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
