runtime: support cgo index into pointer-to-array

We were missing a case for calling a C function with an index
into a pointer-to-array.

Fixes #70016

Change-Id: I9c74d629e58722813c1aaa0f0dc225a5a64d111b
Reviewed-on: https://go-review.googlesource.com/c/go/+/621576
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/cgo/internal/testerrors/ptr_test.go

@@ -472,6 +472,23 @@ var ptrTests = []ptrTest{
 		body:    `s := struct { a [4]byte; p *int }{p: new(int)}; C.f43(unsafe.Pointer(unsafe.SliceData(s.a[:])))`,
 		fail:    false,
 	},
+	{
+		// Passing the address of an element of a pointer-to-array.
+		name:    "arraypointer",
+		c:       `void f44(void* p) {}`,
+		imports: []string{"unsafe"},
+		body:    `a := new([10]byte); C.f44(unsafe.Pointer(&a[0]))`,
+		fail:    false,
+	},
+	{
+		// Passing the address of an element of a pointer-to-array
+		// that contains a Go pointer.
+		name:    "arraypointer2",
+		c:       `void f45(void** p) {}`,
+		imports: []string{"unsafe"},
+		body:    `i := 0; a := &[2]unsafe.Pointer{nil, unsafe.Pointer(&i)}; C.f45(&a[0])`,
+		fail:    true,
+	},
 }
 
 func TestPointerChecks(t *testing.T) {

src/runtime/cgocall.go

@@ -563,6 +563,17 @@ func cgoCheckPointer(ptr any, arg any) {
 			ep = aep
 			t = ep._type
 			top = false
+		case abi.Pointer:
+			// The Go code is indexing into a pointer to an array,
+			// and we have been passed the pointer-to-array.
+			// Check the array rather than the pointer.
+			pt := (*abi.PtrType)(unsafe.Pointer(aep._type))
+			t = pt.Elem
+			if t.Kind_&abi.KindMask != abi.Array {
+				throw("can't happen")
+			}
+			ep = aep
+			top = false
 		default:
 			throw("can't happen")
 		}