support reversing stack trace positions (#287)

In particular, the positions within function declarations, including the
positions of call sites to other functions.

Note that this isn't well tested just yet, particularly not with other
features like -literals. We can extend the tests and code over time.
This gets us the core basics.

The issue will be closed once the feature is documented for users, in a
follow-up PR.

Updates #5.

position.go

@@ -21,11 +21,11 @@ func isDirective(text string) bool {
 // printFile prints a Go file to a buffer, while also removing non-directive
 // comments and adding extra compiler directives to obfuscate position
 // information.
-func printFile(file *ast.File) ([]byte, error) {
+func printFile(file1 *ast.File) ([]byte, error) {
 	printConfig := printer.Config{Mode: printer.RawFormat}
 
 	var buf1 bytes.Buffer
-	if err := printConfig.Fprint(&buf1, fset, file); err != nil {
+	if err := printConfig.Fprint(&buf1, fset, file1); err != nil {
 		return nil, err
 	}
 	src := buf1.Bytes()
@@ -36,8 +36,9 @@ func printFile(file *ast.File) ([]byte, error) {
 		return src, nil
 	}
 
-	filename := fset.Position(file.Pos()).Filename
-	if strings.HasPrefix(filepath.Base(filename), "_cgo_") {
+	absFilename := fset.Position(file1.Pos()).Filename
+	filename := filepath.Base(absFilename)
+	if strings.HasPrefix(filename, "_cgo_") {
 		// cgo-generated files don't need changed line numbers.
 		// Plus, the compiler can complain rather easily.
 		return src, nil
@@ -48,7 +49,7 @@ func printFile(file *ast.File) ([]byte, error) {
 	// and those are the only source of truth that go/printer uses.
 	// So the positions of the comments in the given file are wrong.
 	// The only way we can get the final ones is to parse again.
-	file, err := parser.ParseFile(fset, filename, src, parser.ParseComments)
+	file2, err := parser.ParseFile(fset, absFilename, src, parser.ParseComments)
 	if err != nil {
 		return nil, err
 	}
@@ -63,7 +64,7 @@ func printFile(file *ast.File) ([]byte, error) {
 		toAdd = append(toAdd, commentToAdd{offset, text})
 	}
 	addComment(0, "/*line :1*/")
-	for _, group := range file.Comments {
+	for _, group := range file2.Comments {
 		for _, comment := range group.List {
 			if isDirective(comment.Text) {
 				// TODO(mvdan): merge with the zeroing below
@@ -73,7 +74,9 @@ func printFile(file *ast.File) ([]byte, error) {
 		}
 	}
 	// Remove all existing comments by making them whitespace.
-	for _, group := range file.Comments {
+	// This is superior to removing the comments before printing,
+	// because then the final source would have different line numbers.
+	for _, group := range file2.Comments {
 		for _, comment := range group.List {
 			start := fset.Position(comment.Pos()).Offset
 			end := fset.Position(comment.End()).Offset
@@ -83,9 +86,10 @@ func printFile(file *ast.File) ([]byte, error) {
 		}
 	}
 
-	for _, decl := range file.Decls {
+	for i, decl := range file2.Decls {
 		newName := ""
 		if !opts.Tiny {
+			decl := file1.Decls[i]
 			origPos := fmt.Sprintf("%s:%d", filename, fset.Position(decl.Pos()).Offset)
 			newName = hashWith(curPkg.GarbleActionID, origPos) + ".go"
 			// log.Printf("%q hashed with %x to %q", origPos, curPkg.GarbleActionID, newName)

reverse.go

@@ -5,6 +5,7 @@ package main
 
 import (
 	"bufio"
+	"fmt"
 	"go/ast"
 	"go/parser"
 	"go/token"
@@ -56,12 +57,29 @@ func commandReverse(args []string) error {
 		addReplace(lpkg.ImportPath)
 
 		for _, goFile := range lpkg.GoFiles {
-			goFile = filepath.Join(lpkg.Dir, goFile)
-			file, err := parser.ParseFile(fset, goFile, nil, 0)
+			fullGoFile := filepath.Join(lpkg.Dir, goFile)
+			file, err := parser.ParseFile(fset, fullGoFile, nil, 0)
 			if err != nil {
 				return err
 			}
 			for _, decl := range file.Decls {
+				// Reverse position information.
+				pos := fset.Position(decl.Pos())
+				origPos := fmt.Sprintf("%s:%d", goFile, pos.Offset)
+				newFilename := hashWith(lpkg.GarbleActionID, origPos) + ".go"
+
+				// For each line number within this function,
+				// replace the obfuscated position with its original form.
+				// Remember that the "/*line" directive starts at 1.
+				numLines := fset.Position(decl.End()).Line - pos.Line
+				for i := 0; i < numLines; i++ {
+					replaces = append(replaces,
+						fmt.Sprintf("%s:%d", newFilename, 1+i),
+						fmt.Sprintf("%s/%s:%d", lpkg.ImportPath, goFile, pos.Line+i),
+					)
+				}
+
+				// Reverse the name itself.
 				// TODO: Probably do type names too. What else?
 				switch decl := decl.(type) {
 				case *ast.FuncDecl:

testdata/scripts/reverse.txt

@@ -13,14 +13,13 @@ cp stderr main.stderr
 # This output is not reproducible between 'go test' runs,
 # so we can't use a static golden file.
 grep 'goroutine 1 \[running\]' main.stderr
-! grep 'ExportedLibFunc|unexportedMainFunc|test/main|main.go|lib.go' main.stderr
+! grep 'ExportedLibFunc|unexportedMainFunc|test/main|main\.go|lib\.go' main.stderr
 
 stdin main.stderr
 garble reverse
-stdout -count=1 'test/main/lib\.ExportedLibFunc'
-stdout -count=1 'main\.unexportedMainFunc'
-# TODO: this is what we want when "reverse" is finished
-# cmp stdout reverse.stdout
+cmp stdout reverse.stdout
+
+# TODO: test with -literals too, as it modifies the source.
 
 # Ensure that the reversed output matches the non-garbled output.
 go build -trimpath
@@ -64,6 +63,8 @@ func ExportedLibFunc(w io.Writer) error {
 	// Strip them out here, to have portable static stdout files.
 	rxVariableSuffix := regexp.MustCompile(`0x[0-9a-f]+`)
 
+	// Keep this comment here, because comments affect line numbers.
+
 	stack := debug.Stack()
 	stack = rxVariableSuffix.ReplaceAll(stack, []byte("0x??"))
 	_, err := w.Write(stack)
@@ -74,7 +75,7 @@ goroutine 1 [running]:
 runtime/debug.Stack(0x??, 0x??, 0x??)
 	runtime/debug/stack.go:24 +0x??
 test/main/lib.ExportedLibFunc(0x??, 0x??, 0x??, 0x??)
-	test/main/lib/lib.go:15 +0x??
+	test/main/lib/lib.go:17 +0x??
 main.unexportedMainFunc(...)
 	test/main/main.go:14
 main.main()