net: add a new sockptr_t type

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>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sockptr.h

+/* 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;
+
+static inline bool sockptr_is_kernel(sockptr_t sockptr)
+{
+	return sockptr.is_kernel;
+}
+
+static inline sockptr_t KERNEL_SOCKPTR(void *p)
+{
+	return (sockptr_t) { .kernel = p, .is_kernel = true };
+}
+
+static inline sockptr_t USER_SOCKPTR(void __user *p)
+{
+	return (sockptr_t) { .user = p };
+}
+
+static inline bool sockptr_is_null(sockptr_t sockptr)
+{
+	return !sockptr.user && !sockptr.kernel;
+}
+
+static inline int copy_from_sockptr(void *dst, sockptr_t src, size_t size)
+{
+	if (!sockptr_is_kernel(src))
+		return copy_from_user(dst, src.user, size);
+	memcpy(dst, src.kernel, size);
+	return 0;
+}
+
+static inline int copy_to_sockptr(sockptr_t dst, const void *src, size_t size)
+{
+	if (!sockptr_is_kernel(dst))
+		return copy_to_user(dst.user, src, size);
+	memcpy(dst.kernel, src, size);
+	return 0;
+}
+
+static inline void *memdup_sockptr(sockptr_t src, size_t len)
+{
+	void *p = kmalloc_track_caller(len, GFP_USER | __GFP_NOWARN);
+
+	if (!p)
+		return ERR_PTR(-ENOMEM);
+	if (copy_from_sockptr(p, src, len)) {
+		kfree(p);
+		return ERR_PTR(-EFAULT);
+	}
+	return p;
+}
+
+static inline void *memdup_sockptr_nul(sockptr_t src, size_t len)
+{
+	char *p = kmalloc_track_caller(len + 1, GFP_KERNEL);
+
+	if (!p)
+		return ERR_PTR(-ENOMEM);
+	if (copy_from_sockptr(p, src, len)) {
+		kfree(p);
+		return ERR_PTR(-EFAULT);
+	}
+	p[len] = '\0';
+	return p;
+}
+
+static inline void sockptr_advance(sockptr_t sockptr, size_t len)
+{
+	if (sockptr_is_kernel(sockptr))
+		sockptr.kernel += len;
+	else
+		sockptr.user += len;
+}
+
+static inline long strncpy_from_sockptr(char *dst, sockptr_t src, size_t count)
+{
+	if (sockptr_is_kernel(src)) {
+		size_t len = min(strnlen(src.kernel, count - 1) + 1, count);
+
+		memcpy(dst, src.kernel, len);
+		return len;
+	}
+	return strncpy_from_user(dst, src.user, count);
+}
+
+#endif /* _LINUX_SOCKPTR_H */