This corresponds to the NT syscall NtCreateSemaphore().

Semaphores are one of three types of object to be implemented in this driver,
the others being mutexes and events.

An NT semaphore contains a 32-bit counter, and is signaled and can be acquired
when the counter is nonzero. The counter has a maximum value which is specified
at creation time. The initial value of the semaphore is also specified at
creation time. There are no restrictions on the maximum and initial value.

Each object is exposed as an file, to which any number of fds may be opened.
When all fds are closed, the object is deleted.

Objects hold a pointer to the ntsync_device that created them. The device's
reference count is driven by struct file.

Signed-off-by: Elizabeth Figura <zfigura@codeweavers.com>
---
 .../userspace-api/ioctl/ioctl-number.rst      |   2 +
 drivers/misc/ntsync.c                         | 131 ++++++++++++++++++
 include/uapi/linux/ntsync.h                   |  21 +++
 3 files changed, 154 insertions(+)
 create mode 100644 include/uapi/linux/ntsync.h

diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
index 457e16f06e04..2f5c6994f042 100644
--- a/Documentation/userspace-api/ioctl/ioctl-number.rst
+++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
@@ -173,6 +173,8 @@ Code  Seq#    Include File                                           Comments
 'M'   00-0F  drivers/video/fsl-diu-fb.h                              conflict!
 'N'   00-1F  drivers/usb/scanner.h
 'N'   40-7F  drivers/block/nvme.c
+'N'   80-8F  uapi/linux/ntsync.h                                     NT synchronization primitives
+                                                                     <mailto:wine-devel@winehq.org>
 'O'   00-06  mtd/ubi-user.h                                          UBI
 'P'   all    linux/soundcard.h                                       conflict!
 'P'   60-6F  sound/sscape_ioctl.h                                    conflict!
diff --git a/drivers/misc/ntsync.c b/drivers/misc/ntsync.c
index bd76e653d83e..20158ec148bc 100644
--- a/drivers/misc/ntsync.c
+++ b/drivers/misc/ntsync.c
@@ -5,26 +5,157 @@
  * Copyright (C) 2024 Elizabeth Figura <zfigura@codeweavers.com>
  */
 
+#include <linux/anon_inodes.h>
+#include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/miscdevice.h>
 #include <linux/module.h>
+#include <linux/slab.h>
+#include <uapi/linux/ntsync.h>
 
 #define NTSYNC_NAME	"ntsync"
 
+enum ntsync_type {
+	NTSYNC_TYPE_SEM,
+};
+
+/*
+ * Individual synchronization primitives are represented by
+ * struct ntsync_obj, and each primitive is backed by a file.
+ *
+ * The whole namespace is represented by a struct ntsync_device also
+ * backed by a file.
+ *
+ * Both rely on struct file for reference counting. Individual
+ * ntsync_obj objects take a reference to the device when created.
+ */
+
+struct ntsync_obj {
+	enum ntsync_type type;
+
+	union {
+		struct {
+			__u32 count;
+			__u32 max;
+		} sem;
+	} u;
+
+	struct file *file;
+	struct ntsync_device *dev;
+};
+
+struct ntsync_device {
+	struct file *file;
+};
+
+static int ntsync_obj_release(struct inode *inode, struct file *file)
+{
+	struct ntsync_obj *obj = file->private_data;
+
+	fput(obj->dev->file);
+	kfree(obj);
+
+	return 0;
+}
+
+static const struct file_operations ntsync_obj_fops = {
+	.owner		= THIS_MODULE,
+	.release	= ntsync_obj_release,
+	.llseek		= no_llseek,
+};
+
+static struct ntsync_obj *ntsync_alloc_obj(struct ntsync_device *dev,
+					   enum ntsync_type type)
+{
+	struct ntsync_obj *obj;
+
+	obj = kzalloc(sizeof(*obj), GFP_KERNEL);
+	if (!obj)
+		return NULL;
+	obj->type = type;
+	obj->dev = dev;
+	get_file(dev->file);
+
+	return obj;
+}
+
+static int ntsync_obj_get_fd(struct ntsync_obj *obj)
+{
+	struct file *file;
+	int fd;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+	file = anon_inode_getfile("ntsync", &ntsync_obj_fops, obj, O_RDWR);
+	if (IS_ERR(file)) {
+		put_unused_fd(fd);
+		return PTR_ERR(file);
+	}
+	obj->file = file;
+	fd_install(fd, file);
+
+	return fd;
+}
+
+static int ntsync_create_sem(struct ntsync_device *dev, void __user *argp)
+{
+	struct ntsync_sem_args __user *user_args = argp;
+	struct ntsync_sem_args args;
+	struct ntsync_obj *sem;
+	int fd;
+
+	if (copy_from_user(&args, argp, sizeof(args)))
+		return -EFAULT;
+
+	if (args.count > args.max)
+		return -EINVAL;
+
+	sem = ntsync_alloc_obj(dev, NTSYNC_TYPE_SEM);
+	if (!sem)
+		return -ENOMEM;
+	sem->u.sem.count = args.count;
+	sem->u.sem.max = args.max;
+	fd = ntsync_obj_get_fd(sem);
+	if (fd < 0) {
+		kfree(sem);
+		return fd;
+	}
+
+	return put_user(fd, &user_args->sem);
+}
+
 static int ntsync_char_open(struct inode *inode, struct file *file)
 {
+	struct ntsync_device *dev;
+
+	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
+	if (!dev)
+		return -ENOMEM;
+
+	file->private_data = dev;
+	dev->file = file;
 	return nonseekable_open(inode, file);
 }
 
 static int ntsync_char_release(struct inode *inode, struct file *file)
 {
+	struct ntsync_device *dev = file->private_data;
+
+	kfree(dev);
+
 	return 0;
 }
 
 static long ntsync_char_ioctl(struct file *file, unsigned int cmd,
 			      unsigned long parm)
 {
+	struct ntsync_device *dev = file->private_data;
+	void __user *argp = (void __user *)parm;
+
 	switch (cmd) {
+	case NTSYNC_IOC_CREATE_SEM:
+		return ntsync_create_sem(dev, argp);
 	default:
 		return -ENOIOCTLCMD;
 	}
diff --git a/include/uapi/linux/ntsync.h b/include/uapi/linux/ntsync.h
new file mode 100644
index 000000000000..6a4867a6c97b
--- /dev/null
+++ b/include/uapi/linux/ntsync.h
@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Kernel support for NT synchronization primitive emulation
+ *
+ * Copyright (C) 2021-2022 Elizabeth Figura <zfigura@codeweavers.com>
+ */
+
+#ifndef __LINUX_NTSYNC_H
+#define __LINUX_NTSYNC_H
+
+#include <linux/types.h>
+
+struct ntsync_sem_args {
+	__u32 sem;
+	__u32 count;
+	__u32 max;
+};
+
+#define NTSYNC_IOC_CREATE_SEM		_IOWR('N', 0x80, struct ntsync_sem_args)
+
+#endif
-- 
2.43.0
