Re: FIXMAP-related change to mm/memory.c

From: Linus Torvalds (
Date: Fri Jun 13 2003 - 00:24:27 EST

On Thu, 12 Jun 2003, David Mosberger wrote:
> 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))

should work. In that case you can drop the page table testing, since we
"know" it is safe.

But I'm too lazy to test, so please send a tested patch,


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