Add a uptr_t type that can hold a pointer to either a user or kernel
memory region, and simply helpers to copy to and from it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/linux/sockptr.h | 104 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)
 create mode 100644 include/linux/sockptr.h

diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
new file mode 100644
index 00000000000000..700856e13ea0c4
--- /dev/null
+++ b/include/linux/sockptr.h

@@ -0,0 +1,104 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2020 Christoph Hellwig.
+ *
+ * Support for "universal" pointers that can point to either kernel or userspace
+ * memory.
+ */
+#ifndef _LINUX_SOCKPTR_H
+#define _LINUX_SOCKPTR_H
+
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+
+typedef struct {
+       union {
+               void            *kernel;
+               void __user     *user;
+       };
+       bool            is_kernel : 1;
+} sockptr_t;