[PATCH RFC] Introduce uniptr_t as a generic "universal" pointer

[Takashi Iwai <tiwai@suse.de>]

Although sockptr_t is used already in several places as a "universal"
pointer, it's still too confusing to use it in other subsystems, since
people see it always as if it were a network-related stuff.

This patch defines a more generic type, uniptr_t, that does exactly as
same as sockptr_t for a wider use.  As of now, it's almost 1:1 copy
with renames (just with comprehensive header file inclusions).

Signed-off-by: Takashi Iwai <tiwai@suse.de>
---

This is a RFC patch, or rather a material for bikeshedding.

Initially the discussion started from the use of sockptr_t for the
sound driver in Andy's patch:
  https://lore.kernel.org/r/20230721100146.67293-1-andriy.shevchenko@linux.intel.com
followed by a bigger series of patches by me:
  https://lore.kernel.org/r/20230731154718.31048-1-tiwai@suse.de

The first reaction to the patches (including my own) were
"why sockptr_t?"  Yes, it's just confusing.  So, here it is, a
proposal of defining the new type for the very purpose as sockptr_t.

The name of uniptr_t is nothing but my random pick up, and we can
endlessly discuss for a better name (genptr_t or whatever).
I'm totally open for the name.

After this introduction, sockptr_t can be alias of uniptr_t,
e.g. simply override with "#define sockptr_t uniptr_t" or such.
How can it be is another open question.

Also, we can clean up the macro implementation along with it;
there seem a few (rather minor) issues as suggested by Andy:
  https://lore.kernel.org/r/ZMlGKy7ibjkQ6ii7@smile.fi.intel.com

Honestly speaking, I don't mind to keep using sockptr_t generically
despite of the name, if people agree.  The rename might make sense,
though, if it's more widely used in other subsystems in future.


Takashi

===

 include/linux/uniptr.h | 121 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 121 insertions(+)
 create mode 100644 include/linux/uniptr.h

diff --git a/include/linux/uniptr.h b/include/linux/uniptr.h
new file mode 100644
index 000000000000..f7994d3a45eb
--- /dev/null
+++ b/include/linux/uniptr.h
@@ -0,0 +1,121 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Support for "universal" pointers that can point to either kernel or userspace
+ * memory.
+ *
+ * Original code from sockptr.h
+ *    Copyright (c) 2020 Christoph Hellwig
+ */
+#ifndef _LINUX_UNIPTR_H
+#define _LINUX_UNIPTR_H
+
+#include <linux/err.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/types.h>
+#include <linux/uaccess.h>
+
+typedef struct {
+	union {
+		void		*kernel;
+		void __user	*user;
+	};
+	bool		is_kernel : 1;
+} uniptr_t;
+
+static inline bool uniptr_is_kernel(uniptr_t uniptr)
+{
+	return uniptr.is_kernel;
+}
+
+static inline uniptr_t KERNEL_UNIPTR(void *p)
+{
+	return (uniptr_t) { .kernel = p, .is_kernel = true };
+}
+
+static inline uniptr_t USER_UNIPTR(void __user *p)
+{
+	return (uniptr_t) { .user = p };
+}
+
+static inline bool uniptr_is_null(uniptr_t uniptr)
+{
+	if (uniptr_is_kernel(uniptr))
+		return !uniptr.kernel;
+	return !uniptr.user;
+}
+
+static inline int copy_from_uniptr_offset(void *dst, uniptr_t src,
+					  size_t offset, size_t size)
+{
+	if (!uniptr_is_kernel(src))
+		return copy_from_user(dst, src.user + offset, size);
+	memcpy(dst, src.kernel + offset, size);
+	return 0;
+}
+
+static inline int copy_from_uniptr(void *dst, uniptr_t src, size_t size)
+{
+	return copy_from_uniptr_offset(dst, src, 0, size);
+}
+
+static inline int copy_to_uniptr_offset(uniptr_t dst, size_t offset,
+					const void *src, size_t size)
+{
+	if (!uniptr_is_kernel(dst))
+		return copy_to_user(dst.user + offset, src, size);
+	memcpy(dst.kernel + offset, src, size);
+	return 0;
+}
+
+static inline int copy_to_uniptr(uniptr_t dst, const void *src, size_t size)
+{
+	return copy_to_uniptr_offset(dst, 0, src, size);
+}
+
+static inline void *memdup_uniptr(uniptr_t src, size_t len)
+{
+	void *p = kmalloc_track_caller(len, GFP_USER | __GFP_NOWARN);
+
+	if (!p)
+		return ERR_PTR(-ENOMEM);
+	if (copy_from_uniptr(p, src, len)) {
+		kfree(p);
+		return ERR_PTR(-EFAULT);
+	}
+	return p;
+}
+
+static inline void *memdup_uniptr_nul(uniptr_t src, size_t len)
+{
+	char *p = kmalloc_track_caller(len + 1, GFP_KERNEL);
+
+	if (!p)
+		return ERR_PTR(-ENOMEM);
+	if (copy_from_uniptr(p, src, len)) {
+		kfree(p);
+		return ERR_PTR(-EFAULT);
+	}
+	p[len] = '\0';
+	return p;
+}
+
+static inline long strncpy_from_uniptr(char *dst, uniptr_t src, size_t count)
+{
+	if (uniptr_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);
+}
+
+static inline int check_zeroed_uniptr(uniptr_t src, size_t offset, size_t size)
+{
+	if (!uniptr_is_kernel(src))
+		return check_zeroed_user(src.user + offset, size);
+	return memchr_inv(src.kernel + offset, 0, size) == NULL;
+}
+
+#endif /* _LINUX_UNIPTR_H */
-- 
2.35.3