Implement a reasonable default seed

Fixes #156

hash.go

@@ -204,10 +204,10 @@ func toUpper(b byte) byte { return b - ('a' - 'A') }
 func runtimeHashWithCustomSalt(salt []byte) uint32 {
 	hasher.Reset()
 	if !flagSeed.present() {
-		hasher.Write(sharedCache.ListedPackages["runtime"].GarbleActionID[:])
-	} else {
-		hasher.Write(flagSeed.bytes)
+		panic("runtimeHashWithCustomSalt: no seed")
 	}
+
+	hasher.Write(flagSeed.bytes)
 	hasher.Write(salt)
 	sum := hasher.Sum(sumBuffer[:0])
 	return binary.LittleEndian.Uint32(sum)
@@ -226,12 +226,6 @@ func entryOffKey() uint32 {
 }
 
 func hashWithPackage(pkg *listedPackage, name string) string {
-	// If the user provided us with an obfuscation seed,
-	// we use that with the package import path directly..
-	// Otherwise, we use GarbleActionID as a fallback salt.
-	if !flagSeed.present() {
-		return hashWithCustomSalt(pkg.GarbleActionID[:], name)
-	}
 	// Use a separator at the end of ImportPath as a salt,
 	// to ensure that "pkgfoo.bar" and "pkg.foobar" don't both hash
 	// as the same string "pkgfoobar".
@@ -256,13 +250,6 @@ func hashWithStruct(strct *types.Struct, field *types.Var) string {
 	// TODO: rethink once the proposed go/types.Hash API in https://go.dev/issue/69420 is merged.
 	salt := strconv.AppendUint(nil, uint64(typeutil_hash(strct)), 32)
 
-	// If the user provided us with an obfuscation seed,
-	// we only use the identity struct type as a salt.
-	// Otherwise, we add garble's own inputs to the salt as a fallback.
-	if !flagSeed.present() {
-		withGarbleHash := addGarbleToHash(salt)
-		salt = withGarbleHash[:]
-	}
 	return hashWithCustomSalt(salt, field.Name())
 }
 
@@ -333,6 +320,9 @@ func hashWithCustomSalt(salt []byte, name string) string {
 	if name == "" {
 		panic("hashWithCustomSalt: empty name")
 	}
+	if !flagSeed.present() {
+		panic("hashWithCustomSalt:no seed")
+	}
 
 	hasher.Reset()
 	hasher.Write(salt)

main.go

@@ -9,6 +9,7 @@ import (
 	"bytes"
 	"cmp"
 	cryptorand "crypto/rand"
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/binary"
 	"encoding/gob"
@@ -35,6 +36,7 @@ import (
 	"runtime/debug"
 	"runtime/pprof"
 	"slices"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -562,6 +564,11 @@ This command wraps "go %s". Below is its help:
 		return nil, err
 	}
 
+	// Compute a deterministic default seed if the user did not provide one.
+	if err := computeDefaultSeed(); err != nil {
+		return nil, err
+	}
+
 	sharedTempDir, err = saveSharedCache()
 	if err != nil {
 		return nil, err
@@ -969,10 +976,10 @@ func (tf *transformer) transformCompile(args []string) ([]string, error) {
 	}
 
 	// Literal and control flow obfuscation uses math/rand, so seed it deterministically.
-	randSeed := tf.curPkg.GarbleActionID[:]
-	if flagSeed.present() {
-		randSeed = flagSeed.bytes
+	if !flagSeed.present() {
+		panic("transformCompile: no seed")
 	}
+	randSeed := flagSeed.bytes
 	// log.Printf("seeding math/rand with %x\n", randSeed)
 	tf.obfRand = mathrand.New(mathrand.NewSource(int64(binary.BigEndian.Uint64(randSeed))))
 
@@ -2336,3 +2343,63 @@ To install Go, see: https://go.dev/doc/install
 	sharedCache.GOGARBLE = cmp.Or(os.Getenv("GOGARBLE"), "*") // we default to obfuscating everything
 	return nil
 }
+
+func computeDefaultSeed() error {
+	// If the user has provided a seed explicitly (or random), keep it.
+	if flagSeed.present() {
+		return nil
+	}
+
+	// Step 1: hash module information from `go list -m`.
+	args := []string{"list", "-m", "-json=Path,Version,GoVersion,Sum,GoModSum,Indirect,Time,Main", "all"}
+
+	cmd := exec.Command(sharedCache.GoCmd, args...)
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("go list modules failed: %v\n%s", err, stderr.String())
+	}
+
+	hasher := sha256.New()
+	hasher.Write(stdout.Bytes())
+
+	// Parse the module list to determine the main module path.
+	dec := json.NewDecoder(bytes.NewReader(stdout.Bytes()))
+	mainModulePath := ""
+	for dec.More() {
+		var mod struct {
+			Path string
+			Main bool
+		}
+		if err := dec.Decode(&mod); err != nil {
+			return fmt.Errorf("decode module json: %w", err)
+		}
+		if mod.Main {
+			mainModulePath = mod.Path
+		}
+	}
+	if mainModulePath == "" {
+		return errors.New("could not determine main module path for seed derivation")
+	}
+
+	// Step 2: include BuildIDs of all packages inside the main module.
+	var pkgs []string
+	for path := range sharedCache.ListedPackages {
+		if strings.HasPrefix(path, mainModulePath) {
+			pkgs = append(pkgs, path)
+		}
+	}
+	// Sort for deterministic order.
+	sort.Strings(pkgs)
+	for _, path := range pkgs {
+		if buildID := sharedCache.ListedPackages[path].BuildID; buildID != "" {
+			hasher.Write([]byte(buildID))
+		}
+	}
+
+	seed := hasher.Sum(nil)
+	flagSeed.bytes = seed
+
+	return nil
+}