]> git.ipfire.org Git - thirdparty/systemd.git/blobdiff - src/shared/varlink.c
various: use _NEG_ macros to reduce indentation
[thirdparty/systemd.git] / src / shared / varlink.c
index 5178da19c81596e27deb93c48e5da8fc6da9a126..41c2daf02bd8d21e12338ed405a0adf8fa509839 100644 (file)
@@ -12,6 +12,7 @@
 #include "list.h"
 #include "process-util.h"
 #include "selinux-util.h"
+#include "serialize.h"
 #include "set.h"
 #include "socket-util.h"
 #include "string-table.h"
@@ -21,6 +22,7 @@
 #include "umask-util.h"
 #include "user-util.h"
 #include "varlink.h"
+#include "varlink-internal.h"
 
 #define VARLINK_DEFAULT_CONNECTIONS_MAX 4096U
 #define VARLINK_DEFAULT_CONNECTIONS_PER_UID_MAX 1024U
@@ -79,6 +81,18 @@ typedef enum VarlinkState {
                VARLINK_PENDING_METHOD,                  \
                VARLINK_PENDING_METHOD_MORE)
 
+typedef struct VarlinkJsonQueueItem VarlinkJsonQueueItem;
+
+/* A queued message we shall write into the socket, along with the file descriptors to send at the same
+ * time. This queue item binds them together so that message/fd boundaries are maintained throughout the
+ * whole pipeline. */
+struct VarlinkJsonQueueItem {
+        LIST_FIELDS(VarlinkJsonQueueItem, queue);
+        JsonVariant *data;
+        size_t n_fds;
+        int fds[];
+};
+
 struct Varlink {
         unsigned n_ref;
 
@@ -119,14 +133,35 @@ struct Varlink {
         size_t input_buffer_size;
         size_t input_buffer_unscanned;
 
+        void *input_control_buffer;
+        size_t input_control_buffer_size;
+
         char *output_buffer; /* valid data starts at output_buffer_index, ends at output_buffer_index+output_buffer_size */
         size_t output_buffer_index;
         size_t output_buffer_size;
 
+        int *input_fds; /* file descriptors associated with the data in input_buffer (for fd passing) */
+        size_t n_input_fds;
+
+        int *output_fds; /* file descriptors associated with the data in output_buffer (for fd passing) */
+        size_t n_output_fds;
+
+        /* Further messages to output not yet formatted into text, and thus not included in output_buffer
+         * yet. We keep them separate from output_buffer, to not violate fd message boundaries: we want that
+         * each fd that is sent is associated with its fds, and that fds cannot be accidentally associated
+         * with preceding or following messages. */
+        LIST_HEAD(VarlinkJsonQueueItem, output_queue);
+        VarlinkJsonQueueItem *output_queue_tail;
+
+        /* The fds to associate with the next message that is about to be enqueued. The user first pushes the
+         * fds it intends to send via varlink_push_fd() into this queue, and then once the message data is
+         * submitted we'll combine the fds and the message data into one. */
+        int *pushed_fds;
+        size_t n_pushed_fds;
+
         VarlinkReply reply_callback;
 
         JsonVariant *current;
-        JsonVariant *reply;
 
         struct ucred ucred;
         bool ucred_acquired:1;
@@ -136,6 +171,13 @@ struct Varlink {
         bool prefer_read_write:1;
         bool got_pollhup:1;
 
+        bool allow_fd_passing_input:1;
+        bool allow_fd_passing_output:1;
+
+        bool output_buffer_sensitive:1; /* whether to erase the output buffer after writing it to the socket */
+
+        int af; /* address family if socket; AF_UNSPEC if not socket; negative if not known */
+
         usec_t timestamp;
         usec_t timeout;
 
@@ -221,6 +263,8 @@ DEFINE_PRIVATE_STRING_TABLE_LOOKUP_TO_STRING(varlink_state, VarlinkState);
 #define varlink_server_log(s, fmt, ...) \
         log_debug("%s: " fmt, varlink_server_description(s), ##__VA_ARGS__)
 
+static int varlink_format_queue(Varlink *v);
+
 static inline const char *varlink_description(Varlink *v) {
         return (v ? v->description : NULL) ?: "varlink";
 }
@@ -229,6 +273,36 @@ static inline const char *varlink_server_description(VarlinkServer *s) {
         return (s ? s->description : NULL) ?: "varlink";
 }
 
+static VarlinkJsonQueueItem *varlink_json_queue_item_free(VarlinkJsonQueueItem *q) {
+        if (!q)
+                return NULL;
+
+        json_variant_unref(q->data);
+        close_many(q->fds, q->n_fds);
+
+        return mfree(q);
+}
+
+static VarlinkJsonQueueItem *varlink_json_queue_item_new(JsonVariant *m, const int fds[], size_t n_fds) {
+        VarlinkJsonQueueItem *q;
+
+        assert(m);
+        assert(fds || n_fds == 0);
+
+        q = malloc(offsetof(VarlinkJsonQueueItem, fds) + sizeof(int) * n_fds);
+        if (!q)
+                return NULL;
+
+        *q = (VarlinkJsonQueueItem) {
+                .data = json_variant_ref(m),
+                .n_fds = n_fds,
+        };
+
+        memcpy_safe(q->fds, fds, n_fds * sizeof(int));
+
+        return TAKE_PTR(q);
+}
+
 static void varlink_set_state(Varlink *v, VarlinkState state) {
         assert(v);
         assert(state >= 0 && state < _VARLINK_STATE_MAX);
@@ -256,14 +330,16 @@ static int varlink_new(Varlink **ret) {
 
         *v = (Varlink) {
                 .n_ref = 1,
-                .fd = -1,
+                .fd = -EBADF,
 
                 .state = _VARLINK_STATE_INVALID,
 
                 .ucred = UCRED_INVALID,
 
                 .timestamp = USEC_INFINITY,
-                .timeout = VARLINK_DEFAULT_TIMEOUT_USEC
+                .timeout = VARLINK_DEFAULT_TIMEOUT_USEC,
+
+                .af = -1,
         };
 
         *ret = v;
@@ -287,6 +363,7 @@ int varlink_connect_address(Varlink **ret, const char *address) {
                 return log_debug_errno(errno, "Failed to create AF_UNIX socket: %m");
 
         v->fd = fd_move_above_stdio(v->fd);
+        v->af = AF_UNIX;
 
         r = sockaddr_un_set_path(&sockaddr.un, address);
         if (r < 0) {
@@ -337,6 +414,7 @@ int varlink_connect_fd(Varlink **ret, int fd) {
                 return log_debug_errno(r, "Failed to create varlink object: %m");
 
         v->fd = fd;
+        v->af = -1,
         varlink_set_state(v, VARLINK_IDLE_CLIENT);
 
         /* Note that if this function is called we assume the passed socket (if it is one) is already
@@ -359,6 +437,17 @@ static void varlink_detach_event_sources(Varlink *v) {
         v->defer_event_source = sd_event_source_disable_unref(v->defer_event_source);
 }
 
+static void varlink_clear_current(Varlink *v) {
+        assert(v);
+
+        /* Clears the currently processed incoming message */
+        v->current = json_variant_unref(v->current);
+
+        close_many(v->input_fds, v->n_input_fds);
+        v->input_fds = mfree(v->input_fds);
+        v->n_input_fds = 0;
+}
+
 static void varlink_clear(Varlink *v) {
         assert(v);
 
@@ -366,11 +455,31 @@ static void varlink_clear(Varlink *v) {
 
         v->fd = safe_close(v->fd);
 
+        varlink_clear_current(v);
+
         v->input_buffer = mfree(v->input_buffer);
-        v->output_buffer = mfree(v->output_buffer);
+        v->output_buffer = v->output_buffer_sensitive ? erase_and_free(v->output_buffer) : mfree(v->output_buffer);
 
-        v->current = json_variant_unref(v->current);
-        v->reply = json_variant_unref(v->reply);
+        v->input_control_buffer = mfree(v->input_control_buffer);
+        v->input_control_buffer_size = 0;
+
+        varlink_clear_current(v);
+
+        close_many(v->output_fds, v->n_output_fds);
+        v->output_fds = mfree(v->output_fds);
+        v->n_output_fds = 0;
+
+        close_many(v->pushed_fds, v->n_pushed_fds);
+        v->pushed_fds = mfree(v->pushed_fds);
+        v->n_pushed_fds = 0;
+
+        while (v->output_queue) {
+                VarlinkJsonQueueItem *q = v->output_queue;
+
+                LIST_REMOVE(queue, v->output_queue, q);
+                varlink_json_queue_item_free(q);
+        }
+        v->output_queue_tail = NULL;
 
         v->event = sd_event_unref(v->event);
 }
@@ -421,7 +530,7 @@ static int varlink_test_disconnect(Varlink *v) {
         /* Similar, if are a client that hasn't written anything yet but the write side is dead, also
          * disconnect. We also explicitly check for POLLHUP here since we likely won't notice the write side
          * being down if we never wrote anything. */
-        if (IN_SET(v->state, VARLINK_IDLE_CLIENT) && (v->write_disconnected || v->got_pollhup))
+        if (v->state == VARLINK_IDLE_CLIENT && (v->write_disconnected || v->got_pollhup))
                 goto disconnect;
 
         /* We are on the server side and still want to send out more replies, but we saw POLLHUP already, and
@@ -439,6 +548,7 @@ disconnect:
 
 static int varlink_write(Varlink *v) {
         ssize_t n;
+        int r;
 
         assert(v);
 
@@ -447,25 +557,53 @@ static int varlink_write(Varlink *v) {
         if (v->connecting) /* Writing while we are still wait for a non-blocking connect() to complete will
                             * result in ENOTCONN, hence exit early here */
                 return 0;
-        if (v->output_buffer_size == 0)
-                return 0;
         if (v->write_disconnected)
                 return 0;
 
+        /* If needed let's convert some output queue json variants into text form */
+        r = varlink_format_queue(v);
+        if (r < 0)
+                return r;
+
+        if (v->output_buffer_size == 0)
+                return 0;
+
         assert(v->fd >= 0);
 
-        /* We generally prefer recv()/send() (mostly because of MSG_NOSIGNAL) but also want to be compatible
-         * with non-socket IO, hence fall back automatically.
-         *
-         * Use a local variable to help gcc figure out that we set 'n' in all cases. */
-        bool prefer_write = v->prefer_read_write;
-        if (!prefer_write) {
-                n = send(v->fd, v->output_buffer + v->output_buffer_index, v->output_buffer_size, MSG_DONTWAIT|MSG_NOSIGNAL);
-                if (n < 0 && errno == ENOTSOCK)
-                        prefer_write = v->prefer_read_write = true;
+        if (v->n_output_fds > 0) { /* If we shall send fds along, we must use sendmsg() */
+                struct iovec iov = {
+                        .iov_base = v->output_buffer + v->output_buffer_index,
+                        .iov_len = v->output_buffer_size,
+                };
+                struct msghdr mh = {
+                        .msg_iov = &iov,
+                        .msg_iovlen = 1,
+                        .msg_controllen = CMSG_SPACE(sizeof(int) * v->n_output_fds),
+                };
+
+                mh.msg_control = alloca0(mh.msg_controllen);
+
+                struct cmsghdr *control = CMSG_FIRSTHDR(&mh);
+                control->cmsg_len = CMSG_LEN(sizeof(int) * v->n_output_fds);
+                control->cmsg_level = SOL_SOCKET;
+                control->cmsg_type = SCM_RIGHTS;
+                memcpy(CMSG_DATA(control), v->output_fds, sizeof(int) * v->n_output_fds);
+
+                n = sendmsg(v->fd, &mh, MSG_DONTWAIT|MSG_NOSIGNAL);
+        } else {
+                /* We generally prefer recv()/send() (mostly because of MSG_NOSIGNAL) but also want to be compatible
+                 * with non-socket IO, hence fall back automatically.
+                 *
+                 * Use a local variable to help gcc figure out that we set 'n' in all cases. */
+                bool prefer_write = v->prefer_read_write;
+                if (!prefer_write) {
+                        n = send(v->fd, v->output_buffer + v->output_buffer_index, v->output_buffer_size, MSG_DONTWAIT|MSG_NOSIGNAL);
+                        if (n < 0 && errno == ENOTSOCK)
+                                prefer_write = v->prefer_read_write = true;
+                }
+                if (prefer_write)
+                        n = write(v->fd, v->output_buffer + v->output_buffer_index, v->output_buffer_size);
         }
-        if (prefer_write)
-                n = write(v->fd, v->output_buffer + v->output_buffer_index, v->output_buffer_size);
         if (n < 0) {
                 if (errno == EAGAIN)
                         return 0;
@@ -480,20 +618,32 @@ static int varlink_write(Varlink *v) {
                 return -errno;
         }
 
+        if (v->output_buffer_sensitive)
+                explicit_bzero_safe(v->output_buffer + v->output_buffer_index, n);
+
         v->output_buffer_size -= n;
 
-        if (v->output_buffer_size == 0)
+        if (v->output_buffer_size == 0) {
                 v->output_buffer_index = 0;
-        else
+                v->output_buffer_sensitive = false; /* We can reset the sensitive flag once the buffer is empty */
+        } else
                 v->output_buffer_index += n;
 
+        close_many(v->output_fds, v->n_output_fds);
+        v->n_output_fds = 0;
+
         v->timestamp = now(CLOCK_MONOTONIC);
         return 1;
 }
 
+#define VARLINK_FDS_MAX (16U*1024U)
+
 static int varlink_read(Varlink *v) {
+        struct iovec iov;
+        struct msghdr mh;
         size_t rs;
         ssize_t n;
+        void *p;
 
         assert(v);
 
@@ -537,16 +687,38 @@ static int varlink_read(Varlink *v) {
                 }
         }
 
+        p = v->input_buffer + v->input_buffer_index + v->input_buffer_size;
         rs = MALLOC_SIZEOF_SAFE(v->input_buffer) - (v->input_buffer_index + v->input_buffer_size);
 
-        bool prefer_read = v->prefer_read_write;
-        if (!prefer_read) {
-                n = recv(v->fd, v->input_buffer + v->input_buffer_index + v->input_buffer_size, rs, MSG_DONTWAIT);
-                if (n < 0 && errno == ENOTSOCK)
-                        prefer_read = v->prefer_read_write = true;
+        if (v->allow_fd_passing_input) {
+                iov = IOVEC_MAKE(p, rs);
+
+                /* Allocate the fd buffer on the heap, since we need a lot of space potentially */
+                if (!v->input_control_buffer) {
+                        v->input_control_buffer_size = CMSG_SPACE(sizeof(int) * VARLINK_FDS_MAX);
+                        v->input_control_buffer = malloc(v->input_control_buffer_size);
+                        if (!v->input_control_buffer)
+                                return -ENOMEM;
+                }
+
+                mh = (struct msghdr) {
+                        .msg_iov = &iov,
+                        .msg_iovlen = 1,
+                        .msg_control = v->input_control_buffer,
+                        .msg_controllen = v->input_control_buffer_size,
+                };
+
+                n = recvmsg_safe(v->fd, &mh, MSG_DONTWAIT|MSG_CMSG_CLOEXEC);
+        } else {
+                bool prefer_read = v->prefer_read_write;
+                if (!prefer_read) {
+                        n = recv(v->fd, p, rs, MSG_DONTWAIT);
+                        if (n < 0 && errno == ENOTSOCK)
+                                prefer_read = v->prefer_read_write = true;
+                }
+                if (prefer_read)
+                        n = read(v->fd, p, rs);
         }
-        if (prefer_read)
-                n = read(v->fd, v->input_buffer + v->input_buffer_index + v->input_buffer_size, rs);
         if (n < 0) {
                 if (errno == EAGAIN)
                         return 0;
@@ -559,10 +731,44 @@ static int varlink_read(Varlink *v) {
                 return -errno;
         }
         if (n == 0) { /* EOF */
+
+                if (v->allow_fd_passing_input)
+                        cmsg_close_all(&mh);
+
                 v->read_disconnected = true;
                 return 1;
         }
 
+        if (v->allow_fd_passing_input) {
+                struct cmsghdr* cmsg;
+
+                cmsg = cmsg_find(&mh, SOL_SOCKET, SCM_RIGHTS, (socklen_t) -1);
+                if (cmsg) {
+                        size_t add;
+
+                        /* We only allow file descriptors to be passed along with the first byte of a
+                         * message. If they are passed with any other byte this is a protocol violation. */
+                        if (v->input_buffer_size != 0) {
+                                cmsg_close_all(&mh);
+                                return -EPROTO;
+                        }
+
+                        add = (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int);
+                        if (add > INT_MAX - v->n_input_fds) {
+                                cmsg_close_all(&mh);
+                                return -EBADF;
+                        }
+
+                        if (!GREEDY_REALLOC(v->input_fds, v->n_input_fds + add)) {
+                                cmsg_close_all(&mh);
+                                return -ENOMEM;
+                        }
+
+                        memcpy_safe(v->input_fds + v->n_input_fds, CMSG_TYPED_DATA(cmsg, int), add * sizeof(int));
+                        v->n_input_fds += add;
+                }
+        }
+
         v->input_buffer_size += n;
         v->input_buffer_unscanned += n;
 
@@ -760,7 +966,7 @@ static int varlink_dispatch_reply(Varlink *v) {
                                 log_debug_errno(r, "Reply callback returned error, ignoring: %m");
                 }
 
-                v->current = json_variant_unref(v->current);
+                varlink_clear_current(v);
 
                 if (v->state == VARLINK_PROCESSING_REPLY) {
 
@@ -900,7 +1106,7 @@ static int varlink_dispatch_method(Varlink *v) {
 
         case VARLINK_PROCESSED_METHOD: /* Method call is fully processed */
         case VARLINK_PROCESSING_METHOD_ONEWAY: /* ditto */
-                v->current = json_variant_unref(v->current);
+                varlink_clear_current(v);
                 varlink_set_state(v, VARLINK_IDLE_SERVER);
                 break;
 
@@ -1023,7 +1229,7 @@ static void handle_revents(Varlink *v, int revents) {
                 if ((revents & (POLLOUT|POLLHUP)) == 0)
                         return;
 
-                varlink_log(v, "Anynchronous connection completed.");
+                varlink_log(v, "Asynchronous connection completed.");
                 v->connecting = false;
         } else {
                 /* Note that we don't care much about POLLIN/POLLOUT here, we'll just try reading and writing
@@ -1073,6 +1279,9 @@ int varlink_wait(Varlink *v, usec_t timeout) {
                 return events;
 
         r = fd_wait_for_event(fd, events, t);
+        if (ERRNO_IS_NEG_TRANSIENT(r)) /* Treat EINTR as not a timeout, but also nothing happened, and
+                                        * the caller gets a chance to call back into us */
+                return 1;
         if (r <= 0)
                 return r;
 
@@ -1159,10 +1368,11 @@ int varlink_flush(Varlink *v) {
                 }
 
                 r = fd_wait_for_event(v->fd, POLLOUT, USEC_INFINITY);
+                if (ERRNO_IS_NEG_TRANSIENT(r))
+                        continue;
                 if (r < 0)
                         return varlink_log_errno(v, r, "Poll failed on fd: %m");
-
-                assert(r != 0);
+                assert(r > 0);
 
                 handle_revents(v, r);
         }
@@ -1241,8 +1451,8 @@ Varlink* varlink_flush_close_unref(Varlink *v) {
         return varlink_close_unref(v);
 }
 
-static int varlink_enqueue_json(Varlink *v, JsonVariant *m) {
-        _cleanup_free_ char *text = NULL;
+static int varlink_format_json(Varlink *v, JsonVariant *m) {
+        _cleanup_(erase_and_freep) char *text = NULL;
         int r;
 
         assert(v);
@@ -1272,7 +1482,6 @@ static int varlink_enqueue_json(Varlink *v, JsonVariant *m) {
 
                 memcpy(v->output_buffer + v->output_buffer_size, text, r + 1);
                 v->output_buffer_size += r + 1;
-
         } else {
                 char *n;
                 const size_t new_size = v->output_buffer_size + r + 1;
@@ -1288,6 +1497,75 @@ static int varlink_enqueue_json(Varlink *v, JsonVariant *m) {
                 v->output_buffer_index = 0;
         }
 
+        if (json_variant_is_sensitive(m))
+                v->output_buffer_sensitive = true; /* Propagate sensitive flag */
+        else
+                text = mfree(text); /* No point in the erase_and_free() destructor declared above */
+
+        return 0;
+}
+
+static int varlink_enqueue_json(Varlink *v, JsonVariant *m) {
+        VarlinkJsonQueueItem *q;
+
+        assert(v);
+        assert(m);
+
+        /* If there are no file descriptors to be queued and no queue entries yet we can shortcut things and
+         * append this entry directly to the output buffer */
+        if (v->n_pushed_fds == 0 && !v->output_queue)
+                return varlink_format_json(v, m);
+
+        /* Otherwise add a queue entry for this */
+        q = varlink_json_queue_item_new(m, v->pushed_fds, v->n_pushed_fds);
+        if (!q)
+                return -ENOMEM;
+
+        v->n_pushed_fds = 0; /* fds now belong to the queue entry */
+
+        LIST_INSERT_AFTER(queue, v->output_queue, v->output_queue_tail, q);
+        v->output_queue_tail = q;
+        return 0;
+}
+
+static int varlink_format_queue(Varlink *v) {
+        int r;
+
+        assert(v);
+
+        /* Takes entries out of the output queue and formats them into the output buffer. But only if this
+         * would not corrupt our fd message boundaries */
+
+        while (v->output_queue) {
+                _cleanup_free_ int *array = NULL;
+                VarlinkJsonQueueItem *q = v->output_queue;
+
+                if (v->n_output_fds > 0) /* unwritten fds? if we'd add more we'd corrupt the fd message boundaries, hence wait */
+                        return 0;
+
+                if (q->n_fds > 0) {
+                        array = newdup(int, q->fds, q->n_fds);
+                        if (!array)
+                                return -ENOMEM;
+                }
+
+                r = varlink_format_json(v, q->data);
+                if (r < 0)
+                        return r;
+
+                /* Take possession of the queue element's fds */
+                free(v->output_fds);
+                v->output_fds = TAKE_PTR(array);
+                v->n_output_fds = q->n_fds;
+                q->n_fds = 0;
+
+                LIST_REMOVE(queue, v->output_queue, q);
+                if (!v->output_queue)
+                        v->output_queue_tail = NULL;
+
+                varlink_json_queue_item_free(q);
+        }
+
         return 0;
 }
 
@@ -1464,11 +1742,15 @@ int varlink_call(
 
         if (v->state == VARLINK_DISCONNECTED)
                 return varlink_log_errno(v, SYNTHETIC_ERRNO(ENOTCONN), "Not connected.");
-        if (!IN_SET(v->state, VARLINK_IDLE_CLIENT))
+        if (v->state != VARLINK_IDLE_CLIENT)
                 return varlink_log_errno(v, SYNTHETIC_ERRNO(EBUSY), "Connection busy.");
 
         assert(v->n_pending == 0); /* n_pending can't be > 0 if we are in VARLINK_IDLE_CLIENT state */
 
+        /* If there was still a reply pinned from a previous call, now it's the time to get rid of it, so
+         * that we can assign a new reply shortly. */
+        varlink_clear_current(v);
+
         r = varlink_sanitize_parameters(&parameters);
         if (r < 0)
                 return varlink_log_errno(v, r, "Failed to sanitize parameters: %m");
@@ -1505,17 +1787,14 @@ int varlink_call(
         case VARLINK_CALLED:
                 assert(v->current);
 
-                json_variant_unref(v->reply);
-                v->reply = TAKE_PTR(v->current);
-
                 varlink_set_state(v, VARLINK_IDLE_CLIENT);
                 assert(v->n_pending == 1);
                 v->n_pending--;
 
                 if (ret_parameters)
-                        *ret_parameters = json_variant_by_key(v->reply, "parameters");
+                        *ret_parameters = json_variant_by_key(v->current, "parameters");
                 if (ret_error_id)
-                        *ret_error_id = json_variant_string(json_variant_by_key(v->reply, "error"));
+                        *ret_error_id = json_variant_string(json_variant_by_key(v->current, "error"));
                 if (ret_flags)
                         *ret_flags = 0;
 
@@ -1585,7 +1864,7 @@ int varlink_reply(Varlink *v, JsonVariant *parameters) {
                 /* We just replied to a method call that was let hanging for a while (i.e. we were outside of
                  * the varlink_dispatch_method() stack frame), which means with this reply we are ready to
                  * process further messages. */
-                v->current = json_variant_unref(v->current);
+                varlink_clear_current(v);
                 varlink_set_state(v, VARLINK_IDLE_SERVER);
         } else
                 /* We replied to a method call from within the varlink_dispatch_method() stack frame), which
@@ -1626,6 +1905,13 @@ int varlink_error(Varlink *v, const char *error_id, JsonVariant *parameters) {
                     VARLINK_PENDING_METHOD, VARLINK_PENDING_METHOD_MORE))
                 return varlink_log_errno(v, SYNTHETIC_ERRNO(EBUSY), "Connection busy.");
 
+        /* Reset the list of pushed file descriptors before sending an error reply. We do this here to
+         * simplify code that puts together a complex reply message with fds, and half-way something
+         * fails. In that case the pushed fds need to be flushed out again. Under the assumption that it
+         * never makes sense to send fds along with errors we simply flush them out here beforehand, so that
+         * the callers don't need to do this explicitly. */
+        varlink_reset_fds(v);
+
         r = varlink_sanitize_parameters(&parameters);
         if (r < 0)
                 return varlink_log_errno(v, r, "Failed to sanitize parameters: %m");
@@ -1641,7 +1927,7 @@ int varlink_error(Varlink *v, const char *error_id, JsonVariant *parameters) {
                 return varlink_log_errno(v, r, "Failed to enqueue json message: %m");
 
         if (IN_SET(v->state, VARLINK_PENDING_METHOD, VARLINK_PENDING_METHOD_MORE)) {
-                v->current = json_variant_unref(v->current);
+                varlink_clear_current(v);
                 varlink_set_state(v, VARLINK_IDLE_SERVER);
         } else
                 varlink_set_state(v, VARLINK_PROCESSED_METHOD);
@@ -1861,10 +2147,9 @@ int varlink_set_description(Varlink *v, const char *description) {
 }
 
 static int io_callback(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
-        Varlink *v = userdata;
+        Varlink *v = ASSERT_PTR(userdata);
 
         assert(s);
-        assert(v);
 
         handle_revents(v, revents);
         (void) varlink_process(v);
@@ -1873,33 +2158,30 @@ static int io_callback(sd_event_source *s, int fd, uint32_t revents, void *userd
 }
 
 static int time_callback(sd_event_source *s, uint64_t usec, void *userdata) {
-        Varlink *v = userdata;
+        Varlink *v = ASSERT_PTR(userdata);
 
         assert(s);
-        assert(v);
 
         (void) varlink_process(v);
         return 1;
 }
 
 static int defer_callback(sd_event_source *s, void *userdata) {
-        Varlink *v = userdata;
+        Varlink *v = ASSERT_PTR(userdata);
 
         assert(s);
-        assert(v);
 
         (void) varlink_process(v);
         return 1;
 }
 
 static int prepare_callback(sd_event_source *s, void *userdata) {
-        Varlink *v = userdata;
+        Varlink *v = ASSERT_PTR(userdata);
         int r, e;
         usec_t until;
         bool have_timeout;
 
         assert(s);
-        assert(v);
 
         e = varlink_get_events(v);
         if (e < 0)
@@ -1928,10 +2210,9 @@ static int prepare_callback(sd_event_source *s, void *userdata) {
 }
 
 static int quit_callback(sd_event_source *event, void *userdata) {
-        Varlink *v = userdata;
+        Varlink *v = ASSERT_PTR(userdata);
 
         assert(event);
-        assert(v);
 
         varlink_flush(v);
         varlink_close(v);
@@ -2020,6 +2301,156 @@ sd_event *varlink_get_event(Varlink *v) {
         return v->event;
 }
 
+int varlink_push_fd(Varlink *v, int fd) {
+        int i;
+
+        assert_return(v, -EINVAL);
+        assert_return(fd >= 0, -EBADF);
+
+        /* Takes an fd to send along with the *next* varlink message sent via this varlink connection. This
+         * takes ownership of the specified fd. Use varlink_dup_fd() below to duplicate the fd first. */
+
+        if (!v->allow_fd_passing_output)
+                return -EPERM;
+
+        if (v->n_pushed_fds >= INT_MAX)
+                return -ENOMEM;
+
+        if (!GREEDY_REALLOC(v->pushed_fds, v->n_pushed_fds + 1))
+                return -ENOMEM;
+
+        i = (int) v->n_pushed_fds;
+        v->pushed_fds[v->n_pushed_fds++] = fd;
+        return i;
+}
+
+int varlink_dup_fd(Varlink *v, int fd) {
+        _cleanup_close_ int dp = -1;
+        int r;
+
+        assert_return(v, -EINVAL);
+        assert_return(fd >= 0, -EBADF);
+
+        /* Like varlink_push_fd() but duplicates the specified fd instead of taking possession of it */
+
+        dp = fcntl(fd, F_DUPFD_CLOEXEC, 3);
+        if (dp < 0)
+                return -errno;
+
+        r = varlink_push_fd(v, dp);
+        if (r < 0)
+                return r;
+
+        TAKE_FD(dp);
+        return r;
+}
+
+int varlink_reset_fds(Varlink *v) {
+        assert_return(v, -EINVAL);
+
+        /* Closes all currently pending fds to send. This may be used whenever the caller is in the process
+         * of putting together a message with fds, and then eventually something fails and they need to
+         * rollback the fds. Note that this is implicitly called whenever an error reply is sent, see above. */
+
+        close_many(v->output_fds, v->n_output_fds);
+        v->n_output_fds = 0;
+        return 0;
+}
+
+int varlink_peek_fd(Varlink *v, size_t i) {
+        assert_return(v, -EINVAL);
+
+        /* Returns one of the file descriptors that were received along with the current message. This does
+         * not duplicate the fd nor invalidate it, it hence remains in our possession. */
+
+        if (!v->allow_fd_passing_input)
+                return -EPERM;
+
+        if (i >= v->n_input_fds)
+                return -ENXIO;
+
+        return v->input_fds[i];
+}
+
+int varlink_take_fd(Varlink *v, size_t i) {
+        assert_return(v, -EINVAL);
+
+        /* Similar to varlink_peek_fd() but the file descriptor's ownership is passed to the caller, and
+         * we'll invalidate the reference to it under our possession. If called twice in a row will return
+         * -EBADF */
+
+        if (!v->allow_fd_passing_input)
+                return -EPERM;
+
+        if (i >= v->n_input_fds)
+                return -ENXIO;
+
+        return TAKE_FD(v->input_fds[i]);
+}
+
+static int verify_unix_socket(Varlink *v) {
+        assert(v);
+
+        if (v->af < 0) {
+                struct stat st;
+
+                if (fstat(v->fd, &st) < 0)
+                        return -errno;
+                if (!S_ISSOCK(st.st_mode)) {
+                        v->af = AF_UNSPEC;
+                        return -ENOTSOCK;
+                }
+
+                v->af = socket_get_family(v->fd);
+                if (v->af < 0)
+                        return v->af;
+        }
+
+        return v->af == AF_UNIX ? 0 : -ENOMEDIUM;
+}
+
+int varlink_set_allow_fd_passing_input(Varlink *v, bool b) {
+        int r;
+
+        assert_return(v, -EINVAL);
+
+        if (v->allow_fd_passing_input == b)
+                return 0;
+
+        if (!b) {
+                v->allow_fd_passing_input = false;
+                return 1;
+        }
+
+        r = verify_unix_socket(v);
+        if (r < 0)
+                return r;
+
+        v->allow_fd_passing_input = true;
+        return 0;
+}
+
+int varlink_set_allow_fd_passing_output(Varlink *v, bool b) {
+        int r;
+
+        assert_return(v, -EINVAL);
+
+        if (v->allow_fd_passing_output == b)
+                return 0;
+
+        if (!b) {
+                v->allow_fd_passing_output = false;
+                return 1;
+        }
+
+        r = verify_unix_socket(v);
+        if (r < 0)
+                return r;
+
+        v->allow_fd_passing_output = true;
+        return 0;
+}
+
 int varlink_server_new(VarlinkServer **ret, VarlinkServerFlags flags) {
         VarlinkServer *s;
 
@@ -2190,7 +2621,7 @@ int varlink_server_add_connection(VarlinkServer *server, int fd, Varlink **ret)
                 r = varlink_attach_event(v, server->event, server->event_priority);
                 if (r < 0) {
                         varlink_log_errno(v, r, "Failed to attach new connection: %m");
-                        v->fd = -1; /* take the fd out of the connection again */
+                        v->fd = -EBADF; /* take the fd out of the connection again */
                         varlink_close(v);
                         return r;
                 }
@@ -2202,14 +2633,23 @@ int varlink_server_add_connection(VarlinkServer *server, int fd, Varlink **ret)
         return 0;
 }
 
+static VarlinkServerSocket *varlink_server_socket_free(VarlinkServerSocket *ss) {
+        if (!ss)
+                return NULL;
+
+        free(ss->address);
+        return mfree(ss);
+}
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(VarlinkServerSocket *, varlink_server_socket_free);
+
 static int connect_callback(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
-        VarlinkServerSocket *ss = userdata;
-        _cleanup_close_ int cfd = -1;
+        VarlinkServerSocket *ss = ASSERT_PTR(userdata);
+        _cleanup_close_ int cfd = -EBADF;
         Varlink *v = NULL;
         int r;
 
         assert(source);
-        assert(ss);
 
         varlink_server_log(ss->server, "New incoming connection.");
 
@@ -2239,12 +2679,13 @@ static int connect_callback(sd_event_source *source, int fd, uint32_t revents, v
         return 0;
 }
 
-int varlink_server_listen_fd(VarlinkServer *s, int fd) {
-        _cleanup_free_ VarlinkServerSocket *ss = NULL;
+static int varlink_server_create_listen_fd_socket(VarlinkServer *s, int fd, VarlinkServerSocket **ret_ss) {
+        _cleanup_(varlink_server_socket_freep) VarlinkServerSocket *ss = NULL;
         int r;
 
-        assert_return(s, -EINVAL);
-        assert_return(fd >= 0, -EBADF);
+        assert(s);
+        assert(fd >= 0);
+        assert(ret_ss);
 
         r = fd_nonblock(fd, true);
         if (r < 0)
@@ -2269,14 +2710,30 @@ int varlink_server_listen_fd(VarlinkServer *s, int fd) {
                         return r;
         }
 
+        *ret_ss = TAKE_PTR(ss);
+        return 0;
+}
+
+int varlink_server_listen_fd(VarlinkServer *s, int fd) {
+        _cleanup_(varlink_server_socket_freep) VarlinkServerSocket *ss = NULL;
+        int r;
+
+        assert_return(s, -EINVAL);
+        assert_return(fd >= 0, -EBADF);
+
+        r = varlink_server_create_listen_fd_socket(s, fd, &ss);
+        if (r < 0)
+                return r;
+
         LIST_PREPEND(sockets, s->sockets, TAKE_PTR(ss));
         return 0;
 }
 
 int varlink_server_listen_address(VarlinkServer *s, const char *address, mode_t m) {
+        _cleanup_(varlink_server_socket_freep) VarlinkServerSocket *ss = NULL;
         union sockaddr_union sockaddr;
         socklen_t sockaddr_len;
-        _cleanup_close_ int fd = -1;
+        _cleanup_close_ int fd = -EBADF;
         int r;
 
         assert_return(s, -EINVAL);
@@ -2296,19 +2753,24 @@ int varlink_server_listen_address(VarlinkServer *s, const char *address, mode_t
 
         (void) sockaddr_un_unlink(&sockaddr.un);
 
-        RUN_WITH_UMASK(~m & 0777) {
+        WITH_UMASK(~m & 0777) {
                 r = mac_selinux_bind(fd, &sockaddr.sa, sockaddr_len);
                 if (r < 0)
                         return r;
         }
 
-        if (listen(fd, SOMAXCONN) < 0)
+        if (listen(fd, SOMAXCONN_DELUXE) < 0)
                 return -errno;
 
-        r = varlink_server_listen_fd(s, fd);
+        r = varlink_server_create_listen_fd_socket(s, fd, &ss);
         if (r < 0)
                 return r;
 
+        r = free_and_strdup(&ss->address, address);
+        if (r < 0)
+                return r;
+
+        LIST_PREPEND(sockets, s->sockets, TAKE_PTR(ss));
         TAKE_FD(fd);
         return 0;
 }
@@ -2354,6 +2816,29 @@ int varlink_server_shutdown(VarlinkServer *s) {
         return 0;
 }
 
+static int varlink_server_add_socket_event_source(VarlinkServer *s, VarlinkServerSocket *ss, int64_t priority) {
+        _cleanup_(sd_event_source_unrefp) sd_event_source *es = NULL;
+
+        int r;
+
+        assert(s);
+        assert(s->event);
+        assert(ss);
+        assert(ss->fd >= 0);
+        assert(!ss->event_source);
+
+        r = sd_event_add_io(s->event, &es, ss->fd, EPOLLIN, connect_callback, ss);
+        if (r < 0)
+                return r;
+
+        r = sd_event_source_set_priority(es, priority);
+        if (r < 0)
+                return r;
+
+        ss->event_source = TAKE_PTR(es);
+        return 0;
+}
+
 int varlink_server_attach_event(VarlinkServer *s, sd_event *e, int64_t priority) {
         int r;
 
@@ -2369,13 +2854,7 @@ int varlink_server_attach_event(VarlinkServer *s, sd_event *e, int64_t priority)
         }
 
         LIST_FOREACH(sockets, ss, s->sockets) {
-                assert(!ss->event_source);
-
-                r = sd_event_add_io(s->event, &ss->event_source, ss->fd, EPOLLIN, connect_callback, ss);
-                if (r < 0)
-                        goto fail;
-
-                r = sd_event_source_set_priority(ss->event_source, priority);
+                r = varlink_server_add_socket_event_source(s, ss, priority);
                 if (r < 0)
                         goto fail;
         }
@@ -2534,3 +3013,83 @@ int varlink_server_set_description(VarlinkServer *s, const char *description) {
 
         return free_and_strdup(&s->description, description);
 }
+
+int varlink_server_serialize(VarlinkServer *s, FILE *f, FDSet *fds) {
+        assert(f);
+        assert(fds);
+
+        if (!s)
+                return 0;
+
+        LIST_FOREACH(sockets, ss, s->sockets) {
+                int copy;
+
+                assert(ss->address);
+                assert(ss->fd >= 0);
+
+                fprintf(f, "varlink-server-socket-address=%s", ss->address);
+
+                /* If we fail to serialize the fd, it will be considered an error during deserialization */
+                copy = fdset_put_dup(fds, ss->fd);
+                if (copy < 0)
+                        return copy;
+
+                fprintf(f, " varlink-server-socket-fd=%i", copy);
+
+                fputc('\n', f);
+        }
+
+        return 0;
+}
+
+int varlink_server_deserialize_one(VarlinkServer *s, const char *value, FDSet *fds) {
+        _cleanup_(varlink_server_socket_freep) VarlinkServerSocket *ss = NULL;
+        _cleanup_free_ char *address = NULL;
+        const char *v = ASSERT_PTR(value);
+        int r, fd = -EBADF;
+        char *buf;
+        size_t n;
+
+        assert(s);
+        assert(fds);
+
+        n = strcspn(v, " ");
+        address = strndup(v, n);
+        if (!address)
+                return log_oom_debug();
+
+        if (v[n] != ' ')
+                return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Failed to deserialize VarlinkServerSocket: %s: %m", value);
+        v = startswith(v + n + 1, "varlink-server-socket-fd=");
+        if (!v)
+                return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Failed to deserialize VarlinkServerSocket fd %s: %m", value);
+
+        n = strcspn(v, " ");
+        buf = strndupa_safe(v, n);
+
+        fd = parse_fd(buf);
+        if (fd < 0)
+                return log_debug_errno(fd, "Unable to parse VarlinkServerSocket varlink-server-socket-fd=%s: %m", buf);
+        if (!fdset_contains(fds, fd))
+                return log_debug_errno(SYNTHETIC_ERRNO(EBADF),
+                                       "VarlinkServerSocket varlink-server-socket-fd= has unknown fd %d: %m", fd);
+
+        ss = new(VarlinkServerSocket, 1);
+        if (!ss)
+                return log_oom_debug();
+
+        *ss = (VarlinkServerSocket) {
+                .server = s,
+                .address = TAKE_PTR(address),
+                .fd = fdset_remove(fds, fd),
+        };
+
+        r = varlink_server_add_socket_event_source(s, ss, SD_EVENT_PRIORITY_NORMAL);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to add VarlinkServerSocket event source to the event loop: %m");
+
+        LIST_PREPEND(sockets, s->sockets, TAKE_PTR(ss));
+        return 0;
+}