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

[Andrew Cooper]

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