systemd-vaultd-update-secrets: get service name from MAINPID

cmd/systemd-vaultd-update-secrets/main.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"os"
 	"path"
+	"strings"
 	"syscall"
 	"time"
 )
@@ -85,14 +86,31 @@ func updateSecrets(serviceName, target string) error {
 	return nil
 }
 
+func getSystemdServiceName() (string, error) {
+	mainPid := os.Getenv("MAINPID")
+	if mainPid == "" {
+		return "", fmt.Errorf("MAINPID not set")
+	}
+	p := fmt.Sprintf("/proc/%s/cgroup", mainPid)
+	content, err := os.ReadFile(p)
+	if err != nil {
+		return "", fmt.Errorf("failed to read cgroup file %s: %w", p, err)
+	}
+	line := strings.SplitN(string(content), "\n", 2)[0]
+	if !strings.HasSuffix(line, ".service") {
+		return "", fmt.Errorf("cgroup file %s does not end with .service", p)
+	}
+	return path.Base(line), nil
+}
+
 func main() {
 	if len(os.Args) != 2 {
 		fmt.Println("Usage: systemd-vaultd-update-secrets <target>")
 		os.Exit(1)
 	}
-	serviceName := os.Getenv("SYSTEMD_ACTIVATION_UNIT")
+	serviceName, err := getSystemdServiceName()
-	if serviceName == "" {
+	if err != nil {
-		fmt.Println("SYSTEMD_ACTIVATION_UNIT not set")
+		fmt.Println(err)
 		os.Exit(1)
 	}