[RFC PATCH v4 19/24] util/char_dev: Add open_cdev()

[Zhenzhong Duan <zhenzhong.duan@intel.com>]

From: Yi Liu <yi.l.liu@intel.com>

/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>
---
 MAINTAINERS             |  6 ++++
 include/qemu/char_dev.h | 16 +++++++++++
 util/chardev_open.c     | 61 +++++++++++++++++++++++++++++++++++++++++
 util/meson.build        |  1 +
 4 files changed, 84 insertions(+)
 create mode 100644 include/qemu/char_dev.h
 create mode 100644 util/chardev_open.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 25835caf39..edecf366b8 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3356,6 +3356,12 @@ S: Maintained
 F: include/qemu/iova-tree.h
 F: util/iova-tree.c
 
+cdev Open
+M: Yi Liu <yi.l.liu@intel.com>
+S: Maintained
+F: include/qemu/char_dev.h
+F: util/chardev_open.c
+
 elf2dmp
 M: Viktor Prutyanov <viktor.prutyanov@phystech.edu>
 S: Maintained
diff --git a/include/qemu/char_dev.h b/include/qemu/char_dev.h
new file mode 100644
index 0000000000..6580d351c6
--- /dev/null
+++ b/include/qemu/char_dev.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_HELPERS_H
+#define QEMU_CHARDEV_HELPERS_H
+
+int open_cdev(const char *devpath, dev_t cdev);
+#endif
diff --git a/util/chardev_open.c b/util/chardev_open.c
new file mode 100644
index 0000000000..d03e415131
--- /dev/null
+++ b/util/chardev_open.c
@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2023 Intel Corporation.
+ * Copyright (c) 2019, Mellanox Technologies. All rights reserved.
+ *
+ * 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.
+ *
+ * Copied from
+ * https://github.com/linux-rdma/rdma-core/blob/master/util/open_cdev.c
+ *
+ */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include "qemu/osdep.h"
+#include "qemu/char_dev.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)
+{
+    char *devpath;
+    int ret;
+
+    /*
+     * 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));
+    ret = open_cdev_internal(devpath, cdev);
+    g_free(devpath);
+    return ret;
+}
+
+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;
+}
diff --git a/util/meson.build b/util/meson.build
index a375160286..d5313d858f 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -107,6 +107,7 @@ 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'
-- 
2.34.1