Subject: Re: [patch V5 07/12] uaccess: Provide scoped user access regions

From: David Laight <[email-masked]>
Date: Fri, 7 Nov 2025 19:17:53 +0000

On Mon, 27 Oct 2025 09:43:55 +0100 (CET)
Thomas Gleixner <[email-masked]> wrote:

> User space access regions are tedious and require similar code patterns all
> over the place:
...
> 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_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 code doesn't work if the 'from' (above) is 'const foo __user *from'.
Due to assigning away constness.

The changes below fix the build, I suspect the code is then correct.

...
> +/* 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)				\

#define USER_ACCESS_GUARD(_mode, void)
(but change all the void below to a different name...)

> +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)

USER_ACCESS_GUARD(read, const void)
USER_ACCESS_GUARD(write, void)
USER_ACCESS_GUARD(rw, void)

> +#undef USER_ACCESS_GUARD
...
> +#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 = ...

> +	     !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)
> +

	David