syscall: make TestGetdirentries checkptr safe

Fixes Darwin.

Updates #35092

Change-Id: I045f070c8549d00610b459e3a82cac870d9ddb54
Reviewed-on: https://go-review.googlesource.com/c/go/+/203077
Run-TryBot: Brad Fitzpatrick <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: bradfitz

src/syscall/getdirentries_test.go

@@ -66,7 +66,15 @@ func testGetdirentries(t *testing.T, count int) {
 		}
 		data := buf[:n]
 		for len(data) > 0 {
-			dirent := (*syscall.Dirent)(unsafe.Pointer(&data[0]))
+			// syscall.Getdirentries's return value may be (and usually is) much
+			// smaller than a syscall.Dirent, which has lots of padding for
+			// the name at the end. The compiler's checkptr validation doesn't like
+			// that. So allocate direntMem that's always big enough, and use that
+			// when converting to *syscall.Dirent.
+			var direntMem [unsafe.Sizeof(syscall.Dirent{})]byte
+			copy(direntMem[:], data)
+			dirent := (*syscall.Dirent)(unsafe.Pointer(&direntMem[0]))
+
 			data = data[dirent.Reclen:]
 			name := make([]byte, dirent.Namlen)
 			for i := 0; i < int(dirent.Namlen); i++ {