User space access regions are tedious and require similar code patterns all
over the place:

     	if (!user_read_access_begin(from, sizeof(*from)))
		return -EFAULT;
	unsafe_get_user(val, from, Efault);
	user_read_access_end();
	return 0;
Efault:
	user_read_access_end();
	return -EFAULT;

This got worse with the recent addition of masked user access, which
optimizes the speculation prevention:

	if (can_do_masked_user_access())
		from = masked_user_read_access_begin((from));
	else if (!user_read_access_begin(from, sizeof(*from)))
		return -EFAULT;
	unsafe_get_user(val, from, Efault);
	user_read_access_end();
	return 0;
Efault:
	user_read_access_end();
	return -EFAULT;

There have been issues with using the wrong user_*_access_end() variant in
the error path and other typical Copy&Pasta problems, e.g. using the wrong
fault label in the user accessor which ends up using the wrong accesss end
variant. 

These patterns beg for scopes with automatic cleanup. The resulting outcome
is:
    	scoped_masked_user_read_access(from, Efault)
		unsafe_get_user(val, from, Efault);
	return 0;
  Efault:
	return -EFAULT;

The scope guarantees the proper cleanup for the access mode is invoked both
in the success and the failure (fault) path.

The scoped_masked_user_$MODE_access() macros are implemented as self
terminating nested for() loops. Thanks to Andrew Cooper for pointing me at
them. The scope can therefore be left with 'break', 'goto' and 'return'.
Even 'continue' "works" due to the self termination mechanism. Both GCC and
clang optimize all the convoluted macro maze out and the above results with
clang in:

 b80:	f3 0f 1e fa          	       endbr64
 b84:	48 b8 ef cd ab 89 67 45 23 01  movabs $0x123456789abcdef,%rax
 b8e:	48 39 c7    	               cmp    %rax,%rdi
 b91:	48 0f 47 f8          	       cmova  %rax,%rdi
 b95:	90                   	       nop
 b96:	90                   	       nop
 b97:	90                   	       nop
 b98:	31 c9                	       xor    %ecx,%ecx
 b9a:	8b 07                	       mov    (%rdi),%eax
 b9c:	89 06                	       mov    %eax,(%rsi)
 b9e:	85 c9                	       test   %ecx,%ecx
 ba0:	0f 94 c0             	       sete   %al
 ba3:	90                   	       nop
 ba4:	90                   	       nop
 ba5:	90                   	       nop
 ba6:	c3                   	       ret

Which looks as compact as it gets. The NOPs are placeholder for STAC/CLAC.
GCC emits the fault path seperately:

 bf0:	f3 0f 1e fa          	       endbr64
 bf4:	48 b8 ef cd ab 89 67 45 23 01  movabs $0x123456789abcdef,%rax
 bfe:	48 39 c7             	       cmp    %rax,%rdi
 c01:	48 0f 47 f8          	       cmova  %rax,%rdi
 c05:	90                   	       nop
 c06:	90                   	       nop
 c07:	90                   	       nop
 c08:	31 d2                	       xor    %edx,%edx
 c0a:	8b 07                	       mov    (%rdi),%eax
 c0c:	89 06                	       mov    %eax,(%rsi)
 c0e:	85 d2                	       test   %edx,%edx
 c10:	75 09                	       jne    c1b <afoo+0x2b>
 c12:	90                   	       nop
 c13:	90                   	       nop
 c14:	90                   	       nop
 c15:	b8 01 00 00 00       	       mov    $0x1,%eax
 c1a:	c3                   	       ret
 c1b:	90                   	       nop
 c1c:	90                   	       nop
 c1d:	90                   	       nop
 c1e:	31 c0                	       xor    %eax,%eax
 c20:	c3                   	       ret


The fault labels for the scoped*() macros and the fault labels for the
actual user space accessors can be shared and must be placed outside of the
scope.

If masked user access is enabled on an architecture, then the pointer
handed in to scoped_masked_user_$MODE_access() can be modified to point to
a guaranteed faulting user address. This modification is only scope local
as the pointer is aliased inside the scope. When the scope is left the
alias is not longer in effect. IOW the original pointer value is preserved
so it can be used e.g. for fixup or diagnostic purposes in the fault path.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
---
V3: Make it a nested for() loop
    Get rid of the code in macro parameters - Linus
    Provide sized variants - Mathieu
V2: Remove the shady wrappers around the opening and use scopes with automatic cleanup
---
 include/linux/uaccess.h |  197 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 197 insertions(+)

--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -2,6 +2,7 @@
 #ifndef __LINUX_UACCESS_H__
 #define __LINUX_UACCESS_H__
 
+#include <linux/cleanup.h>
 #include <linux/fault-inject-usercopy.h>
 #include <linux/instrumented.h>
 #include <linux/minmax.h>
@@ -35,9 +36,17 @@
 
 #ifdef masked_user_access_begin
  #define can_do_masked_user_access() 1
+# ifndef masked_user_write_access_begin
+#  define masked_user_write_access_begin masked_user_access_begin
+# endif
+# ifndef masked_user_read_access_begin
+#  define masked_user_read_access_begin masked_user_access_begin
+#endif
 #else
  #define can_do_masked_user_access() 0
  #define masked_user_access_begin(src) NULL
+ #define masked_user_read_access_begin(src) NULL
+ #define masked_user_write_access_begin(src) NULL
  #define mask_user_address(src) (src)
 #endif
 
@@ -633,6 +642,194 @@ static inline void user_access_restore(u
 #define user_read_access_end user_access_end
 #endif
 
+/* 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_masked_user_##_mode##_begin(void __user *ptr)			\
+{									\
+	return ptr;							\
+}									\
+									\
+static __always_inline void						\
+class_masked_user_##_mode##_end(void __user *ptr)			\
+{									\
+	user_##_mode##_access_end();					\
+}									\
+									\
+DEFINE_CLASS(masked_user_ ##_mode## _access, void __user *,		\
+	     class_masked_user_##_mode##_end(_T),			\
+	     class_masked_user_##_mode##_begin(ptr), void __user *ptr)	\
+									\
+static __always_inline class_masked_user_##_mode##_access_t		\
+class_masked_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
+
+/**
+ * __scoped_user_access_begin - Start the masked user access
+ * @_mode:	The mode of the access class (read, write, rw)
+ * @_uptr:	The pointer to access user space memory
+ * @_size:	Size of the access
+ * @_elbl:	Error label to goto when the access region is rejected.
+ *
+ * Internal helper for __scoped_masked_user_access(). Don't use directly
+ */
+#define __scoped_user_access_begin(_mode, _uptr, _size, _elbl)		\
+({									\
+	typeof((_uptr)) ____ret;					\
+									\
+	if (can_do_masked_user_access()) {				\
+		____ret = masked_user_##_mode##_access_begin((_uptr));	\
+	} else {							\
+		____ret = _uptr;					\
+		if (!user_##_mode##_access_begin(_uptr, (_size)))	\
+			goto _elbl;					\
+	}								\
+	____ret;							\
+})
+
+/**
+ * __scoped_masked_user_access - Open a scope for masked user access
+ * @_mode:	The mode of the access class (read, write, rw)
+ * @_uptr:	The pointer to access user space memory
+ * @_size:	Size of the access
+ * @_elbl:	Error label to goto when the access region is rejected. It
+ *		must be placed outside the scope.
+ *
+ * If the user access function inside the scope requires a fault label, it
+ * can use @_elvl or a difference label outside the scope, which requires
+ * that user access which is implemented with ASM GOTO has been properly
+ * wrapped. See unsafe_get_user() for reference.
+ *
+ *	scoped_masked_user_rw_access(ptr, efault) {
+ *		unsafe_get_user(rval, &ptr->rval, efault);
+ *		unsafe_put_user(wval, &ptr->wval, efault);
+ *	}
+ *	return 0;
+ *  efault:
+ *	return -EFAULT;
+ *
+ * The scope is internally implemented as a autoterminating nested for()
+ * loop, which can be left with 'return', 'break' and 'goto' at any
+ * point.
+ *
+ * When the scope is left user_##@_mode##_access_end() is automatically
+ * invoked.
+ *
+ * When the architecture supports masked user access and the access region
+ * which is determined by @_uptr and @_size is not a valid user space
+ * address, i.e. < TASK_SIZE, the scope sets the pointer to a faulting user
+ * space address and does not terminate early. This optimizes for the good
+ * case and lets the performance uncritical bad case go through the fault.
+ *
+ * The eventual modification of the pointer is limited to the scope.
+ * Outside of the scope the original pointer value is unmodified, so that
+ * the original pointer value is available for diagnostic purposes in an
+ * out of scope fault path.
+ *
+ * Nesting scoped masked user access into a masked user access scope is
+ * invalid and fails the build. Nesting into other guards, e.g. pagefault
+ * is safe.
+ *
+ * Don't use directly. Use the scoped_masked_user_$MODE_access() instead.
+*/
+#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
+for (bool ____stop = false; !____stop; ____stop = true)						\
+	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\
+	     !____stop; ____stop = true)							\
+		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
+		     ____stop = true)					\
+			/* Force modified pointer usage within the scope */			\
+			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\
+				if (1)
+
+/**
+ * scoped_masked_user_read_access_size - Start a scoped user read access with given size
+ * @_usrc:	Pointer to the user space address to read from
+ * @_size:	Size of the access starting from @_usrc
+ * @_elbl:	Error label to goto when the access region is rejected.
+ *
+ * For further information see __scoped_masked_user_access() above.
+ */
+#define scoped_masked_user_read_access_size(_usrc, _size, _elbl)		\
+	__scoped_masked_user_access(read, (_usrc), (_size), _elbl)
+
+/**
+ * scoped_masked_user_read_access - Start a scoped user read access
+ * @_usrc:	Pointer to the user space address to read from
+ * @_elbl:	Error label to goto when the access region is rejected.
+ *
+ * The size of the access starting from @_usrc is determined via sizeof(*@_usrc)).
+ *
+ * For further information see __scoped_masked_user_access() above.
+ */
+#define scoped_masked_user_read_access(_usrc, _elbl)				\
+	scoped_masked_user_read_access_size((_usrc), sizeof(*(_usrc)), _elbl)
+
+/**
+ * scoped_masked_user_read_end - End a scoped user read access
+ *
+ * Ends the scope opened with scoped_masked_user_read_access[_size]()
+ */
+#define scoped_masked_user_read_end()	__scoped_masked_user_end()
+
+/**
+ * scoped_masked_user_write_access_size - Start a scoped user write access with given size
+ * @_udst:	Pointer to the user space address to write to
+ * @_size:	Size of the access starting from @_udst
+ * @_elbl:	Error label to goto when the access region is rejected.
+ *
+ * For further information see __scoped_masked_user_access() above.
+ */
+#define scoped_masked_user_write_access_size(_udst, _size, _elbl)		\
+	__scoped_masked_user_access(write, (_udst),  (_size), _elbl)
+
+/**
+ * scoped_masked_user_write_access - Start a scoped user write access
+ * @_udst:	Pointer to the user space address to write to
+ * @_elbl:	Error label to goto when the access region is rejected.
+ *
+ * The size of the access starting from @_udst is determined via sizeof(*@_udst)).
+ *
+ * For further information see __scoped_masked_user_access() above.
+ */
+#define scoped_masked_user_write_access(_udst, _elbl)				\
+	scoped_masked_user_write_access_size((_udst), sizeof(*(_udst)), _elbl)
+
+/**
+ * scoped_masked_user_rw_access_size - Start a scoped user read/write access with given size
+ * @_uptr	Pointer to the user space address to read from and write to
+ * @_size:	Size of the access starting from @_uptr
+ * @_elbl:	Error label to goto when the access region is rejected.
+ *
+ * For further information see __scoped_masked_user_access() above.
+ */
+#define scoped_masked_user_rw_access_size(_uptr, _size, _elbl)			\
+	__scoped_masked_user_access(rw, (_uptr), (_size), _elbl)
+
+/**
+ * scoped_masked_user_rw_access - Start a scoped user read/write access
+ * @_uptr	Pointer to the user space address to read from and write to
+ * @_elbl:	Error label to goto when the access region is rejected.
+ *
+ * The size of the access starting from @_uptr is determined via sizeof(*@_uptr)).
+ *
+ * For further information see __scoped_masked_user_access() above.
+ */
+#define scoped_masked_user_rw_access(_uptr, _elbl)				\
+	scoped_masked_user_rw_access_size((_uptr), sizeof(*(_uptr)), _elbl)
+
 #ifdef CONFIG_HARDENED_USERCOPY
 void __noreturn usercopy_abort(const char *name, const char *detail,
 			       bool to_user, unsigned long offset,

On Fri, 17 Oct 2025 12:09:08 +0200 (CEST)
Thomas Gleixner <tglx@linutronix.de> wrote:

> User space access regions are tedious and require similar code patterns all
> over the place:
> 
>      	if (!user_read_access_begin(from, sizeof(*from)))
> 		return -EFAULT;
> 	unsafe_get_user(val, from, Efault);
> 	user_read_access_end();
> 	return 0;
> Efault:
> 	user_read_access_end();
> 	return -EFAULT;
> 
> This got worse with the recent addition of masked user access, which
> optimizes the speculation prevention:
> 
> 	if (can_do_masked_user_access())
> 		from = masked_user_read_access_begin((from));
> 	else if (!user_read_access_begin(from, sizeof(*from)))
> 		return -EFAULT;
> 	unsafe_get_user(val, from, Efault);
> 	user_read_access_end();
> 	return 0;
> Efault:
> 	user_read_access_end();
> 	return -EFAULT;
> 
> There have been issues with using the wrong user_*_access_end() variant in
> the error path and other typical Copy&Pasta problems, e.g. using the wrong
> fault label in the user accessor which ends up using the wrong accesss end
> variant. 
> 
> These patterns beg for scopes with automatic cleanup. The resulting outcome
> is:
>     	scoped_masked_user_read_access(from, Efault)
> 		unsafe_get_user(val, from, Efault);
> 	return 0;
>   Efault:
> 	return -EFAULT;

That definitely looks better than the earlier versions.
Even if the implementation looks like an entry in the obfuscated C competition.

I don't think you need the 'masked' in that name.
Since it works in all cases.

(I don't like the word 'masked' at all, not sure where it came from.
Probably because the first version used logical operators.
'Masking' a user address ought to be the operation of removing high-order
address bits that the hardware is treating as 'don't care'.
The canonical operation here is uaddr = min(uaddr, guard_page) - likely to be
a conditional move.
I think that s/masked/sanitised/ would make more sense (the patch to do
that isn't very big at the moment). I might post it.)

> 
> The scope guarantees the proper cleanup for the access mode is invoked both
> in the success and the failure (fault) path.
> 
> The scoped_masked_user_$MODE_access() macros are implemented as self
> terminating nested for() loops. Thanks to Andrew Cooper for pointing me at
> them. The scope can therefore be left with 'break', 'goto' and 'return'.
> Even 'continue' "works" due to the self termination mechanism. Both GCC and
> clang optimize all the convoluted macro maze out and the above results with
> clang in:
> 
>  b80:	f3 0f 1e fa          	       endbr64
>  b84:	48 b8 ef cd ab 89 67 45 23 01  movabs $0x123456789abcdef,%rax
>  b8e:	48 39 c7    	               cmp    %rax,%rdi
>  b91:	48 0f 47 f8          	       cmova  %rax,%rdi
>  b95:	90                   	       nop
>  b96:	90                   	       nop
>  b97:	90                   	       nop
>  b98:	31 c9                	       xor    %ecx,%ecx
>  b9a:	8b 07                	       mov    (%rdi),%eax
>  b9c:	89 06                	       mov    %eax,(%rsi)
>  b9e:	85 c9                	       test   %ecx,%ecx
>  ba0:	0f 94 c0             	       sete   %al
>  ba3:	90                   	       nop
>  ba4:	90                   	       nop
>  ba5:	90                   	       nop
>  ba6:	c3                   	       ret
> 
> Which looks as compact as it gets. The NOPs are placeholder for STAC/CLAC.
> GCC emits the fault path seperately:
> 
>  bf0:	f3 0f 1e fa          	       endbr64
>  bf4:	48 b8 ef cd ab 89 67 45 23 01  movabs $0x123456789abcdef,%rax
>  bfe:	48 39 c7             	       cmp    %rax,%rdi
>  c01:	48 0f 47 f8          	       cmova  %rax,%rdi
>  c05:	90                   	       nop
>  c06:	90                   	       nop
>  c07:	90                   	       nop
>  c08:	31 d2                	       xor    %edx,%edx
>  c0a:	8b 07                	       mov    (%rdi),%eax
>  c0c:	89 06                	       mov    %eax,(%rsi)
>  c0e:	85 d2                	       test   %edx,%edx
>  c10:	75 09                	       jne    c1b <afoo+0x2b>
>  c12:	90                   	       nop
>  c13:	90                   	       nop
>  c14:	90                   	       nop
>  c15:	b8 01 00 00 00       	       mov    $0x1,%eax
>  c1a:	c3                   	       ret
>  c1b:	90                   	       nop
>  c1c:	90                   	       nop
>  c1d:	90                   	       nop
>  c1e:	31 c0                	       xor    %eax,%eax
>  c20:	c3                   	       ret
> 
> 
> The fault labels for the scoped*() macros and the fault labels for the
> actual user space accessors can be shared and must be placed outside of the
> scope.
> 
> If masked user access is enabled on an architecture, then the pointer
> handed in to scoped_masked_user_$MODE_access() can be modified to point to
> a guaranteed faulting user address. This modification is only scope local
> as the pointer is aliased inside the scope. When the scope is left the
> alias is not longer in effect. IOW the original pointer value is preserved
> so it can be used e.g. for fixup or diagnostic purposes in the fault path.

I think you need to add (in the kerndoc somewhere):

There is no requirement to do the accesses in strict memory order
(or to access the lowest address first).
The only constraint is that gaps must be significantly less than 4k.

Basically the architectures have to support code accessing uptr[4]
before uptr[0] (so using ~0 as the 'bad address' isn't a good idea).
Otherwise you have to go through 'hoops' to double check that all code
accesses the first member of a structure before the second one.
(I've looked through likely users of this and something like poll
or epoll does the 2nd access first - and it isn't obvious.)

There always has to be a guard page at the top of valid user addresses.
Otherwise sequential accesses run into kernel space.
So the code just has to generate the base of the guard page for kernel
addresses (see the horrid ppc code for cpu that have broken conditional move).

> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> ---
> V3: Make it a nested for() loop
>     Get rid of the code in macro parameters - Linus
>     Provide sized variants - Mathieu
> V2: Remove the shady wrappers around the opening and use scopes with automatic cleanup
> ---
>  include/linux/uaccess.h |  197 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 197 insertions(+)
> 
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -2,6 +2,7 @@
>  #ifndef __LINUX_UACCESS_H__
>  #define __LINUX_UACCESS_H__
>  
> +#include <linux/cleanup.h>
>  #include <linux/fault-inject-usercopy.h>
>  #include <linux/instrumented.h>
>  #include <linux/minmax.h>
> @@ -35,9 +36,17 @@
>  
>  #ifdef masked_user_access_begin
>   #define can_do_masked_user_access() 1
> +# ifndef masked_user_write_access_begin
> +#  define masked_user_write_access_begin masked_user_access_begin
> +# endif
> +# ifndef masked_user_read_access_begin
> +#  define masked_user_read_access_begin masked_user_access_begin
> +#endif
>  #else
>   #define can_do_masked_user_access() 0
>   #define masked_user_access_begin(src) NULL
> + #define masked_user_read_access_begin(src) NULL
> + #define masked_user_write_access_begin(src) NULL
>   #define mask_user_address(src) (src)
>  #endif
>  
> @@ -633,6 +642,194 @@ static inline void user_access_restore(u
>  #define user_read_access_end user_access_end
>  #endif
>  
> +/* 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_masked_user_##_mode##_begin(void __user *ptr)			\
> +{									\
> +	return ptr;							\
> +}									\
> +									\
> +static __always_inline void						\
> +class_masked_user_##_mode##_end(void __user *ptr)			\
> +{									\
> +	user_##_mode##_access_end();					\
> +}									\
> +									\
> +DEFINE_CLASS(masked_user_ ##_mode## _access, void __user *,		\
> +	     class_masked_user_##_mode##_end(_T),			\
> +	     class_masked_user_##_mode##_begin(ptr), void __user *ptr)	\
> +									\
> +static __always_inline class_masked_user_##_mode##_access_t		\
> +class_masked_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
> +
> +/**
> + * __scoped_user_access_begin - Start the masked user access
> + * @_mode:	The mode of the access class (read, write, rw)
> + * @_uptr:	The pointer to access user space memory
> + * @_size:	Size of the access
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * Internal helper for __scoped_masked_user_access(). Don't use directly
> + */
> +#define __scoped_user_access_begin(_mode, _uptr, _size, _elbl)		\
> +({									\
> +	typeof((_uptr)) ____ret;					\
> +									\
> +	if (can_do_masked_user_access()) {				\
> +		____ret = masked_user_##_mode##_access_begin((_uptr));	\
> +	} else {							\
> +		____ret = _uptr;					\
> +		if (!user_##_mode##_access_begin(_uptr, (_size)))	\
> +			goto _elbl;					\
> +	}								\
> +	____ret;							\
> +})
> +
> +/**
> + * __scoped_masked_user_access - Open a scope for masked user access
> + * @_mode:	The mode of the access class (read, write, rw)
> + * @_uptr:	The pointer to access user space memory
> + * @_size:	Size of the access
> + * @_elbl:	Error label to goto when the access region is rejected. It
> + *		must be placed outside the scope.
> + *
> + * If the user access function inside the scope requires a fault label, it
> + * can use @_elvl or a difference label outside the scope, which requires
> + * that user access which is implemented with ASM GOTO has been properly
> + * wrapped. See unsafe_get_user() for reference.
> + *
> + *	scoped_masked_user_rw_access(ptr, efault) {
> + *		unsafe_get_user(rval, &ptr->rval, efault);
> + *		unsafe_put_user(wval, &ptr->wval, efault);
> + *	}
> + *	return 0;
> + *  efault:
> + *	return -EFAULT;
> + *
> + * The scope is internally implemented as a autoterminating nested for()
> + * loop, which can be left with 'return', 'break' and 'goto' at any
> + * point.
> + *
> + * When the scope is left user_##@_mode##_access_end() is automatically
> + * invoked.
> + *
> + * When the architecture supports masked user access and the access region
> + * which is determined by @_uptr and @_size is not a valid user space
> + * address, i.e. < TASK_SIZE, the scope sets the pointer to a faulting user
> + * space address and does not terminate early. This optimizes for the good
> + * case and lets the performance uncritical bad case go through the fault.
> + *
> + * The eventual modification of the pointer is limited to the scope.
> + * Outside of the scope the original pointer value is unmodified, so that
> + * the original pointer value is available for diagnostic purposes in an
> + * out of scope fault path.
> + *
> + * Nesting scoped masked user access into a masked user access scope is
> + * invalid and fails the build. Nesting into other guards, e.g. pagefault
> + * is safe.
> + *
> + * Don't use directly. Use the scoped_masked_user_$MODE_access() instead.
> +*/
> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
> +for (bool ____stop = false; !____stop; ____stop = true)						\
> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\

Can you use 'auto' instead of typeof() ?

> +	     !____stop; ____stop = true)							\
> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
> +		     ____stop = true)					\
> +			/* Force modified pointer usage within the scope */			\
> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\

gcc 15.1 also seems to support 'const auto _uptr = _tmpptr;'

	David

> +				if (1)
> +
> +/**
> + * scoped_masked_user_read_access_size - Start a scoped user read access with given size
> + * @_usrc:	Pointer to the user space address to read from
> + * @_size:	Size of the access starting from @_usrc
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * For further information see __scoped_masked_user_access() above.
> + */
> +#define scoped_masked_user_read_access_size(_usrc, _size, _elbl)		\
> +	__scoped_masked_user_access(read, (_usrc), (_size), _elbl)
> +
> +/**
> + * scoped_masked_user_read_access - Start a scoped user read access
> + * @_usrc:	Pointer to the user space address to read from
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * The size of the access starting from @_usrc is determined via sizeof(*@_usrc)).
> + *
> + * For further information see __scoped_masked_user_access() above.
> + */
> +#define scoped_masked_user_read_access(_usrc, _elbl)				\
> +	scoped_masked_user_read_access_size((_usrc), sizeof(*(_usrc)), _elbl)
> +
> +/**
> + * scoped_masked_user_read_end - End a scoped user read access
> + *
> + * Ends the scope opened with scoped_masked_user_read_access[_size]()
> + */
> +#define scoped_masked_user_read_end()	__scoped_masked_user_end()
> +
> +/**
> + * scoped_masked_user_write_access_size - Start a scoped user write access with given size
> + * @_udst:	Pointer to the user space address to write to
> + * @_size:	Size of the access starting from @_udst
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * For further information see __scoped_masked_user_access() above.
> + */
> +#define scoped_masked_user_write_access_size(_udst, _size, _elbl)		\
> +	__scoped_masked_user_access(write, (_udst),  (_size), _elbl)
> +
> +/**
> + * scoped_masked_user_write_access - Start a scoped user write access
> + * @_udst:	Pointer to the user space address to write to
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * The size of the access starting from @_udst is determined via sizeof(*@_udst)).
> + *
> + * For further information see __scoped_masked_user_access() above.
> + */
> +#define scoped_masked_user_write_access(_udst, _elbl)				\
> +	scoped_masked_user_write_access_size((_udst), sizeof(*(_udst)), _elbl)
> +
> +/**
> + * scoped_masked_user_rw_access_size - Start a scoped user read/write access with given size
> + * @_uptr	Pointer to the user space address to read from and write to
> + * @_size:	Size of the access starting from @_uptr
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * For further information see __scoped_masked_user_access() above.
> + */
> +#define scoped_masked_user_rw_access_size(_uptr, _size, _elbl)			\
> +	__scoped_masked_user_access(rw, (_uptr), (_size), _elbl)
> +
> +/**
> + * scoped_masked_user_rw_access - Start a scoped user read/write access
> + * @_uptr	Pointer to the user space address to read from and write to
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * The size of the access starting from @_uptr is determined via sizeof(*@_uptr)).
> + *
> + * For further information see __scoped_masked_user_access() above.
> + */
> +#define scoped_masked_user_rw_access(_uptr, _elbl)				\
> +	scoped_masked_user_rw_access_size((_uptr), sizeof(*(_uptr)), _elbl)
> +
>  #ifdef CONFIG_HARDENED_USERCOPY
>  void __noreturn usercopy_abort(const char *name, const char *detail,
>  			       bool to_user, unsigned long offset,
> 
>

On Mon, Oct 20 2025 at 19:28, David Laight wrote:
> On Fri, 17 Oct 2025 12:09:08 +0200 (CEST)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> That definitely looks better than the earlier versions.
> Even if the implementation looks like an entry in the obfuscated C
> competition.

It has too many characters for that. The contest variant would be:

for(u8 s=0;!s;s=1)for(typeof(u) t= S(m,u,s,e);!s;s=1)for(C(u##m##a,c)(t);!s;s=1)for(const typeof(u) u=t;!s;s=1)

> I don't think you need the 'masked' in that name.
> Since it works in all cases.
>
> (I don't like the word 'masked' at all, not sure where it came from.

It's what Linus named it and I did not think about the name much so far.

> Probably because the first version used logical operators.
> 'Masking' a user address ought to be the operation of removing high-order
> address bits that the hardware is treating as 'don't care'.
> The canonical operation here is uaddr = min(uaddr, guard_page) - likely to be
> a conditional move.

That's how it's implemented for x86:

>>  b84:	48 b8 ef cd ab 89 67 45 23 01  movabs $0x123456789abcdef,%rax
>>  b8e:	48 39 c7    	               cmp    %rax,%rdi
>>  b91:	48 0f 47 f8          	       cmova  %rax,%rdi

0x123456789abcdef is a compile time placeholder for $USR_PTR_MAX which is
replaced during early boot by the real user space topmost address. See below.

> I think that s/masked/sanitised/ would make more sense (the patch to do
> that isn't very big at the moment). I might post it.)

The real point is that it is optimized. It does not have to use the
speculation fence if the architecture supports "masking" because the CPU
can't speculate on the input address as the actual read/write address
depends on the cmova. That's similar to the array_nospec() magic which
masks the input index unconditionally so it's in the valid range before
it can be used for speculatively accessing the array.

So yes, the naming is a bit awkward.

In principle most places which use user_$MODE_access_begin() could
benefit from that. Also under the hood the scope magic actually falls
back to that when the architecture does not support the "masked"
variant.

So simply naming it scoped_user_$MODE_access() is probably the least
confusing of all.

>> If masked user access is enabled on an architecture, then the pointer
>> handed in to scoped_masked_user_$MODE_access() can be modified to point to
>> a guaranteed faulting user address. This modification is only scope local
>> as the pointer is aliased inside the scope. When the scope is left the
>> alias is not longer in effect. IOW the original pointer value is preserved
>> so it can be used e.g. for fixup or diagnostic purposes in the fault path.
>
> I think you need to add (in the kerndoc somewhere):
>
> There is no requirement to do the accesses in strict memory order
> (or to access the lowest address first).
> The only constraint is that gaps must be significantly less than 4k.

The requirement is that the access is not spilling over into the kernel
address space, which means:

       USR_PTR_MAX <= address < (1U << 63)

USR_PTR_MAX on x86 is either
            (1U << 47) - PAGE_SIZE (4-level page tables)
         or (1U << 57) - PAGE_SIZE (5-level page tables)

Which means at least ~8 EiB of unmapped space in both cases.

The access order does not matter at all.

>> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
>> +for (bool ____stop = false; !____stop; ____stop = true)						\
>> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\
>
> Can you use 'auto' instead of typeof() ?

Compilers are mightily unhappy about that unless I do typecasting on the
assignment, which is not really buying anything.

>> +	     !____stop; ____stop = true)							\
>> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
>> +		     ____stop = true)					\
>> +			/* Force modified pointer usage within the scope */			\
>> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\
>
> gcc 15.1 also seems to support 'const auto _uptr = _tmpptr;'

Older compilers not so much.

Thanks,

        tglx

On Tue, 21 Oct 2025 16:29:58 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> On Mon, Oct 20 2025 at 19:28, David Laight wrote:
> > On Fri, 17 Oct 2025 12:09:08 +0200 (CEST)
> > Thomas Gleixner <tglx@linutronix.de> wrote:
> > That definitely looks better than the earlier versions.
> > Even if the implementation looks like an entry in the obfuscated C
> > competition.  
> 
> It has too many characters for that. The contest variant would be:
> 
> for(u8 s=0;!s;s=1)for(typeof(u) t= S(m,u,s,e);!s;s=1)for(C(u##m##a,c)(t);!s;s=1)for(const typeof(u) u=t;!s;s=1)
> 
> > I don't think you need the 'masked' in that name.
> > Since it works in all cases.
> >
> > (I don't like the word 'masked' at all, not sure where it came from.  
> 
> It's what Linus named it and I did not think about the name much so far.
> 
> > Probably because the first version used logical operators.
> > 'Masking' a user address ought to be the operation of removing high-order
> > address bits that the hardware is treating as 'don't care'.
> > The canonical operation here is uaddr = min(uaddr, guard_page) - likely to be
> > a conditional move.  
> 
> That's how it's implemented for x86:

I know - I suggested using cmov.

> 
> >>  b84:	48 b8 ef cd ab 89 67 45 23 01  movabs $0x123456789abcdef,%rax
> >>  b8e:	48 39 c7    	               cmp    %rax,%rdi
> >>  b91:	48 0f 47 f8          	       cmova  %rax,%rdi  
> 
> 0x123456789abcdef is a compile time placeholder for $USR_PTR_MAX which is
> replaced during early boot by the real user space topmost address. See below.
> 
> > I think that s/masked/sanitised/ would make more sense (the patch to do
> > that isn't very big at the moment). I might post it.)  
> 
> The real point is that it is optimized. It does not have to use the
> speculation fence if the architecture supports "masking" because the CPU
> can't speculate on the input address as the actual read/write address
> depends on the cmova. That's similar to the array_nospec() magic which
> masks the input index unconditionally so it's in the valid range before
> it can be used for speculatively accessing the array.
> 
> So yes, the naming is a bit awkward.
> 
> In principle most places which use user_$MODE_access_begin() could
> benefit from that. Also under the hood the scope magic actually falls
> back to that when the architecture does not support the "masked"
> variant.
> 
> So simply naming it scoped_user_$MODE_access() is probably the least
> confusing of all.
> 
> >> If masked user access is enabled on an architecture, then the pointer
> >> handed in to scoped_masked_user_$MODE_access() can be modified to point to
> >> a guaranteed faulting user address. This modification is only scope local
> >> as the pointer is aliased inside the scope. When the scope is left the
> >> alias is not longer in effect. IOW the original pointer value is preserved
> >> so it can be used e.g. for fixup or diagnostic purposes in the fault path.  
> >
> > I think you need to add (in the kerndoc somewhere):
> >
> > There is no requirement to do the accesses in strict memory order
> > (or to access the lowest address first).
> > The only constraint is that gaps must be significantly less than 4k.  
> 
> The requirement is that the access is not spilling over into the kernel
> address space, which means:
> 
>        USR_PTR_MAX <= address < (1U << 63)
> 
> USR_PTR_MAX on x86 is either
>             (1U << 47) - PAGE_SIZE (4-level page tables)
>          or (1U << 57) - PAGE_SIZE (5-level page tables)
> 
> Which means at least ~8 EiB of unmapped space in both cases.
> 
> The access order does not matter at all.

But consider the original x86-64 version.
While it relied on the guard page for accesses that started with a user
address, kernel addresses were converted to ~0.
While a byte access at ~0 fails because it isn't mapped, an access
at 'addr + 4' wraps to the bottom of userspace which can be mapped.
So the first access had to be at the requested address, although
subsequent ones only have to be 'reasonably sequential'.

Not all code that is an obvious candidate for this code accesses
the base address first.
So it is best to require that the implementations allow for this,
and then explicitly document that it is allowed behaviour.

The ppc patches do convert kernel addresses to the base on an
invalid page - so they are fine.
I've not seen patches for other architectures.

32bit x86 has a suitable guard page, but the code really needs 'cmov'
and the recent removal of old cpu (including the 486) didn't quite
go that far.


> 
> >> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\

Thinking about it there is no need for leading _ on #define parameter names.
It is only variables defined inside #define that have 'issues' if the caller
passes in the same name.

> >> +for (bool ____stop = false; !____stop; ____stop = true)						\
> >> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\  
> >
> > Can you use 'auto' instead of typeof() ?  
> 
> Compilers are mightily unhappy about that unless I do typecasting on the
> assignment, which is not really buying anything.

ok - I did a very quick check and thought it might work.

If you can't use auto for the third definition, then I think tmpptr can be 'void _user *'.

	David

> 
> >> +	     !____stop; ____stop = true)							\
> >> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
> >> +		     ____stop = true)					\
> >> +			/* Force modified pointer usage within the scope */			\
> >> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\  
> >
> > gcc 15.1 also seems to support 'const auto _uptr = _tmpptr;'  
> 
> Older compilers not so much.
> 
> Thanks,
> 
>         tglx

On Tue, Oct 21, 2025 at 04:29:58PM +0200, Thomas Gleixner wrote:

> So simply naming it scoped_user_$MODE_access() is probably the least
> confusing of all.

Yeah, that makes sense.

On Tue, 21 Oct 2025 at 04:30, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Mon, Oct 20 2025 at 19:28, David Laight wrote:
> >
> > (I don't like the word 'masked' at all, not sure where it came from.
>
> It's what Linus named it and I did not think about the name much so far.

The original implementation was a mask application, so it made sense
at the time.

We could still change it since there aren't that many users, but I'm
not sure what would be a better name...

                   Linus

On Tue, Oct 21 2025 at 05:06, Linus Torvalds wrote:
> On Tue, 21 Oct 2025 at 04:30, Thomas Gleixner <tglx@linutronix.de> wrote:
>>
>> On Mon, Oct 20 2025 at 19:28, David Laight wrote:
>> >
>> > (I don't like the word 'masked' at all, not sure where it came from.
>>
>> It's what Linus named it and I did not think about the name much so far.
>
> The original implementation was a mask application, so it made sense
> at the time.
>
> We could still change it since there aren't that many users, but I'm
> not sure what would be a better name...

I couldn't come up with something sensible for the architecture side.

But for the scope guards I think the simple scoped_user_$MODE_access()
is fine as for the usage site it's just a user access, no?

the scope magic resolves either to the "masked" variant or to the
regular user_access_begin() + speculation barrier depending on
architecture support. But that's under the hood an implementation detail
of the scope...() macros.

Thanks,

        tglx

On Tue, 21 Oct 2025 at 05:46, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Tue, Oct 21 2025 at 05:06, Linus Torvalds wrote:
> >
> > We could still change it since there aren't that many users, but I'm
> > not sure what would be a better name...
>
> I couldn't come up with something sensible for the architecture side.
>
> But for the scope guards I think the simple scoped_user_$MODE_access()
> is fine as for the usage site it's just a user access, no?

Ack.

               Linus

On Tue, Oct 21 2025 at 16:29, Thomas Gleixner wrote:
> On Mon, Oct 20 2025 at 19:28, David Laight wrote:
>> There is no requirement to do the accesses in strict memory order
>> (or to access the lowest address first).
>> The only constraint is that gaps must be significantly less than 4k.
>
> The requirement is that the access is not spilling over into the kernel
> address space, which means:
>
>        USR_PTR_MAX <= address < (1U << 63)
>
> USR_PTR_MAX on x86 is either
>             (1U << 47) - PAGE_SIZE (4-level page tables)
>          or (1U << 57) - PAGE_SIZE (5-level page tables)
>
> Which means at least ~8 EiB of unmapped space in both cases.
>
> The access order does not matter at all.

I just noticed that LAM reduces that gap to one page, but then the
kernel has a 8EiB gap right at the kernel/user boundary, which means
even in the LAM case an access with less than 8EiB offset from
USR_PTR_MAX is guaranteed to fault and not to be able to speculatively
access actual kernel memory.

Thanks,

        tglx

On Tue, 21 Oct 2025 16:42:22 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> On Tue, Oct 21 2025 at 16:29, Thomas Gleixner wrote:
> > On Mon, Oct 20 2025 at 19:28, David Laight wrote:  
> >> There is no requirement to do the accesses in strict memory order
> >> (or to access the lowest address first).
> >> The only constraint is that gaps must be significantly less than 4k.  
> >
> > The requirement is that the access is not spilling over into the kernel
> > address space, which means:
> >
> >        USR_PTR_MAX <= address < (1U << 63)
> >
> > USR_PTR_MAX on x86 is either
> >             (1U << 47) - PAGE_SIZE (4-level page tables)
> >          or (1U << 57) - PAGE_SIZE (5-level page tables)
> >
> > Which means at least ~8 EiB of unmapped space in both cases.
> >
> > The access order does not matter at all.  
> 
> I just noticed that LAM reduces that gap to one page, but then the
> kernel has a 8EiB gap right at the kernel/user boundary, which means
> even in the LAM case an access with less than 8EiB offset from
> USR_PTR_MAX is guaranteed to fault and not to be able to speculatively
> access actual kernel memory.

It wouldn't be a speculative access, it would be a real access.
But 4k (eg a single page) is plenty for 'reasonably sequential'.

Pretty much the only thing that has to be disallowed is a reverse
order memcpy() (or one that accesses the last bytes first) for
copy_to/from_user() if the length parameter is ignored completely.
Linus wasn't brave enough to remove it from the current version
of access_ok().

I do wonder if any other cpu have the same architectural issues
that required the guard page between user and kernel on 32bit x86.
(One is a system call at the end of the last page.)

LAM is one reason why 'masked_user_access' is such a bad name.

	David 

> 
> Thanks,
> 
>         tglx

On 2025-10-17 06:09, Thomas Gleixner wrote:

> +/**
> + * __scoped_user_access_begin - Start the masked user access
> + * @_mode:	The mode of the access class (read, write, rw)
> + * @_uptr:	The pointer to access user space memory
> + * @_size:	Size of the access
> + * @_elbl:	Error label to goto when the access region is rejected.
> + *
> + * Internal helper for __scoped_masked_user_access(). Don't use directly
> + */

^ general comment about ending sentences with '.' across this patch
(nit).


> +#define __scoped_user_access_begin(_mode, _uptr, _size, _elbl)		\
> +({									\
> +	typeof((_uptr)) ____ret;					\
> +									\
> +	if (can_do_masked_user_access()) {				\
> +		____ret = masked_user_##_mode##_access_begin((_uptr));	\

I don't think the extra () are needed here, or is there something
special happening within this macro that requires it ?

> +	} else {							\
> +		____ret = _uptr;					\
> +		if (!user_##_mode##_access_begin(_uptr, (_size)))	\

likewise around _size.

> +*/
> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
> +for (bool ____stop = false; !____stop; ____stop = true)						\
> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\

The extra () around _uptr seems useless.

> +	     !____stop; ____stop = true)							\
> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\

Removing the space before (_tmpptr) would make it clearer that it
behaves as arguments to CLASS(masked_user_##_mode##_access, scope),
similarly to what is done in cleanup.h:scoped_class().

Nesting those constructs will cause variables to be hidden by inner
definitions. I recommend using __UNIQUE_ID() to make sure the "stop" and
"tmpptr" variables don't clash with external ones rather than trying to
solve the issue with a random amount of leading underscores.

> +		     ____stop = true)					\
> +			/* Force modified pointer usage within the scope */			\
> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\

I'm puzzled that it does not trigger compiler warnings as it shadows
_uptr if _uptr is a variable defined outside of this scope.

> +				if (1)
> +

^ can be removed (as pointed out by someone else already).

[...]
> +#define scoped_masked_user_read_access_size(_usrc, _size, _elbl)		\
> +	__scoped_masked_user_access(read, (_usrc), (_size), _elbl)

Useless () around _usrc and _size.


> +#define scoped_masked_user_read_access(_usrc, _elbl)				\
> +	scoped_masked_user_read_access_size((_usrc), sizeof(*(_usrc)), _elbl)

() around the first argument are useless.


> +#define scoped_masked_user_write_access_size(_udst, _size, _elbl)		\
> +	__scoped_masked_user_access(write, (_udst),  (_size), _elbl)

Useless () around _udst and _size.

> + */
> +#define scoped_masked_user_write_access(_udst, _elbl)				\
> +	scoped_masked_user_write_access_size((_udst), sizeof(*(_udst)), _elbl)

() around the first argument are useless.

> +#define scoped_masked_user_rw_access_size(_uptr, _size, _elbl)			\
> +	__scoped_masked_user_access(rw, (_uptr), (_size), _elbl)

Useless () around _uptr and _size.

> +#define scoped_masked_user_rw_access(_uptr, _elbl)				\
> +	scoped_masked_user_rw_access_size((_uptr), sizeof(*(_uptr)), _elbl)

() around the first argument are useless.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

On 17/10/2025 11:09 am, Thomas Gleixner wrote:
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
> +for (bool ____stop = false; !____stop; ____stop = true)						\
> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\
> +	     !____stop; ____stop = true)							\
> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
> +		     ____stop = true)					\
> +			/* Force modified pointer usage within the scope */			\
> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\
> +				if (1)
> +

Truly a thing of beauty.  At least the end user experience is nice.

One thing to be aware of is that:

    scoped_masked_user_rw_access(ptr, efault) {
        unsafe_get_user(rval, &ptr->rval, efault);
        unsafe_put_user(wval, &ptr->wval, efault);
    } else {
        // unreachable
    }

will compile.  Instead, I think you want the final line of the macro to
be "if (0) {} else" to prevent this.


While we're on the subject, can we find some C standards people to lobby.

C2Y has a proposal to introduce "if (int foo =" syntax to generalise the
for() loop special case.  Can we please see about fixing the restriction
of only allowing a single type per loop?   This example could be a
single loop if it weren't for that restriction.

Thanks,

~Andrew

On Fri, Oct 17, 2025 at 12:08:24PM +0100, Andrew Cooper wrote:
> On 17/10/2025 11:09 am, Thomas Gleixner wrote:
> > --- a/include/linux/uaccess.h
> > +++ b/include/linux/uaccess.h
> > +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
> > +for (bool ____stop = false; !____stop; ____stop = true)						\
> > +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\
> > +	     !____stop; ____stop = true)							\
> > +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
> > +		     ____stop = true)					\
> > +			/* Force modified pointer usage within the scope */			\
> > +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\
> > +				if (1)
> > +
> 
> Truly a thing of beauty.  At least the end user experience is nice.
> 
> One thing to be aware of is that:
> 
>     scoped_masked_user_rw_access(ptr, efault) {
>         unsafe_get_user(rval, &ptr->rval, efault);
>         unsafe_put_user(wval, &ptr->wval, efault);
>     } else {
>         // unreachable
>     }
> 
> will compile.  Instead, I think you want the final line of the macro to
> be "if (0) {} else" to prevent this.
> 
> 
> While we're on the subject, can we find some C standards people to lobby.
> 
> C2Y has a proposal to introduce "if (int foo =" syntax to generalise the
> for() loop special case.  Can we please see about fixing the restriction
> of only allowing a single type per loop?   This example could be a
> single loop if it weren't for that restriction.

So elsewhere, Linus suggested to use a struct to get around that. See
for example this lovely thing:

  https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=locking/core&id=1bc5d8cefd0d9768dc03c83140dd54c552bea470

On Fri, Oct 17 2025 at 12:08, Andrew Cooper wrote:

> On 17/10/2025 11:09 am, Thomas Gleixner wrote:
>> --- a/include/linux/uaccess.h
>> +++ b/include/linux/uaccess.h
>> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
>> +for (bool ____stop = false; !____stop; ____stop = true)						\
>> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\
>> +	     !____stop; ____stop = true)							\
>> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
>> +		     ____stop = true)					\
>> +			/* Force modified pointer usage within the scope */			\
>> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\
>> +				if (1)
>> +
>
> Truly a thing of beauty.  At least the end user experience is nice.
>
> One thing to be aware of is that:
>
>     scoped_masked_user_rw_access(ptr, efault) {
>         unsafe_get_user(rval, &ptr->rval, efault);
>         unsafe_put_user(wval, &ptr->wval, efault);
>     } else {
>         // unreachable
>     }
>
> will compile.  Instead, I think you want the final line of the macro to
> be "if (0) {} else" to prevent this.

Duh. yes. But I can just remove the 'if (1)' completely. That's a
leftover from some earlier iteration of this.

> While we're on the subject, can we find some C standards people to lobby.
>
> C2Y has a proposal to introduce "if (int foo =" syntax to generalise the
> for() loop special case.  Can we please see about fixing the restriction
> of only allowing a single type per loop?   This example could be a
> single loop if it weren't for that restriction.

That'd be nice. But we can't have nice things, can we?

Thanks,

        tglx

On 17/10/2025 12:21 pm, Thomas Gleixner wrote:
> On Fri, Oct 17 2025 at 12:08, Andrew Cooper wrote:
>
>> On 17/10/2025 11:09 am, Thomas Gleixner wrote:
>>> --- a/include/linux/uaccess.h
>>> +++ b/include/linux/uaccess.h
>>> +#define __scoped_masked_user_access(_mode, _uptr, _size, _elbl)					\
>>> +for (bool ____stop = false; !____stop; ____stop = true)						\
>>> +	for (typeof((_uptr)) _tmpptr = __scoped_user_access_begin(_mode, _uptr, _size, _elbl);	\
>>> +	     !____stop; ____stop = true)							\
>>> +		for (CLASS(masked_user_##_mode##_access, scope) (_tmpptr); !____stop;		\
>>> +		     ____stop = true)					\
>>> +			/* Force modified pointer usage within the scope */			\
>>> +			for (const typeof((_uptr)) _uptr = _tmpptr; !____stop; ____stop = true)	\
>>> +				if (1)
>>> +
>> Truly a thing of beauty.  At least the end user experience is nice.
>>
>> One thing to be aware of is that:
>>
>>     scoped_masked_user_rw_access(ptr, efault) {
>>         unsafe_get_user(rval, &ptr->rval, efault);
>>         unsafe_put_user(wval, &ptr->wval, efault);
>>     } else {
>>         // unreachable
>>     }
>>
>> will compile.  Instead, I think you want the final line of the macro to
>> be "if (0) {} else" to prevent this.
> Duh. yes. But I can just remove the 'if (1)' completely. That's a
> leftover from some earlier iteration of this.

Oh, of course.  That works too.

>
>> While we're on the subject, can we find some C standards people to lobby.
>>
>> C2Y has a proposal to introduce "if (int foo =" syntax to generalise the
>> for() loop special case.  Can we please see about fixing the restriction
>> of only allowing a single type per loop?   This example could be a
>> single loop if it weren't for that restriction.
> That'd be nice. But we can't have nice things, can we?

Well, the worst they can say is no :)

~Andrew