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

David Laight
2020-08-17T08:23:51Z

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.

David

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