util/char_dev: Add open_cdev()

/dev/vfio/devices/vfioX may not exist. In that case it is still possible
to open /dev/char/$major:$minor instead. Add helper function to abstract
the cdev open.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

MAINTAINERS

@@ -2174,6 +2174,8 @@ M: Zhenzhong Duan <zhenzhong.duan@intel.com>
 S: Supported
 F: backends/iommufd.c
 F: include/sysemu/iommufd.h
+F: include/qemu/chardev_open.h
+F: util/chardev_open.c
 
 vhost
 M: Michael S. Tsirkin <mst@redhat.com>

include/qemu/chardev_open.h

@@ -0,0 +1,16 @@
+/*
+ * QEMU Chardev Helper
+ *
+ * Copyright (C) 2023 Intel Corporation.
+ *
+ * Authors: Yi Liu <yi.l.liu@intel.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ */
+
+#ifndef QEMU_CHARDEV_OPEN_H
+#define QEMU_CHARDEV_OPEN_H
+
+int open_cdev(const char *devpath, dev_t cdev);
+#endif

util/chardev_open.c

@@ -0,0 +1,81 @@
+/*
+ * Copyright (c) 2019, Mellanox Technologies. All rights reserved.
+ * Copyright (C) 2023 Intel Corporation.
+ *
+ * This software is available to you under a choice of one of two
+ * licenses.  You may choose to be licensed under the terms of the GNU
+ * General Public License (GPL) Version 2, available from the file
+ * COPYING in the main directory of this source tree, or the
+ * OpenIB.org BSD license below:
+ *
+ *      Redistribution and use in source and binary forms, with or
+ *      without modification, are permitted provided that the following
+ *      conditions are met:
+ *
+ *      - Redistributions of source code must retain the above
+ *        copyright notice, this list of conditions and the following
+ *        disclaimer.
+ *
+ *      - Redistributions in binary form must reproduce the above
+ *        copyright notice, this list of conditions and the following
+ *        disclaimer in the documentation and/or other materials
+ *        provided with the distribution.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * Authors: Yi Liu <yi.l.liu@intel.com>
+ *
+ * Copied from
+ * https://github.com/linux-rdma/rdma-core/blob/master/util/open_cdev.c
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/chardev_open.h"
+
+static int open_cdev_internal(const char *path, dev_t cdev)
+{
+    struct stat st;
+    int fd;
+
+    fd = qemu_open_old(path, O_RDWR);
+    if (fd == -1) {
+        return -1;
+    }
+    if (fstat(fd, &st) || !S_ISCHR(st.st_mode) ||
+        (cdev != 0 && st.st_rdev != cdev)) {
+        close(fd);
+        return -1;
+    }
+    return fd;
+}
+
+static int open_cdev_robust(dev_t cdev)
+{
+    g_autofree char *devpath = NULL;
+
+    /*
+     * This assumes that udev is being used and is creating the /dev/char/
+     * symlinks.
+     */
+    devpath = g_strdup_printf("/dev/char/%u:%u", major(cdev), minor(cdev));
+    return open_cdev_internal(devpath, cdev);
+}
+
+int open_cdev(const char *devpath, dev_t cdev)
+{
+    int fd;
+
+    fd = open_cdev_internal(devpath, cdev);
+    if (fd == -1 && cdev != 0) {
+        return open_cdev_robust(cdev);
+    }
+    return fd;
+}

util/meson.build

@@ -108,6 +108,7 @@ if have_block
     util_ss.add(files('filemonitor-stub.c'))
   endif
   util_ss.add(when: 'CONFIG_LINUX', if_true: files('vfio-helpers.c'))
+  util_ss.add(when: 'CONFIG_LINUX', if_true: files('chardev_open.c'))
 endif
 
 if cpu == 'aarch64'