Introduce and use EscapeBusAddressValue

D-Bus specification [1] requires that the values in server address need
to be escaped in a special way, and other clients perform the needed
escaping (e.g. systemd [2] does that).

More to say, if dbus sees a character that should have been escaped, it
returns an error [3].

Add EscapeBusAddressValue function (with some tests and a benchmark),
and use it from tryDiscoverDbusSessionBusAddress().

The function is exported as it might be of use to other packages (in
particular, runc [4]).

NOTE that the function does not escape '*' symbol, because de-facto it
is the list of optionally-escaped characters since 2007, but that is not
yet documented in D-Bus spec (see [5] for more details).

[1] https://dbus.freedesktop.org/doc/dbus-specification.html#addresses
[2] https://github.com/systemd/systemd/blob/5efbd0bf897a990ebe43d7dc69141d87c404ac9a/src/libsystemd/sd-bus/bus-internal.c#L294-L318
[3] https://gitlab.freedesktop.org/dbus/dbus/-/blob/37b76d13738e782fe2eb12abdd0179745c0b3f81/dbus/dbus-address.c#L330
[4] opencontainers/runc#3356
[5] https://gitlab.freedesktop.org/dbus/dbus/-/merge_requests/248

Signed-off-by: Kir Kolyshkin <[email protected]>

conn_other.go

@@ -54,7 +54,7 @@ func tryDiscoverDbusSessionBusAddress() string {
 		if runUserBusFile := path.Join(runtimeDirectory, "bus"); fileExists(runUserBusFile) {
 			// if /run/user/<uid>/bus exists, that file itself
 			// *is* the unix socket, so return its path
-			return fmt.Sprintf("unix:path=%s", runUserBusFile)
+			return fmt.Sprintf("unix:path=%s", EscapeBusAddressValue(runUserBusFile))
 		}
 		if runUserSessionDbusFile := path.Join(runtimeDirectory, "dbus-session"); fileExists(runUserSessionDbusFile) {
 			// if /run/user/<uid>/dbus-session exists, it's a

escape.go

@@ -0,0 +1,68 @@
+package dbus
+
+import "strings"
+
+// EscapeBusAddressValue implements a requirement to escape the values
+// in D-Bus server addresses, as defined by the D-Bus specification at
+// https://dbus.freedesktop.org/doc/dbus-specification.html#addresses.
+func EscapeBusAddressValue(val string) string {
+	toEsc := strNeedsEscape(val)
+	if toEsc == 0 {
+		// Avoid unneeded allocation/copying.
+		return val
+	}
+
+	var out strings.Builder
+	// Every to-be-escaped byte needs 3 bytes, i.e. 2 extra bytes.
+	out.Grow(len(val) + 2*toEsc)
+
+	for i := 0; i < len(val); i++ {
+		ch := val[i]
+		if !needsEscape(ch) {
+			_ = out.WriteByte(ch)
+
+			continue
+		}
+		// Convert ch to %xx, where xx is hex value.
+		_ = out.WriteByte('%')
+		_ = out.WriteByte(hexchar(ch >> 4))
+		_ = out.WriteByte(hexchar(ch & 0x0F))
+	}
+
+	return out.String()
+}
+
+// hexchar returns an octal representation of a n, where n < 16.
+// For invalid values of n, the function panics.
+func hexchar(n byte) byte {
+	const hex = "0123456789abcdef"
+
+	// For n >= len(hex), runtime will panic.
+	return hex[n]
+}
+
+// needsEscape tells if a byte is NOT one of optionally-escaped bytes.
+func needsEscape(c byte) bool {
+	if 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || '0' <= c && c <= '9' {
+		return false
+	}
+	switch c {
+	case '-', '_', '/', '\\', '.', '*':
+		return false
+	}
+
+	return true
+}
+
+// strNeedsEscape tells how many bytes in the string need escaping.
+func strNeedsEscape(val string) int {
+	count := 0
+
+	for i := 0; i < len(val); i++ {
+		if needsEscape(val[i]) {
+			count++
+		}
+	}
+
+	return count
+}

escape_test.go

@@ -0,0 +1,47 @@
+package dbus
+
+import (
+	"net/url"
+	"testing"
+)
+
+var escapeTestCases = []struct {
+	in, out string
+}{
+	{in: "", out: ""},
+	{in: "ABCDabcdZYXzyx01289", out: "ABCDabcdZYXzyx01289"},
+	{in: `_-/\*`, out: `_-/\*`},
+	{in: `=+:~!`, out: `%3d%2b%3a%7e%21`},
+	{in: `space here`, out: `space%20here`},
+	{in: `Привет`, out: `%d0%9f%d1%80%d0%b8%d0%b2%d0%b5%d1%82`},
+	{in: `ჰეი`, out: `%e1%83%b0%e1%83%94%e1%83%98`},
+	{in: `你好`, out: `%e4%bd%a0%e5%a5%bd`},
+	{in: `こんにちは`, out: `%e3%81%93%e3%82%93%e3%81%ab%e3%81%a1%e3%81%af`},
+}
+
+func TestEscapeBusAddressValue(t *testing.T) {
+	for _, tc := range escapeTestCases {
+		out := EscapeBusAddressValue(tc.in)
+		if out != tc.out {
+			t.Errorf("input: %q; want %q, got %q", tc.in, tc.out, out)
+		}
+		// Use QueryUnescape to unescape %xx back to bytes.
+		in, err := url.QueryUnescape(out)
+		if err != nil {
+			t.Errorf("unescape error: %v", err)
+		} else if in != tc.in {
+			t.Errorf("unescape: want %q, got %q", tc.in, in)
+		}
+	}
+}
+
+func BenchmarkEscapeBusAddressValue(b *testing.B) {
+	var out string
+
+	for i := 0; i < b.N; i++ {
+		for _, tc := range escapeTestCases {
+			out = EscapeBusAddressValue(tc.in)
+		}
+	}
+	b.Log("out:", out)
+}