core: add new logic for services to store file descriptors in PID 1

With this change it is possible to send file descriptors to PID 1, via
sd_pid_notify_with_fds() which PID 1 will store individually for each
service, and pass via the usual fd passing logic on next invocation.
This is useful for enable daemon reload schemes where daemons serialize
their state to /run, push their fds into PID 1 and terminate, restoring
their state on next start from the data in /run and passed in from PID
1.

The fds are kept by PID 1 as long as no POLLHUP or POLLERR is seen on
them, and the service they belong to are either not dead or failed, or
have a job queued.

1 files changed, 55 insertions, 23 deletions

diff --git a/src/core/manager.c b/src/core/manager.c
index 519b374382..c18312a369 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -84,6 +84,9 @@
 #define JOBS_IN_PROGRESS_PERIOD_USEC (USEC_PER_SEC / 3)
 #define JOBS_IN_PROGRESS_PERIOD_DIVISOR 3
 
+#define NOTIFY_FD_MAX 768
+#define NOTIFY_BUFFER_MAX PIPE_BUF
+
 static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata);
 static int manager_dispatch_signal_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata);
 static int manager_dispatch_time_change_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata);
@@ -1449,7 +1452,7 @@ static unsigned manager_dispatch_dbus_queue(Manager *m) {
         return n;
 }
 
-static void manager_invoke_notify_message(Manager *m, Unit *u, pid_t pid, char *buf, size_t n) {
+static void manager_invoke_notify_message(Manager *m, Unit *u, pid_t pid, char *buf, size_t n, FDSet *fds) {
         _cleanup_strv_free_ char **tags = NULL;
 
         assert(m);
@@ -1466,12 +1469,13 @@ static void manager_invoke_notify_message(Manager *m, Unit *u, pid_t pid, char *
         log_unit_debug(u->id, "Got notification message for unit %s", u->id);
 
         if (UNIT_VTABLE(u)->notify_message)
-                UNIT_VTABLE(u)->notify_message(u, pid, tags);
+                UNIT_VTABLE(u)->notify_message(u, pid, tags, fds);
 }
 
 static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t revents, void *userdata) {
         Manager *m = userdata;
         ssize_t n;
+        int r;
 
         assert(m);
         assert(m->notify_fd == fd);
@@ -1482,73 +1486,101 @@ static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t
         }
 
         for (;;) {
-                char buf[4096];
+                _cleanup_fdset_free_ FDSet *fds = NULL;
+                char buf[NOTIFY_BUFFER_MAX+1];
                 struct iovec iovec = {
                         .iov_base = buf,
                         .iov_len = sizeof(buf)-1,
                 };
-                bool found = false;
-
                 union {
                         struct cmsghdr cmsghdr;
-                        uint8_t buf[CMSG_SPACE(sizeof(struct ucred))];
+                        uint8_t buf[CMSG_SPACE(sizeof(struct ucred)) +
+                                    CMSG_SPACE(sizeof(int) * NOTIFY_FD_MAX)];
                 } control = {};
-
                 struct msghdr msghdr = {
                         .msg_iov = &iovec,
                         .msg_iovlen = 1,
                         .msg_control = &control,
                         .msg_controllen = sizeof(control),
                 };
-                struct ucred *ucred;
+                struct cmsghdr *cmsg;
+                struct ucred *ucred = NULL;
+                bool found = false;
                 Unit *u1, *u2, *u3;
+                int *fd_array = NULL;
+                unsigned n_fds = 0;
 
-                n = recvmsg(m->notify_fd, &msghdr, MSG_DONTWAIT);
-                if (n <= 0) {
-                        if (n == 0)
-                                return -EIO;
-
+                n = recvmsg(m->notify_fd, &msghdr, MSG_DONTWAIT|MSG_CMSG_CLOEXEC);
+                if (n < 0) {
                         if (errno == EAGAIN || errno == EINTR)
                                 break;
 
                         return -errno;
                 }
+                if (n == 0)
+                        return -ECONNRESET;
+
+                for (cmsg = CMSG_FIRSTHDR(&msghdr); cmsg; cmsg = CMSG_NXTHDR(&msghdr, cmsg)) {
+                        if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
+
+                                fd_array = (int*) CMSG_DATA(cmsg);
+                                n_fds = (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int);
+
+                        } else if (cmsg->cmsg_level == SOL_SOCKET &&
+                                   cmsg->cmsg_type == SCM_CREDENTIALS &&
+                                   cmsg->cmsg_len == CMSG_LEN(sizeof(struct ucred))) {
 
-                if (msghdr.msg_controllen < CMSG_LEN(sizeof(struct ucred)) ||
-                    control.cmsghdr.cmsg_level != SOL_SOCKET ||
-                    control.cmsghdr.cmsg_type != SCM_CREDENTIALS ||
-                    control.cmsghdr.cmsg_len != CMSG_LEN(sizeof(struct ucred))) {
-                        log_warning("Received notify message without credentials. Ignoring.");
+                                ucred = (struct ucred*) CMSG_DATA(cmsg);
+                        }
+                }
+
+                if (n_fds > 0) {
+                        assert(fd_array);
+
+                        r = fdset_new_array(&fds, fd_array, n_fds);
+                        if (r < 0) {
+                                close_many(fd_array, n_fds);
+                                return log_oom();
+                        }
+                }
+
+                if (!ucred || ucred->pid <= 0) {
+                        log_warning("Received notify message without valid credentials. Ignoring.");
                         continue;
                 }
 
-                ucred = (struct ucred*) CMSG_DATA(&control.cmsghdr);
+                if ((size_t) n >= sizeof(buf)) {
+                        log_warning("Received notify message exceeded maximum size. Ignoring.");
+                        continue;
+                }
 
-                assert((size_t) n < sizeof(buf));
                 buf[n] = 0;
 
                 /* Notify every unit that might be interested, but try
                  * to avoid notifying the same one multiple times. */
                 u1 = manager_get_unit_by_pid(m, ucred->pid);
                 if (u1) {
-                        manager_invoke_notify_message(m, u1, ucred->pid, buf, n);
+                        manager_invoke_notify_message(m, u1, ucred->pid, buf, n, fds);
                         found = true;
                 }
 
                 u2 = hashmap_get(m->watch_pids1, LONG_TO_PTR(ucred->pid));
                 if (u2 && u2 != u1) {
-                        manager_invoke_notify_message(m, u2, ucred->pid, buf, n);
+                        manager_invoke_notify_message(m, u2, ucred->pid, buf, n, fds);
                         found = true;
                 }
 
                 u3 = hashmap_get(m->watch_pids2, LONG_TO_PTR(ucred->pid));
                 if (u3 && u3 != u2 && u3 != u1) {
-                        manager_invoke_notify_message(m, u3, ucred->pid, buf, n);
+                        manager_invoke_notify_message(m, u3, ucred->pid, buf, n, fds);
                         found = true;
                 }
 
                 if (!found)
                         log_warning("Cannot find unit for notify message of PID "PID_FMT".", ucred->pid);
+
+                if (fdset_size(fds) > 0)
+                        log_warning("Got auxiliary fds with notification message, closing all.");
         }
 
         return 0;