core: serialize/deserialize varlink sockets for pid1

Fixes #20330

src/shared/meson.build

@@ -326,6 +326,7 @@ shared_sources = files(
         'utmp-wtmp.h',
         'varlink.c',
         'varlink.h',
+        'varlink-internal.h',
         'verb-log-control.c',
         'verb-log-control.h',
         'verbs.c',

src/shared/varlink-internal.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <stdio.h>
+
+#include "fdset.h"
+#include "varlink.h"
+
+int varlink_server_serialize(VarlinkServer *s, FILE *f, FDSet *fds);
+int varlink_server_deserialize_one(VarlinkServer *s, const char *value, FDSet *fds);

src/shared/varlink.c

@@ -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
@@ -2577,3 +2579,84 @@ 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 = -1;
+        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);
+
+        r = safe_atoi(buf, &fd);
+        if (r < 0)
+                return log_debug_errno(r, "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;
+}