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SubjectRe: [BUG] FIXMAP-related change to mm/memory.c
Date
Hi Linus, all.

>> Is it possible to constrain the FIXADDR range on x86/x86-64
>> (FIXADDR_START-FIXADDR_TOP) such that the entire range is
>> read-only by user-level? If so, we could simplify the
>> permission test like this:

> Well, you could replace the uses of FIXADDR_START/FIXADDR_TOP
> with something like FIXADDR_USER_START/FIXADDR_USER_TOP, and
> then force those to cover only the _one_ user-accessible page.
>
> Something like
>
> #define FIXADDR_USER_START (fix_to_virt(FIX_VSYSCALL))
> #define FIXADDR_USER_END (FIXADDR_USER_START + PAGE_SIZE)
>
> should work. In that case you can drop the page table testing,
> since we "know" it is safe.

Should FIXADDR_USER_END point to the last byte of the relevant page,
or to the first byte of the following page as per Linus's suggestion?
The above looks like an off-by-one bug to me?

Best wishes from Riley.
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