arena: align allocs to pointer size

malloc.go

@@ -45,6 +45,10 @@ func (a *arena) Alloc(length uintptr) unsafe.Pointer {
 	if length == 0 {
 		panic("can't allocate zero bytes")
 	}
+	// Round up to pointer alignment: callers store pointer-containing
+	// structs into returned memory, and the Go runtime's write barrier
+	// throws on unaligned destinations (issue #25).
+	length = (length + 7) &^ 7
 	if length > a.remaining {
 		a.grow()
 	}

malloc_test.go

@@ -0,0 +1,107 @@
+package wf
+
+import (
+	"net/netip"
+	"runtime"
+	"testing"
+	"unsafe"
+)
+
+// TestArenaAllocAligned is the deterministic regression test for issue #25:
+// every arena allocation must be pointer-aligned, because callers store
+// pointer-containing structs (fwpmFilter0 etc.) into the returned memory,
+// and the Go runtime's write barrier throws on unaligned destinations.
+func TestArenaAllocAligned(t *testing.T) {
+	var a arena
+	defer a.Dispose()
+
+	// 17 mirrors unsafe.Sizeof(fwpV6AddrAndMask{}) — the allocation that
+	// breaks alignment in any rule with IPv6 prefix conditions.
+	for i := 0; i < 64; i++ {
+		p := uintptr(a.Alloc(17))
+		if p%unsafe.Alignof(uintptr(0)) != 0 {
+			t.Fatalf("Alloc #%d returned unaligned pointer %#x", i, p)
+		}
+	}
+}
+
+// TestToFilter0UnalignedWriteBarrier reproduces the intermittent
+// "fatal error: bulkBarrierPreWrite: unaligned arguments" crash from
+// issue #25 and makes it near-deterministic.
+//
+// Mechanism: IPv6 prefix conditions make toCondition0 allocate 17-byte
+// fwpV6AddrAndMask values, after which the arena cursor is no longer
+// 8-aligned; toFilter0 then stores a pointer-containing struct
+// (`*ret = fwpmFilter0{...}`) at the unaligned address. The compiler-emitted
+// write barrier (wbMove -> bulkBarrierPreWrite) throws only while a GC mark
+// phase is active, which is why the crash is flaky in the wild. Here a
+// background goroutine runs back-to-back GC cycles over a pointer-dense
+// retained heap, so mark phases cover most of the wall clock and the loop
+// below hits one with near certainty.
+//
+// On a broken arena this is a process-level fatal error, not a catchable
+// panic: the test binary dies with the exact production stack. With the
+// alignment fix in arena.Alloc it passes.
+func TestToFilter0UnalignedWriteBarrier(t *testing.T) {
+	keepGCOccupied(t)
+
+	lt := layerTypes{
+		LayerALEAuthConnectV6: fieldTypes{
+			FieldIPRemoteAddress: typeIP,
+		},
+	}
+	rule := &Rule{
+		ID:     RuleID{Data1: 0xdeadbeef, Data2: 0xcafe, Data3: 0xf00d},
+		Name:   "issue #25 repro",
+		Layer:  LayerALEAuthConnectV6,
+		Weight: 2000,
+		Action: ActionPermit,
+	}
+	for _, p := range []string{"::1/128", "fe80::/10", "fc00::/7", "ff00::/8"} {
+		rule.Conditions = append(rule.Conditions, &Match{
+			Field: FieldIPRemoteAddress,
+			Op:    MatchTypeEqual,
+			Value: netip.MustParsePrefix(p),
+		})
+	}
+
+	for i := 0; i < 10000; i++ {
+		var a arena
+		_, err := toFilter0(&a, rule, lt)
+		a.Dispose()
+		if err != nil {
+			t.Fatalf("toFilter0: %v", err)
+		}
+	}
+}
+
+func keepGCOccupied(t *testing.T) {
+	type node struct {
+		next *node
+		pad  *[64]byte
+	}
+	nodes := make([]*node, 1<<18)
+	for i := range nodes {
+		nodes[i] = &node{next: &node{}, pad: new([64]byte)}
+	}
+	runtime.KeepAlive(nodes)
+
+	stop := make(chan struct{})
+	gcDone := make(chan struct{})
+	go func() {
+		defer close(gcDone)
+		for {
+			select {
+			case <-stop:
+				return
+			default:
+				runtime.GC()
+			}
+		}
+	}()
+
+	t.Cleanup(func() {
+		close(stop)
+		<-gcDone
+	})
+}