SystemServer+LibCore: Implement socket takeover

SystemServer can now create sockets on behalf of services before spawning any
of them, and pass the open socket fd as fd 3. CLocalServer gains a method to
complete the takeover and listen on the passed fd.

This is not used by any services at the moment.

Libraries/LibCore/CLocalServer.cpp

@@ -3,18 +3,67 @@
 #include <LibCore/CNotifier.h>
 #include <stdio.h>
 #include <sys/socket.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
 CLocalServer::CLocalServer(CObject* parent)
     : CObject(parent)
 {
-    m_fd = socket(AF_LOCAL, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
-    ASSERT(m_fd >= 0);
 }
 
 CLocalServer::~CLocalServer()
 {
 }
 
+bool CLocalServer::take_over_from_system_server()
+{
+    if (m_listening)
+        return false;
+
+    constexpr auto socket_takeover = "SOCKET_TAKEOVER";
+
+    if (getenv(socket_takeover)) {
+        dbg() << "Taking the socket over from SystemServer";
+
+        // Sanity check: it has to be a socket.
+        struct stat stat;
+        int rc = fstat(3, &stat);
+        if (rc == 0 && S_ISSOCK(stat.st_mode)) {
+            // The SystemServer has passed us the socket as fd 3,
+            // so use that instead of creating our own.
+            m_fd = 3;
+            // It had to be !CLOEXEC for obvious reasons, but we
+            // don't need it to be !CLOEXEC anymore, so set the
+            // CLOEXEC flag now.
+            fcntl(m_fd, F_SETFD, FD_CLOEXEC);
+            // We wouldn't want our children to think we're passing
+            // them a socket either, so unset the env variable.
+            unsetenv(socket_takeover);
+
+            m_listening = true;
+            setup_notifier();
+            return true;
+        } else {
+            if (rc != 0)
+                perror("fstat");
+            dbg() << "It's not a socket, what the heck??";
+        }
+    }
+
+    dbg() << "Failed to take the socket over from SystemServer";
+
+    return false;
+}
+
+void CLocalServer::setup_notifier()
+{
+    m_notifier = CNotifier::construct(m_fd, CNotifier::Event::Read, this);
+    m_notifier->on_ready_to_read = [this] {
+        if (on_ready_to_accept)
+            on_ready_to_accept();
+    };
+}
+
 bool CLocalServer::listen(const String& address)
 {
     if (m_listening)
@@ -22,20 +71,25 @@ bool CLocalServer::listen(const String& address)
 
     int rc;
 
+    m_fd = socket(AF_LOCAL, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
+    ASSERT(m_fd >= 0);
+
     auto socket_address = CSocketAddress::local(address);
     auto un = socket_address.to_sockaddr_un();
     rc = ::bind(m_fd, (const sockaddr*)&un, sizeof(un));
-    ASSERT(rc == 0);
+    if (rc < 0) {
+        perror("bind");
+        ASSERT_NOT_REACHED();
+    }
 
     rc = ::listen(m_fd, 5);
-    ASSERT(rc == 0);
-    m_listening = true;
+    if (rc < 0) {
+        perror("listen");
+        ASSERT_NOT_REACHED();
+    }
 
-    m_notifier = CNotifier::construct(m_fd, CNotifier::Event::Read, this);
-    m_notifier->on_ready_to_read = [this] {
-        if (on_ready_to_accept)
-            on_ready_to_accept();
-    };
+    m_listening = true;
+    setup_notifier();
     return true;
 }
 

Libraries/LibCore/CLocalServer.h

@@ -10,6 +10,7 @@ class CLocalServer : public CObject {
 public:
     virtual ~CLocalServer() override;
 
+    bool take_over_from_system_server();
     bool is_listening() const { return m_listening; }
     bool listen(const String& address);
 
@@ -20,6 +21,8 @@ public:
 private:
     explicit CLocalServer(CObject* parent = nullptr);
 
+    void setup_notifier();
+
     int m_fd { -1 };
     bool m_listening { false };
     RefPtr<CNotifier> m_notifier;

Servers/SystemServer/Service.cpp

@@ -3,7 +3,9 @@
 #include <AK/JsonArray.h>
 #include <AK/JsonObject.h>
 #include <LibCore/CConfigFile.h>
+#include <LibCore/CLocalSocket.h>
 #include <fcntl.h>
+#include <libgen.h>
 #include <pwd.h>
 #include <sched.h>
 #include <stdio.h>
@@ -44,6 +46,61 @@ Service* Service::find_by_pid(pid_t pid)
     return (*it).value;
 }
 
+static int ensure_parent_directories(const char* path)
+{
+    ASSERT(path[0] == '/');
+
+    char* parent_buffer = strdup(path);
+    const char* parent = dirname(parent_buffer);
+
+    int rc = 0;
+    while (true) {
+        int rc = mkdir(parent, 0755);
+
+        if (rc == 0)
+            break;
+
+        if (errno != ENOENT)
+            break;
+
+        ensure_parent_directories(parent);
+    };
+
+    free(parent_buffer);
+    return rc;
+}
+
+void Service::setup_socket()
+{
+    ASSERT(!m_socket_path.is_null());
+    ASSERT(m_socket_fd == -1);
+
+    ensure_parent_directories(m_socket_path.characters());
+
+    // Note: we use SOCK_CLOEXEC here to make sure we don't leak every socket to
+    // all the clients. We'll make the one we do need to pass down !CLOEXEC later
+    // after forking off the process.
+    m_socket_fd = socket(AF_LOCAL, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0);
+    if (m_socket_fd < 0) {
+        perror("socket");
+        ASSERT_NOT_REACHED();
+    }
+
+    auto socket_address = CSocketAddress::local(m_socket_path);
+    auto un = socket_address.to_sockaddr_un();
+    int rc = bind(m_socket_fd, (const sockaddr*)&un, sizeof(un));
+    if (rc < 0) {
+        perror("bind");
+        ASSERT_NOT_REACHED();
+    }
+
+    rc = listen(m_socket_fd, 5);
+    if (rc < 0) {
+        perror("listen");
+        ASSERT_NOT_REACHED();
+    }
+}
+
 void Service::spawn()
 {
     dbg() << "Spawning " << name();
@@ -76,6 +133,14 @@ void Service::spawn()
             dup2(0, 2);
         }
 
+        if (!m_socket_path.is_null()) {
+            ASSERT(m_socket_fd > 2);
+            dup2(m_socket_fd, 3);
+            // The new descriptor is !CLOEXEC here.
+            // This is true even if m_socket_fd == 3.
+            setenv("SOCKET_TAKEOVER", "1", true);
+        }
+
         if (!m_user.is_null()) {
             setuid(m_uid);
             setgid(m_gid);
@@ -134,6 +199,11 @@ Service::Service(const CConfigFile& config, const StringView& name)
 
     m_keep_alive = config.read_bool_entry(name, "KeepAlive");
 
+    m_socket_path = config.read_entry(name, "Socket");
+    if (!m_socket_path.is_null()) {
+        setup_socket();
+    }
+
     m_user = config.read_entry(name, "User");
     if (!m_user.is_null())
         resolve_user();
@@ -156,6 +226,7 @@ void Service::save_to(JsonObject& json)
     json.set("stdio_file_path", m_stdio_file_path);
     json.set("priority", m_priority);
     json.set("keep_alive", m_keep_alive);
+    json.set("socket_path", m_socket_path);
     json.set("user", m_user);
     json.set("uid", m_uid);
     json.set("gid", m_gid);

Servers/SystemServer/Service.h

@@ -2,6 +2,7 @@
 
 #include <AK/RefPtr.h>
 #include <AK/String.h>
+#include <LibCore/CNotifier.h>
 #include <LibCore/CObject.h>
 
 class CConfigFile;
@@ -33,6 +34,8 @@ private:
     int m_priority { 1 };
     // Whether we should re-launch it if it exits.
     bool m_keep_alive { false };
+    // Path to the socket to create and listen on on behalf of this service.
+    String m_socket_path;
     // The name of the user we should run this service as.
     String m_user;
     uid_t m_uid { 0 };
@@ -40,6 +43,9 @@ private:
 
     // PID of the running instance of this service.
     pid_t m_pid { -1 };
+    // An open fd to the socket.
+    int m_socket_fd { -1 };
 
     void resolve_user();
+    void setup_socket();
 };

Servers/SystemServer/main.cpp

@@ -89,11 +89,13 @@ int main(int, char**)
     CEventLoop event_loop;
 
     // Read our config and instantiate services.
+    // This takes care of setting up sockets.
     Vector<RefPtr<Service>> services;
     auto config = CConfigFile::get_for_system("SystemServer");
     for (auto name : config->groups())
         services.append(Service::construct(*config, name));
 
+    // After we've set them all up, spawn them!
     for (auto& service : services)
         service->spawn();