Subject: Re: [PATCH] uaccess: Fix build of scoped user access with const pointer

From: David Laight <[email-masked]>
Date: Sun, 1 Mar 2026 21:59:50 +0000

On Sun, 1 Mar 2026 12:01:08 -0800
Linus Torvalds <[email-masked]> wrote:

> On Sun, 1 Mar 2026 at 11:34, Christophe Leroy (CS GROUP)
> <[email-masked]> wrote:
> >
> > -       for (void __user *_tmpptr = __scoped_user_access_begin(mode, uptr, size, elbl); \
> > +       for (void __user *_tmpptr = (void __user *)                                     \
> > +                                   __scoped_user_access_begin(mode, uptr, size, elbl); \  
> 
> Why are you casting this return value? Wouldn't it be a lot better to
> just make the types be the CORRECT ones?
> 
> I didn't test this, so maybe I'm missing something, but why isn't that
> just doing
> 
>         for (auto _tmpptr = __scoped_user_access_begin(mode, uptr,
> size, elbl);         \
> 
> instead? No cast, just a "use the right type automatically".
> 
> That macro actually does something similar just a few lines later, in
> that the innermost loop uses
> 
>          for (const typeof(uptr) uptr = _tmpptr; !done; done = true)
> 
> which picks up the type automatically from the argument (and then it
> uses the argument both for the type and name, which is horrendously
> confusing, but that's a separate thing).
> 
> Does that simple "auto" approach break something else?

This is what I needed to do:
(Note that is pre-dates 'auto', but it should work.)
Send at 21:56 on dec 20.

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 typeof(uptr) in that assignment and changing the 'read' functions
to use 'const void __user *ptr' rather than 'void __user *ptr'.

Fixes: e497310b4ffb "(uaccess: Provide scoped user access regions)"
Signed-off-by: David Laight <[email-masked]>
---
 include/linux/uaccess.h | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 1f3804245c06..c5d5f2d395bc 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -650,32 +650,32 @@ static inline void user_access_restore(unsigned long flags) { }
 #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)			\
+#define USER_ACCESS_GUARD(_mode, type)				\
+static __always_inline type __user *				\
+class_user_##_mode##_begin(type __user *ptr)			\
 {								\
 	return ptr;						\
 }								\
 								\
 static __always_inline void					\
-class_user_##_mode##_end(void __user *ptr)			\
+class_user_##_mode##_end(type __user *ptr)			\
 {								\
 	user_##_mode##_access_end();				\
 }								\
 								\
-DEFINE_CLASS(user_ ##_mode## _access, void __user *,		\
+DEFINE_CLASS(user_ ##_mode## _access, type __user *,		\
 	     class_user_##_mode##_end(_T),			\
-	     class_user_##_mode##_begin(ptr), void __user *ptr)	\
+	     class_user_##_mode##_begin(ptr), type __user *ptr)	\
 								\
 static __always_inline class_user_##_mode##_access_t		\
-class_user_##_mode##_access_ptr(void __user *scope)		\
+class_user_##_mode##_access_ptr(type __user *scope)		\
 {								\
 	return scope;						\
 }
 
-USER_ACCESS_GUARD(read)
-USER_ACCESS_GUARD(write)
-USER_ACCESS_GUARD(rw)
+USER_ACCESS_GUARD(read, const void)
+USER_ACCESS_GUARD(write, void)
+USER_ACCESS_GUARD(rw, void)
 #undef USER_ACCESS_GUARD
 
 /**
@@ -752,7 +752,7 @@ USER_ACCESS_GUARD(rw)
  */
 #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); \
+	for (typeof(uptr) _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 */		\
-- 
2.39.5

	David

>                    Linus