NAPI broken on old kernels and also not helpful

overlay/tio/tio_gso_linux.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"runtime"
 	"sync/atomic"
 	"syscall"
 	"unsafe"
@@ -264,24 +263,12 @@ func (r *Offload) writeWithScratch(buf []byte, iovs *[2]unix.Iovec) (int, error)
 	// to validVnetHdr during Offload construction so we don't rebuild it here.
 	iovs[1].Base = &buf[0]
 	iovs[1].SetLen(len(buf))
-	iovPtr := unsafe.Pointer(&iovs[0])
-	// Pin the caller's buffer AND the iovec array through the syscall.
-	return r.rawWrite(iovPtr, 2, buf, iovs)
+	return r.rawWrite(unsafe.Slice(&iovs[0], len(iovs)))
 }
 
-func (r *Offload) rawWrite(iovs unsafe.Pointer, iovcnt int, keepAlive ...interface{}) (int, error) {
+func (r *Offload) rawWrite(iovs []unix.Iovec) (int, error) {
 	for {
-		n, _, errno := syscall.Syscall(unix.SYS_WRITEV, uintptr(r.fd), uintptr(iovs), uintptr(iovcnt))
-		// Anchor the iovec array + every user-supplied payload slice
-		// through the syscall return. Without these, Go's GC may move or
-		// collect the underlying backing arrays while the kernel is still
-		// reading them via DMA (we pass the iovec as uintptr, so the
-		// compiler does not keep it live). Observed in practice as a
-		// kernel refcount underflow on tun_chr_write_iter / sock_wfree.
-		runtime.KeepAlive(iovs)
-		for _, ka := range keepAlive {
-			runtime.KeepAlive(ka)
-		}
+		n, _, errno := syscall.Syscall(unix.SYS_WRITEV, uintptr(r.fd), uintptr(unsafe.Pointer(&iovs[0])), uintptr(len(iovs)))
 		if errno == 0 {
 			if int(n) < virtioNetHdrLen {
 				return 0, io.ErrShortWrite
@@ -364,13 +351,7 @@ func (r *Offload) WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, isV6 bool,
 		r.gsoIovs[2+i].SetLen(len(p))
 	}
 
-	iovPtr := unsafe.Pointer(&r.gsoIovs[0])
-	iovCnt := len(r.gsoIovs)
-	// Pin EVERYTHING the kernel might still read via DMA: the backing iovec
-	// slice, the IP/TCP header buffer, and every individual payload
-	// fragment. Skipping any of these risks a use-after-free in
-	// tun_chr_write_iter if GC runs mid-syscall.
-	_, err := r.rawWrite(iovPtr, iovCnt, r.gsoIovs, hdr, pays)
+	_, err := r.rawWrite(r.gsoIovs)
 	return err
 }
 

overlay/tio/tio_poll_linux.go

@@ -3,7 +3,6 @@ package tio
 import (
 	"fmt"
 	"os"
-	"runtime"
 	"sync/atomic"
 	"syscall"
 	"unsafe"
@@ -121,13 +120,6 @@ func (t *Poll) readOne(to []byte) (int, error) {
 	}
 	for {
 		n, _, errno := syscall.Syscall(syscall.SYS_READV, uintptr(t.fd), uintptr(unsafe.Pointer(&iovecs[0])), 2)
-		// Pin the iovec + destination buffer backing array across the syscall.
-		// Without these the Go runtime may move/GC them while the kernel is
-		// still writing via DMA (we pass the iovec as uintptr, which hides it
-		// from escape analysis). Same class of bug as rawWrite in the Offload
-		// path.
-		runtime.KeepAlive(iovecs)
-		runtime.KeepAlive(to)
 		if errno == 0 {
 			bytesRead := int(n)
 			if bytesRead < 4 {
@@ -174,10 +166,6 @@ func (t *Poll) Write(from []byte) (int, error) {
 	}
 	for {
 		n, _, errno := syscall.Syscall(syscall.SYS_WRITEV, uintptr(t.fd), uintptr(unsafe.Pointer(&iovecs[0])), 2)
-		// Pin the iovec + source buffer backing array across the syscall.
-		// See readOne's KeepAlive comment for rationale.
-		runtime.KeepAlive(iovecs)
-		runtime.KeepAlive(from)
 		if errno == 0 {
 			return int(n) - 4, nil
 		}

overlay/tun_linux.go

@@ -139,7 +139,7 @@ func newTun(c *config.C, l *logrus.Logger, vpnNetworks []netip.Prefix, multiqueu
 		return nil, err
 	}
 	vnetHdr := true
-	name, err := tunSetIff(fd, nameStr, baseFlags|unix.IFF_VNET_HDR|unix.IFF_NAPI)
+	name, err := tunSetIff(fd, nameStr, baseFlags|unix.IFF_VNET_HDR)
 	if err != nil {
 		_ = unix.Close(fd)
 		vnetHdr = false
@@ -307,7 +307,7 @@ func (t *tun) NewMultiQueueReader() error {
 
 	flags := uint16(unix.IFF_TUN | unix.IFF_NO_PI | unix.IFF_MULTI_QUEUE)
 	if t.vnetHdr {
-		flags |= unix.IFF_VNET_HDR | unix.IFF_NAPI
+		flags |= unix.IFF_VNET_HDR
 	}
 	if _, err = tunSetIff(fd, t.Device, flags); err != nil {
 		_ = unix.Close(fd)