Re: [PATCH] iov_iter: separate direction from flavour

[Christoph Hellwig]

On Sun, Jul 04, 2021 at 01:41:51PM -0700, Linus Torvalds wrote:
> On Sun, Jul 4, 2021 at 1:28 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> >
> > Turns out that, at least on m68k/nommu, USER_DS and KERNEL_DS are the same.
> >
> > #define USER_DS         MAKE_MM_SEG(TASK_SIZE)
> > #define KERNEL_DS       MAKE_MM_SEG(0xFFFFFFFF)
> 
> Ahh. So the code is fine, it's just that "uaccess_kernel()" isn't
> something that can be reliably even tested for, and it will always
> return true on those nommu platforms.

Yes, I think m68knommu and armnommu have this problems.  They really
need to be converted to stop implementing set_fs ASAP, as there is no
point for them.

> And we don't have a "uaccess_user()" macro that would test if it
> matches USER_DS (and that also would always return true on those
> configurations), so we can't just change the
> 
>         WARN_ON_ONCE(uaccess_kernel());
> 
> into a
> 
>         WARN_ON_ONCE(!uaccess_user());
> 
> instead.
> 
> Very annoying. Basically, every single use of "uaccess_kernel()" is unreliable.

Yes.

> The other alternative would be to just make nommu platforms that have
> KERNEL_DS==USER_DS simply do
> 
>     #define uaccess_kernel() (false)
> 
> and avoid it that way, since that's closer to what the modern
> non-CONFIG_SET_FS world view is, and is what include/linux/uaccess.h
> does for that case..

Maybe that is the best short-term bandaid.