capped_file: increase test coverage, fix out-of-bounds check

Author: holiman

capped_file.go

@@ -64,7 +64,9 @@ func (cf *cappedFile) WriteAt(data []byte, off int64) (n int, err error) {
 		totalLength = len(data)
 		fileNum     = offset / cf.cap
 	)
-	if fileNum >= uint64(len(cf.files)) {
+	// Check if the write starts or ends out of bounds
+	if fileNum >= uint64(len(cf.files)) ||
+		(offset+uint64(len(data)))/cf.cap >= uint64(len(cf.files)) {
 		return 0, ErrBadIndex
 	}
 	for ; written < totalLength; fileNum++ {
@@ -98,7 +100,9 @@ func (cf *cappedFile) ReadAt(b []byte, off int64) (n int, err error) {
 		read    int // no of bytes read
 		fileNum = offset / cf.cap
 	)
-	if fileNum >= uint64(len(cf.files)) {
+	// Check if the read starts or ends out of bounds
+	if fileNum >= uint64(len(cf.files)) ||
+		(offset+uint64(len(b)))/cf.cap >= uint64(len(cf.files)) {
 		return 0, ErrBadIndex
 	}
 	for ; read < len(b); fileNum++ {

capped_file_test.go

@@ -6,6 +6,7 @@ package billy
 
 import (
 	"bytes"
+	"errors"
 	"os"
 	"testing"
 )
@@ -132,3 +133,31 @@ func TestReadonly(t *testing.T) {
 		t.Fatalf("want error trying to write files in readonly, got none")
 	}
 }
+
+func TestOutOfBounds(t *testing.T) {
+	f, err := newCappedFile("multifile-ro-test", 10, 50, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		f.Close()
+		wipe(t, f)
+	})
+	// Read starting OOB
+	if _, err := f.ReadAt(make([]byte, 10), 501); !errors.Is(err, ErrBadIndex) {
+		t.Fatalf("want %v have %v", ErrBadIndex, err)
+	}
+	// Read reaching into OOB
+	if _, err := f.ReadAt(make([]byte, 10), 495); !errors.Is(err, ErrBadIndex) {
+		t.Fatalf("want %v have %v", ErrBadIndex, err)
+	}
+	// Write starting OOB
+	if _, err := f.WriteAt(make([]byte, 10), 501); !errors.Is(err, ErrBadIndex) {
+		t.Fatalf("want %v have %v", ErrBadIndex, err)
+	}
+	// Write reaching into OOB
+	if _, err := f.WriteAt(make([]byte, 10), 495); !errors.Is(err, ErrBadIndex) {
+		t.Fatalf("want %v have %v", ErrBadIndex, err)
+	}
+
+}