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

From: Thomas Gleixner <[email-masked]>
Date: Fri, 17 Oct 2025 13:21:14 +0200

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