syscall: point to x/sys in DLL loading docs, update syscall generator

Updates the syscall generator for patchset 4 of https://golang.org/cl/21388.

Updates #14959

Change-Id: Icbd6df489887d3dcc076dfc73d4feb1376abaf8b
Reviewed-on: https://go-review.googlesource.com/21428
Reviewed-by: Alex Brainman <[email protected]>
Reviewed-on: https://go-review.googlesource.com/21697
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz

src/syscall/dll_windows.go

@@ -37,6 +37,13 @@ type DLL struct {
 }
 
 // LoadDLL loads the named DLL file into memory.
+//
+// If name is not an absolute path and is not a known system DLL used by
+// Go, Windows will search for the named DLL in many locations, causing
+// potential DLL preloading attacks.
+//
+// Use LazyDLL in golang.org/x/sys/windows for a secure way to
+// load system DLLs.
 func LoadDLL(name string) (*DLL, error) {
 	namep, err := UTF16PtrFromString(name)
 	if err != nil {
@@ -174,6 +181,12 @@ func (p *Proc) Call(a ...uintptr) (r1, r2 uintptr, lastErr error) {
 // It will delay the load of the DLL until the first
 // call to its Handle method or to one of its
 // LazyProc's Addr method.
+//
+// LazyDLL is subject to the same DLL preloading attacks as documented
+// on LoadDLL.
+//
+// Use LazyDLL in golang.org/x/sys/windows for a secure way to
+// load system DLLs.
 type LazyDLL struct {
 	mu   sync.Mutex
 	dll  *DLL // non nil once DLL is loaded

src/syscall/mksyscall_windows.go

@@ -707,9 +707,9 @@ func (src *Source) Generate(w io.Writer) error {
 			}
 			if *sysRepo {
 				if packageName == "windows" {
-					return "&LazyDLL{Name: " + arg + ", Flags: LoadLibrarySearchSystem32}"
+					return "&LazyDLL{Name: " + arg + ", System: true}"
 				} else {
-					return "&windows.LazyDLL{Name: " + arg + ", Flags: windows.LoadLibrarySearchSystem32}"
+					return "&windows.LazyDLL{Name: " + arg + ", System: true}"
 				}
 			} else {
 				return syscalldot() + "NewLazyDLL(" + arg + ")"