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