runtime/secret: fix cgo crashes inside of secret.Do

Fixes #78999

Change-Id: I9fba1fd771f06fc57de1ecb70d9f9cb8bc8078fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/771640
Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/runtime/asm_amd64.s

@@ -942,6 +942,11 @@ TEXT ·asmcgocall(SB),NOSPLIT,$0-20
 	JEQ	nosecret
 	CALL	·secretEraseRegisters(SB)
 
+	get_tls(CX)
+	MOVQ    g(CX), DI
+	MOVQ    g_m(DI), R8
+	MOVQ    m_g0(R8), SI
+
 nosecret:
 #endif
 	MOVQ	fn+0(FP), AX

src/runtime/crash_cgo_test.go

@@ -15,6 +15,7 @@ import (
 	"internal/testenv"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"runtime"
 	"strconv"
 	"strings"
@@ -118,6 +119,27 @@ func TestCgoCallbackX15(t *testing.T) {
 	}
 }
 
+func TestSecretCgo(t *testing.T) {
+	t.Parallel()
+	testenv.MustHaveGoBuild(t)
+	testenv.MustHaveCGO(t)
+
+	exe := filepath.Join(t.TempDir(), "secretcgo.exe")
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", exe)
+	cmd.Dir = "testdata/testprogcgo"
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Env, "GOEXPERIMENT=runtimesecret")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("building testprogcgo with runtimesecret: %v\n%s", err, out)
+	}
+
+	got := runBuiltTestProg(t, exe, "SecretCgo")
+	if want := "OK\n"; got != want {
+		t.Fatalf("expected %q, got:\n%s", want, got)
+	}
+}
+
 func TestCgoExternalThreadPanic(t *testing.T) {
 	t.Parallel()
 	if runtime.GOOS == "plan9" {

src/runtime/testdata/testprogcgo/secretcgo.go

@@ -0,0 +1,31 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build goexperiment.runtimesecret
+
+package main
+
+/*
+static int cAdd(int a, int b) { return a + b; }
+*/
+import "C"
+
+import (
+	"fmt"
+	"runtime/secret"
+)
+
+func init() {
+	register("SecretCgo", SecretCgo)
+}
+
+func SecretCgo() {
+	secret.Do(func() {
+		r := C.cAdd(1, 2)
+		if r != 3 {
+			panic(fmt.Sprintf("got %d, want 3", r))
+		}
+	})
+	fmt.Println("OK")
+}