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SubjectRe: [PATCH] iov_iter: separate direction from flavour
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@roeck-us.net> 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.

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