uaccess: Fix scoped_user_read_access() for 'pointer to const'

If a 'const struct foo __user *ptr' is used for the address passed to
scoped_user_read_access() then you get a warning/error

  uaccess.h:691:1: error: initialization discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]

for the

  void __user *_tmpptr = __scoped_user_access_begin(mode, uptr, size, elbl)

assignment.

Fix by using 'auto' for both _tmpptr and the redeclaration of uptr.
Replace the CLASS() with explicit __cleanup() functions on uptr.

Fixes: e497310 ("uaccess: Provide scoped user access regions")
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-and-tested-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: David Laight

include/linux/uaccess.h

@@ -647,36 +647,22 @@ static inline void user_access_restore(unsigned long flags) { }
 /* Define RW variant so the below _mode macro expansion works */
 #define masked_user_rw_access_begin(u)	masked_user_access_begin(u)
 #define user_rw_access_begin(u, s)	user_access_begin(u, s)
-#define user_rw_access_end()		user_access_end()
 
 /* Scoped user access */
-#define USER_ACCESS_GUARD(_mode)				\
-static __always_inline void __user *				\
-class_user_##_mode##_begin(void __user *ptr)			\
-{								\
-	return ptr;						\
-}								\
-								\
-static __always_inline void					\
-class_user_##_mode##_end(void __user *ptr)			\
-{								\
-	user_##_mode##_access_end();				\
-}								\
-								\
-DEFINE_CLASS(user_ ##_mode## _access, void __user *,		\
-	     class_user_##_mode##_end(_T),			\
-	     class_user_##_mode##_begin(ptr), void __user *ptr)	\
-								\
-static __always_inline class_user_##_mode##_access_t		\
-class_user_##_mode##_access_ptr(void __user *scope)		\
-{								\
-	return scope;						\
-}
 
-USER_ACCESS_GUARD(read)
-USER_ACCESS_GUARD(write)
-USER_ACCESS_GUARD(rw)
-#undef USER_ACCESS_GUARD
+/* Cleanup wrapper functions */
+static __always_inline void __scoped_user_read_access_end(const void *p)
+{
+	user_read_access_end();
+};
+static __always_inline void __scoped_user_write_access_end(const void *p)
+{
+	user_write_access_end();
+};
+static __always_inline void __scoped_user_rw_access_end(const void *p)
+{
+	user_access_end();
+};
 
 /**
  * __scoped_user_access_begin - Start a scoped user access
@@ -750,13 +736,13 @@ USER_ACCESS_GUARD(rw)
  *
  * Don't use directly. Use scoped_masked_user_$MODE_access() instead.
  */
-#define __scoped_user_access(mode, uptr, size, elbl)					\
-for (bool done = false; !done; done = true)						\
-	for (void __user *_tmpptr = __scoped_user_access_begin(mode, uptr, size, elbl); \
-	     !done; done = true)							\
-		for (CLASS(user_##mode##_access, scope)(_tmpptr); !done; done = true)	\
-			/* Force modified pointer usage within the scope */		\
-			for (const typeof(uptr) uptr = _tmpptr; !done; done = true)
+#define __scoped_user_access(mode, uptr, size, elbl)				\
+for (bool done = false; !done; done = true)					\
+	for (auto _tmpptr = __scoped_user_access_begin(mode, uptr, size, elbl);	\
+	     !done; done = true)						\
+		/* Force modified pointer usage within the scope */		\
+		for (const auto uptr  __cleanup(__scoped_user_##mode##_access_end) = \
+		     _tmpptr; !done; done = true)
 
 /**
  * scoped_user_read_access_size - Start a scoped user read access with given size