drivers/xen/xenbus/xenbus_dev_backend.c

// SPDX-License-Identifier: GPL-2.0
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/slab.h>
#include <linux/types.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/miscdevice.h>
#include <linux/init.h>
#include <linux/capability.h>

#include <xen/xen.h>
#include <xen/page.h>
#include <xen/xenbus.h>
#include <xen/xenbus_dev.h>
#include <xen/grant_table.h>
#include <xen/events.h>
#include <asm/xen/hypervisor.h>

#include "xenbus.h"

static int xenbus_backend_open(struct inode *inode, struct file *filp)
{
	if (!capable(CAP_SYS_ADMIN))
		return -EPERM;

	return nonseekable_open(inode, filp);
}

static long xenbus_alloc(domid_t domid)
{
	struct evtchn_alloc_unbound arg;
	int err = -EEXIST;

	xs_suspend();

	/* If xenstored_ready is nonzero, that means we have already talked to
	 * xenstore and set up watches. These watches will be restored by
	 * xs_resume, but that requires communication over the port established
	 * below that is not visible to anyone until the ioctl returns.
	 *
	 * This can be resolved by splitting the ioctl into two parts
	 * (postponing the resume until xenstored is active) but this is
	 * unnecessarily complex for the intended use where xenstored is only
	 * started once - so return -EEXIST if it's already running.
	 */
	if (xenstored_ready)
		goto out_err;

	gnttab_grant_foreign_access_ref(GNTTAB_RESERVED_XENSTORE, domid,
			virt_to_gfn(xen_store_interface), 0 /* writable */);

	arg.dom = DOMID_SELF;
	arg.remote_dom = domid;

	err = HYPERVISOR_event_channel_op(EVTCHNOP_alloc_unbound, &arg);
	if (err)
		goto out_err;

	if (xen_store_evtchn > 0)
		xb_deinit_comms();

	xen_store_evtchn = arg.port;

	xs_resume();

	return arg.port;

 out_err:
	xs_suspend_cancel();
	return err;
}

static long xenbus_backend_ioctl(struct file *file, unsigned int cmd,
				 unsigned long data)
{
	if (!capable(CAP_SYS_ADMIN))
		return -EPERM;

	switch (cmd) {
	case IOCTL_XENBUS_BACKEND_EVTCHN:
		if (xen_store_evtchn > 0)
			return xen_store_evtchn;
		return -ENODEV;
	case IOCTL_XENBUS_BACKEND_SETUP:
		return xenbus_alloc(data);
	default:
		return -ENOTTY;
	}
}

static int xenbus_backend_mmap(struct file *file, struct vm_area_struct *vma)
{
	size_t size = vma->vm_end - vma->vm_start;

	if (!capable(CAP_SYS_ADMIN))
		return -EPERM;

	if ((size > PAGE_SIZE) || (vma->vm_pgoff != 0))
		return -EINVAL;

	if (remap_pfn_range(vma, vma->vm_start,
			    virt_to_pfn(xen_store_interface),
			    size, vma->vm_page_prot))
		return -EAGAIN;

	return 0;
}

static const struct file_operations xenbus_backend_fops = {
	.open = xenbus_backend_open,
	.mmap = xenbus_backend_mmap,
	.unlocked_ioctl = xenbus_backend_ioctl,
};

static struct miscdevice xenbus_backend_dev = {
	.minor = MISC_DYNAMIC_MINOR,
	.name = "xen/xenbus_backend",
	.fops = &xenbus_backend_fops,
};

static int __init xenbus_backend_init(void)
{
	int err;

	if (!xen_initial_domain())
		return -ENODEV;

	err = misc_register(&xenbus_backend_dev);
	if (err)
		pr_err("Could not register xenbus backend device\n");
	return err;
}
device_initcall(xenbus_backend_init);