Re: [patch V3 07/12] uaccess: Provide scoped masked user access regions

[Andrew Cooper]

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