start supporting asm functions better

Spotted while trying to link a program using unix.Syscall, since its
implementation is assembly.

Telling if a function couldn't be garbled isn't trivial. If that
function belongs to an imported package, we only load its export data
instead of type-checking from source, so we don't have all the
information needed.

Instead, use the gc export data importer to import two versions of each
dependency: its original version, for the initial type-checking, and its
garbled version, to check if any of its exported names weren't garbled.

Updates #9.

main.go

@@ -52,19 +52,21 @@ var (
 	fset      = token.NewFileSet()
 	emptyFset = token.NewFileSet()
 
-	b64           = base64.NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_z")
-	printerConfig = printer.Config{Mode: printer.RawFormat}
-	typesConfig   = types.Config{Importer: importer.ForCompiler(fset, "gc", objLookup)}
-
-	buildInfo = packageInfo{imports: make(map[string]importedPkg)}
+	b64             = base64.NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_z")
+	printerConfig   = printer.Config{Mode: printer.RawFormat}
+	origTypesConfig = types.Config{Importer: importer.ForCompiler(fset, "gc", origLookup)}
+
+	buildInfo       = packageInfo{imports: make(map[string]importedPkg)}
+	garbledImporter = importer.ForCompiler(fset, "gc", func(path string) (io.ReadCloser, error) {
+		return os.Open(buildInfo.imports[path].packagefile)
+	}).(types.ImporterFrom)
 )
 
-type jsonExport struct {
-	Export string
-}
-
-func objLookup(path string) (io.ReadCloser, error) {
-	// objPath := buildInfo.imports[path].packagefile
+// origLookup helps implement a types.Importer which finds the export data for
+// the original dependencies, not their garbled counterparts. This is useful to
+// typecheck a package before it's garbled, so we can make decisions on how to
+// garble it.
+func origLookup(path string) (io.ReadCloser, error) {
 	cmd := exec.Command("go", "list", "-json", "-export", path)
 	dir := os.Getenv("GARBLE_DIR")
 	if dir == "" {
@@ -84,6 +86,26 @@ func objLookup(path string) (io.ReadCloser, error) {
 	return os.Open(res.Export)
 }
 
+func garbledImport(path string) (*types.Package, error) {
+	ipkg, ok := buildInfo.imports[path]
+	if !ok {
+		return nil, fmt.Errorf("could not find imported package %q", path)
+	}
+	if ipkg.pkg != nil {
+		return ipkg.pkg, nil // cached
+	}
+	dir := os.Getenv("GARBLE_DIR")
+	if dir == "" {
+		return nil, fmt.Errorf("$GARBLE_DIR unset; did you run via 'garble build'?")
+	}
+	pkg, err := garbledImporter.ImportFrom(path, dir, 0)
+	if err != nil {
+		return nil, err
+	}
+	ipkg.pkg = pkg // cache for later use
+	return pkg, nil
+}
+
 type packageInfo struct {
 	buildID string
 	imports map[string]importedPkg
@@ -92,6 +114,8 @@ type packageInfo struct {
 type importedPkg struct {
 	packagefile string
 	buildID     string
+
+	pkg *types.Package
 }
 
 func main1() int {
@@ -245,7 +269,7 @@ func transformCompile(args []string) ([]string, error) {
 		Uses: make(map[*ast.Ident]types.Object),
 	}
 	pkgPath := flagValue(flags, "-p")
-	if _, err := typesConfig.Check(pkgPath, fset, files, info); err != nil {
+	if _, err := origTypesConfig.Check(pkgPath, fset, files, info); err != nil {
 		return nil, fmt.Errorf("typecheck error: %v", err)
 	}
 
@@ -386,6 +410,9 @@ func transformGo(node ast.Node, info *types.Info) ast.Node {
 				if obj.Exported() && sign.Recv() != nil {
 					return true // might implement an interface
 				}
+				if implementedOutsideGo(x) {
+					return true // implemented elsewhere, like assembly
+				}
 				switch node.Name {
 				case "main", "init", "TestMain":
 					return true // don't break them
@@ -414,11 +441,21 @@ func transformGo(node ast.Node, info *types.Info) ast.Node {
 					return true // std isn't transformed
 				}
 				if id := buildInfo.imports[path].buildID; id != "" {
+					garbledPkg, err := garbledImport(path)
+					if err != nil {
+						panic(err) // shouldn't happen
+					}
+					// Check if the imported name wasn't
+					// garbled, e.g. if it's assembly.
+					if garbledPkg.Scope().Lookup(obj.Name()) != nil {
+						return true
+					}
 					buildID = id
 				}
 			}
-			// log.Printf("%q hashed with %q", node.Name, buildID)
+			// orig := node.Name
 			node.Name = hashWith(buildID, node.Name)
+			// log.Printf("%q hashed with %q to %q", orig, buildID, node.Name)
 		}
 		return true
 	}
@@ -435,6 +472,18 @@ func isStandardLibrary(path string) bool {
 	return !strings.Contains(path, ".")
 }
 
+// implementedOutsideGo returns whether a *types.Func does not have a body, for
+// example when it's implemented in assembly.
+//
+// Note that this function can only return true if the obj parameter was
+// type-checked from source - that is, if it's the top-level package we're
+// building. Dependency packages, whose type information comes from export data,
+// do not differentiate these "external funcs" in any way.
+func implementedOutsideGo(obj *types.Func) bool {
+	return obj.Type().(*types.Signature).Recv() == nil &&
+		(obj.Scope() != nil && obj.Scope().Pos() == token.NoPos)
+}
+
 func objOf(t types.Type) types.Object {
 	switch t := t.(type) {
 	case *types.Named:

testdata/scripts/asm.txt

@@ -0,0 +1,49 @@
+garble build
+exec ./main
+cmp stdout main.stdout
+binsubstr main$exe 'privateAdd' 'PublicAdd'
+
+[short] stop # no need to verify this with -short
+
+go build
+exec ./main
+cmp stdout main.stdout
+
+-- go.mod --
+module foo.com/main
+-- main.go --
+package main
+
+import (
+	"fmt"
+
+	"foo.com/main/imported"
+)
+
+func privateAdd(x, y int64) int64
+
+func main() {
+	fmt.Println(privateAdd(1, 2))
+	fmt.Println(imported.PublicAdd(3, 4))
+}
+-- main.s --
+TEXT ·privateAdd(SB),$0-24
+	MOVQ x+0(FP), BX
+	MOVQ y+8(FP), BP
+	ADDQ BP, BX
+	MOVQ BX, ret+16(FP)
+	RET
+-- imported/imported.go --
+package imported
+
+func PublicAdd(x, y int64) int64
+-- imported/imported.s --
+TEXT ·PublicAdd(SB),$0-24
+	MOVQ x+0(FP), BX
+	MOVQ y+8(FP), BP
+	ADDQ BP, BX
+	MOVQ BX, ret+16(FP)
+	RET
+-- main.stdout --
+3
+7

testdata/scripts/implement.txt

@@ -2,7 +2,7 @@ garble build main.go
 exec ./main
 cmp stdout main.stdout
 
-! binsubstr main$exe 'unexportedMethod'
+! binsubstr main$exe 'unexportedMethod' 'privateIface'
 
 -- main.go --
 package main
@@ -19,6 +19,14 @@ func (t T) unexportedMethod() string {
 	return "unexported method for " + string(t)
 }
 
+type privateInterface interface {
+	privateIface()
+}
+
+func (T) privateIface() {}
+
+var _ privateInterface = T("")
+
 func main() {
 	fmt.Println(T("foo"))
 	fmt.Println(T("foo").unexportedMethod())