+		return fmt.Errorf("load bpf objects: %w", err)
 	}
 
 	tf.link, err = link.AttachXDP(link.XDPOptions{
 		Program:   tf.bpfObjs.NbXdpProg,
-		Interface: iFace.Index,
+		Interface: iFaceIndex,
 	})
-
 	if err != nil {
-		_ = tf.bpfObjs.Close()
+		if closeErr := tf.bpfObjs.Close(); closeErr != nil {
+			log.Debugf("failed to close bpf objects after xdp attach error: %s", closeErr)
+		}
 		tf.link = nil
-		return err
+		return fmt.Errorf("attach xdp: %w", err)
 	}
 	return nil
 }