Prevent RandReader.Read argument from escaping to the heap

cgo_go124.go

@@ -0,0 +1,18 @@
+//go:build go1.24 && !cmd_go_bootstrap
+
+package openssl
+
+// The following noescape and nocallback directives are used to prevent the Go
+// compiler from allocating function parameters on the heap. See
+// https://github.com/golang/go/blob/0733682e5ff4cd294f5eccb31cbe87a543147bc6/src/cmd/cgo/doc.go#L439-L461
+//
+// If possible, write a C wrapper function to optimize a call rather than using
+// this feature so the optimization will work for all supported Go versions.
+//
+// This is just a performance optimization. Only add functions that have been
+// observed to benefit from these directives, not every function that is merely
+// expected to meet the noescape/nocallback criteria.
+
+// #cgo noescape go_openssl_RAND_bytes
+// #cgo nocallback go_openssl_RAND_bytes
+import "C"

openssl_test.go

@@ -2,14 +2,19 @@ package openssl_test
 
 import (
 	"fmt"
+	"go/version"
 	"os"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 
 	"github.com/golang-fips/openssl/v2"
 )
 
+// sink is used to prevent the compiler from optimizing out the allocations.
+var sink uint8
+
 // getVersion returns the OpenSSL version to use for testing.
 func getVersion() string {
 	v := os.Getenv("GO_OPENSSL_VERSION_OVERRIDE")
@@ -78,3 +83,11 @@ func TestCheckVersion(t *testing.T) {
 		t.Fatalf("FIPS mismatch: want %v, got %v", want, fips)
 	}
 }
+
+// compareCurrentVersion compares v with [runtime.Version].
+// See [go/versions.Compare] for information about
+// v format and comparison rules.
+func compareCurrentVersion(v string) int {
+	ver := strings.TrimPrefix(runtime.Version(), "devel ")
+	return version.Compare(ver, v)
+}

pbkdf2_test.go

@@ -166,8 +166,6 @@ func TestWithUnsupportedHash(t *testing.T) {
 	}
 }
 
-var sink uint8
-
 func benchmark(b *testing.B, h func() hash.Hash) {
 	password := make([]byte, h().Size())
 	salt := make([]byte, 8)

rand_test.go

@@ -12,3 +12,20 @@ func TestRand(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestAllocations(t *testing.T) {
+	n := int(testing.AllocsPerRun(10, func() {
+		buf := make([]byte, 32)
+		openssl.RandReader.Read(buf)
+		sink ^= buf[0]
+	}))
+	want := 1
+	if compareCurrentVersion("go1.24") >= 0 {
+		// The go1.24 compiler is able to optimize the allocation away.
+		// See cgo_go124.go for more information.
+		want = 0
+	}
+	if n > want {
+		t.Errorf("allocs = %d, want %d", n, want)
+	}
+}