container/mount: mount data escape helper function

For formatting user-supplied path strings into overlayfs mount data.

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

container/mount.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 	. "syscall"
 
 	"hakurei.app/container/vfs"
@@ -183,3 +184,20 @@ func parentPerm(perm os.FileMode) os.FileMode {
 	}
 	return os.FileMode(pperm)
 }
+
+// escapeOverlayDataSegment escapes a string for formatting into the data argument of an overlay mount call.
+func escapeOverlayDataSegment(s string) string {
+	if s == zeroString {
+		return zeroString
+	}
+
+	if f := strings.SplitN(s, "\x00", 2); len(f) > 0 {
+		s = f[0]
+	}
+
+	return strings.NewReplacer(
+		`\`, `\\`,
+		`,`, `\,`,
+		`:`, `\:`,
+	).Replace(s)
+}

container/mount_test.go

@@ -0,0 +1,49 @@
+package container
+
+import (
+	"os"
+	"testing"
+)
+
+func TestParentPerm(t *testing.T) {
+	testCases := []struct {
+		perm os.FileMode
+		want os.FileMode
+	}{
+		{0755, 0755},
+		{0750, 0750},
+		{0705, 0705},
+		{0700, 0700},
+		{050, 0750},
+		{05, 0705},
+		{0, 0700},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.perm.String(), func(t *testing.T) {
+			if got := parentPerm(tc.perm); got != tc.want {
+				t.Errorf("parentPerm: %#o, want %#o", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestEscapeOverlayDataSegment(t *testing.T) {
+	testCases := []struct {
+		name string
+		s    string
+		want string
+	}{
+		{"zero", zeroString, zeroString},
+		{"multi", `\\\:,:,\\\`, `\\\\\\\:\,\:\,\\\\\\`},
+		{"bwrap", `/path :,\`, `/path \:\,\\`},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := escapeOverlayDataSegment(tc.s); got != tc.want {
+				t.Errorf("escapeOverlayDataSegment: %s, want %s", got, tc.want)
+			}
+		})
+	}
+}