virtiofsd: add --fd=FDNUM fd passing option

Although --socket-path=PATH is useful for manual invocations, management
tools typically create the UNIX domain socket themselves and pass it to
the vhost-user device backend.  This way QEMU can be launched
immediately with a valid socket.  No waiting for the vhost-user device
backend is required when fd passing is used.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/fuse_i.h

@@ -68,6 +68,7 @@ struct fuse_session {
     size_t bufsize;
     int error;
     char *vu_socket_path;
+    int   vu_listen_fd;
     int   vu_socketfd;
     struct fv_VuDev *virtio_dev;
 };

tools/virtiofsd/fuse_lowlevel.c

@@ -2130,6 +2130,7 @@ static const struct fuse_opt fuse_ll_opts[] = {
     LL_OPTION("--debug", debug, 1),
     LL_OPTION("allow_root", deny_others, 1),
     LL_OPTION("--socket-path=%s", vu_socket_path, 0),
+    LL_OPTION("--fd=%d", vu_listen_fd, 0),
     FUSE_OPT_END
 };
 
@@ -2147,7 +2148,8 @@ void fuse_lowlevel_help(void)
      */
     printf(
         "    -o allow_root              allow access by root\n"
-        "    --socket-path=PATH         path for the vhost-user socket\n");
+        "    --socket-path=PATH         path for the vhost-user socket\n"
+        "    --fd=FDNUM                 fd number of vhost-user socket\n");
 }
 
 void fuse_session_destroy(struct fuse_session *se)
@@ -2191,6 +2193,7 @@ struct fuse_session *fuse_session_new(struct fuse_args *args,
         goto out1;
     }
     se->fd = -1;
+    se->vu_listen_fd = -1;
     se->conn.max_write = UINT_MAX;
     se->conn.max_readahead = UINT_MAX;
 
@@ -2212,8 +2215,13 @@ struct fuse_session *fuse_session_new(struct fuse_args *args,
         goto out4;
     }
 
-    if (!se->vu_socket_path) {
+    if (!se->vu_socket_path && se->vu_listen_fd < 0) {
-        fprintf(stderr, "fuse: missing -o vhost_user_socket option\n");
+        fuse_log(FUSE_LOG_ERR, "fuse: missing --socket-path or --fd option\n");
+        goto out4;
+    }
+    if (se->vu_socket_path && se->vu_listen_fd >= 0) {
+        fuse_log(FUSE_LOG_ERR,
+                 "fuse: --socket-path and --fd cannot be given together\n");
         goto out4;
     }
 
@@ -2253,7 +2261,7 @@ void fuse_session_unmount(struct fuse_session *se)
 
 int fuse_lowlevel_is_virtio(struct fuse_session *se)
 {
-    return se->vu_socket_path != NULL;
+    return !!se->virtio_dev;
 }
 
 #ifdef linux

tools/virtiofsd/fuse_virtio.c

@@ -638,18 +638,21 @@ int virtio_loop(struct fuse_session *se)
     return 0;
 }
 
-int virtio_session_mount(struct fuse_session *se)
+static int fv_create_listen_socket(struct fuse_session *se)
 {
     struct sockaddr_un un;
     mode_t old_umask;
 
+    /* Nothing to do if fd is already initialized */
+    if (se->vu_listen_fd >= 0) {
+        return 0;
+    }
+
     if (strlen(se->vu_socket_path) >= sizeof(un.sun_path)) {
         fuse_log(FUSE_LOG_ERR, "Socket path too long\n");
         return -1;
     }
 
-    se->fd = -1;
-
     /*
      * Create the Unix socket to communicate with qemu
      * based on QEMU's vhost-user-bridge
@@ -682,15 +685,31 @@ int virtio_session_mount(struct fuse_session *se)
         return -1;
     }
 
+    se->vu_listen_fd = listen_sock;
+    return 0;
+}
+
+int virtio_session_mount(struct fuse_session *se)
+{
+    int ret;
+
+    ret = fv_create_listen_socket(se);
+    if (ret < 0) {
+        return ret;
+    }
+
+    se->fd = -1;
+
     fuse_log(FUSE_LOG_INFO, "%s: Waiting for vhost-user socket connection...\n",
              __func__);
-    int data_sock = accept(listen_sock, NULL, NULL);
+    int data_sock = accept(se->vu_listen_fd, NULL, NULL);
     if (data_sock == -1) {
         fuse_log(FUSE_LOG_ERR, "vhost socket accept: %m\n");
-        close(listen_sock);
+        close(se->vu_listen_fd);
         return -1;
     }
-    close(listen_sock);
+    close(se->vu_listen_fd);
+    se->vu_listen_fd = -1;
     fuse_log(FUSE_LOG_INFO, "%s: Received vhost-user socket connection\n",
              __func__);