Re: [PATCH 09/11] x86: remove address space overrides using set_fs()

'Christoph Hellwig'
2020-08-27T09:37:57Z

On Mon, Aug 17, 2020 at 08:23:11AM +0000, David Laight wrote:
> From: Christoph Hellwig
> > Sent: 17 August 2020 08:32
> >
> > Stop providing the possibility to override the address space using
> > set_fs() now that there is no need for that any more. To properly
> > handle the TASK_SIZE_MAX checking for 4 vs 5-level page tables on
> > x86 a new alternative is introduced, which just like the one in
> > entry_64.S has to use the hardcoded virtual address bits to escape
> > the fact that TASK_SIZE_MAX isn't actually a constant when 5-level
> > page tables are enabled.
> ....
> > @@ -93,7 +69,7 @@ static inline bool pagefault_disabled(void);
> > #define access_ok(addr, size) \
> > ({ \
> > WARN_ON_IN_IRQ(); \
> > - likely(!__range_not_ok(addr, size, user_addr_max())); \
> > + likely(!__range_not_ok(addr, size, TASK_SIZE_MAX)); \
> > })
>
> Can't that always compare against a constant even when 5-levl
> page tables are enabled on x86-64?
>
> On x86-64 it can (probably) reduce to (addr | (addr + size)) < 0.

I'll leave that to the x86 maintainers as a future cleanup if wanted.