diff --git a/include/qemu/sockets.h b/include/qemu/sockets.h
index 57cd049d6edd..7d1f8135767d 100644
--- a/include/qemu/sockets.h
+++ b/include/qemu/sockets.h
@@ -18,6 +18,7 @@ int qemu_accept(int s, struct sockaddr *addr, socklen_t *addrlen);
 int socket_set_cork(int fd, int v);
 int socket_set_nodelay(int fd);
 void qemu_set_block(int fd);
+int qemu_try_set_nonblock(int fd);
 void qemu_set_nonblock(int fd);
 int socket_set_fast_reuse(int fd);
 
diff --git a/net/tap.c b/net/tap.c
index 6207f61f84ab..fb04c9044ce2 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -766,6 +766,7 @@ int net_init_tap(const Netdev *netdev, const char *name,
     Error *err = NULL;
     const char *vhostfdname;
     char ifname[128];
+    int ret = 0;
 
     assert(netdev->type == NET_CLIENT_DRIVER_TAP);
     tap = &netdev->u.tap;
@@ -795,7 +796,12 @@ int net_init_tap(const Netdev *netdev, const char *name,
             return -1;
         }
 
-        qemu_set_nonblock(fd);
+        ret = qemu_try_set_nonblock(fd);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "%s: Can't use file descriptor %d",
+                             name, fd);
+            return -1;
+        }
 
         vnet_hdr = tap_probe_vnet_hdr(fd);
 
@@ -810,7 +816,6 @@ int net_init_tap(const Netdev *netdev, const char *name,
         char **fds;
         char **vhost_fds;
         int nfds = 0, nvhosts = 0;
-        int ret = 0;
 
         if (tap->has_ifname || tap->has_script || tap->has_downscript ||
             tap->has_vnet_hdr || tap->has_helper || tap->has_queues ||
@@ -843,7 +848,12 @@ int net_init_tap(const Netdev *netdev, const char *name,
                 goto free_fail;
             }
 
-            qemu_set_nonblock(fd);
+            ret = qemu_try_set_nonblock(fd);
+            if (ret < 0) {
+                error_setg_errno(errp, -ret, "%s: Can't use file descriptor %d",
+                                 name, fd);
+                goto free_fail;
+            }
 
             if (i == 0) {
                 vnet_hdr = tap_probe_vnet_hdr(fd);
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index 916f1be2243a..149254cd691f 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -253,25 +253,35 @@ void qemu_set_block(int fd)
     assert(f != -1);
 }
 
-void qemu_set_nonblock(int fd)
+int qemu_try_set_nonblock(int fd)
 {
     int f;
     f = fcntl(fd, F_GETFL);
-    assert(f != -1);
-    f = fcntl(fd, F_SETFL, f | O_NONBLOCK);
-#ifdef __OpenBSD__
     if (f == -1) {
+        return -errno;
+    }
+    if (fcntl(fd, F_SETFL, f | O_NONBLOCK) == -1) {
+#ifdef __OpenBSD__
         /*
          * Previous to OpenBSD 6.3, fcntl(F_SETFL) is not permitted on
          * memory devices and sets errno to ENODEV.
          * It's OK if we fail to set O_NONBLOCK on devices like /dev/null,
          * because they will never block anyway.
          */
-        assert(errno == ENODEV);
-    }
-#else
-    assert(f != -1);
+        if (errno == ENODEV) {
+            return 0;
+        }
 #endif
+        return -errno;
+    }
+    return 0;
+}
+
+void qemu_set_nonblock(int fd)
+{
+    int f;
+    f = qemu_try_set_nonblock(fd);
+    assert(f == 0);
 }
 
 int socket_set_fast_reuse(int fd)
diff --git a/util/oslib-win32.c b/util/oslib-win32.c
index e9b14ab17847..5548ce6038f3 100644
--- a/util/oslib-win32.c
+++ b/util/oslib-win32.c
@@ -132,31 +132,6 @@ struct tm *localtime_r(const time_t *timep, struct tm *result)
 }
 #endif /* CONFIG_LOCALTIME_R */
 
-void qemu_set_block(int fd)
-{
-    unsigned long opt = 0;
-    WSAEventSelect(fd, NULL, 0);
-    ioctlsocket(fd, FIONBIO, &opt);
-}
-
-void qemu_set_nonblock(int fd)
-{
-    unsigned long opt = 1;
-    ioctlsocket(fd, FIONBIO, &opt);
-    qemu_fd_register(fd);
-}
-
-int socket_set_fast_reuse(int fd)
-{
-    /* Enabling the reuse of an endpoint that was used by a socket still in
-     * TIME_WAIT state is usually performed by setting SO_REUSEADDR. On Windows
-     * fast reuse is the default and SO_REUSEADDR does strange things. So we
-     * don't have to do anything here. More info can be found at:
-     * http://msdn.microsoft.com/en-us/library/windows/desktop/ms740621.aspx */
-    return 0;
-}
-
-
 static int socket_error(void)
 {
     switch (WSAGetLastError()) {
@@ -233,6 +208,38 @@ static int socket_error(void)
     }
 }
 
+void qemu_set_block(int fd)
+{
+    unsigned long opt = 0;
+    WSAEventSelect(fd, NULL, 0);
+    ioctlsocket(fd, FIONBIO, &opt);
+}
+
+int qemu_try_set_nonblock(int fd)
+{
+    unsigned long opt = 1;
+    if (ioctlsocket(fd, FIONBIO, &opt) != NO_ERROR) {
+        return -socket_error();
+    }
+    qemu_fd_register(fd);
+    return 0;
+}
+
+void qemu_set_nonblock(int fd)
+{
+    (void)qemu_try_set_nonblock(fd);
+}
+
+int socket_set_fast_reuse(int fd)
+{
+    /* Enabling the reuse of an endpoint that was used by a socket still in
+     * TIME_WAIT state is usually performed by setting SO_REUSEADDR. On Windows
+     * fast reuse is the default and SO_REUSEADDR does strange things. So we
+     * don't have to do anything here. More info can be found at:
+     * http://msdn.microsoft.com/en-us/library/windows/desktop/ms740621.aspx */
+    return 0;
+}
+
 int inet_aton(const char *cp, struct in_addr *ia)
 {
     uint32_t addr = inet_addr(cp);
