On Fri, Oct 17, 2025 at 12:08:24PM +0100, 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.
>
>
> 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.
So elsewhere, Linus suggested to use a struct to get around that. See
for example this lovely thing:
https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=locking/core&id=1bc5d8cefd0d9768dc03c83140dd54c552bea470
