internal/pkg: optionally validate flat pathnames

This makes the decoder safe against untrusted input without hurting performance for a trusted stream. This should still not be called against untrusted input though.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/dir.go

@@ -8,6 +8,7 @@ import (
 	"io/fs"
 	"math"
 	"os"
+	"path/filepath"
 	"syscall"
 
 	"hakurei.app/container/check"
@@ -52,8 +53,12 @@ func (ent *FlatEntry) Encode(w io.Writer) (n int, err error) {
 	return w.Write(payload)
 }
 
+// ErrInsecurePath is returned by [FlatEntry.Decode] if validation is requested
+// and a nonlocal path is encountered in the stream.
+var ErrInsecurePath = errors.New("insecure file path")
+
 // Decode decodes the entry from its representation produced by Encode.
-func (ent *FlatEntry) Decode(r io.Reader) (n int, err error) {
+func (ent *FlatEntry) Decode(r io.Reader, validate bool) (n int, err error) {
 	var nr int
 
 	header := make([]byte, wordSize*2)
@@ -92,6 +97,11 @@ func (ent *FlatEntry) Decode(r io.Reader) (n int, err error) {
 	} else {
 		ent.Data = buf[pPathSize : pPathSize+dataSize]
 	}
+
+	if validate && !filepath.IsLocal(ent.Path) {
+		err = ErrInsecurePath
+	}
+
 	return
 }
 
@@ -108,11 +118,16 @@ type DirScanner struct {
 	// Entry to store results in. Its address is returned by the Entry method
 	// and is updated on every call to Scan.
 	ent FlatEntry
+
+	// Validate pathnames during decoding.
+	validate bool
 }
 
 // NewDirScanner returns the address of a new instance of [DirScanner] reading
 // from r. The caller must no longer read from r after this function returns.
-func NewDirScanner(r io.Reader) *DirScanner { return &DirScanner{r: r} }
+func NewDirScanner(r io.Reader, validate bool) *DirScanner {
+	return &DirScanner{r: r, validate: validate}
+}
 
 // Err returns the first non-EOF I/O error.
 func (s *DirScanner) Err() error {
@@ -132,7 +147,7 @@ func (s *DirScanner) Scan() bool {
 	}
 
 	var n int
-	n, s.err = s.ent.Decode(s.r)
+	n, s.err = s.ent.Decode(s.r, s.validate)
 	if errors.Is(s.err, io.EOF) {
 		return n != 0
 	}

internal/pkg/dir_test.go

@@ -85,7 +85,7 @@ func TestFlatten(t *testing.T) {
 					t.Fatalf("Flatten: error = %v", err)
 				}
 
-				s := pkg.NewDirScanner(bytes.NewReader(buf.Bytes()))
+				s := pkg.NewDirScanner(bytes.NewReader(buf.Bytes()), true)
 				var got []pkg.FlatEntry
 				for s.Scan() {
 					got = append(got, *s.Entry())