linux-kernel - Re: [PATCH 22/35] arm64/mm: Implement map_shadow

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Message-ID: <7a4c97f68347d4188286c543cdccaa12577cdb9e.camel@intel.com>
Date:   Tue, 18 Jul 2023 15:49:28 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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        "Szabolcs.Nagy@....com" <Szabolcs.Nagy@....com>
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Subject: Re: [PATCH 22/35] arm64/mm: Implement map_shadow_stack()

On Tue, 2023-07-18 at 14:55 +0100, Mark Brown wrote:
> On Tue, Jul 18, 2023 at 10:10:04AM +0100, Szabolcs Nagy wrote:
> 
> >    uint64_t *p = map_shadow_stack(0, N*8, 0);
> 
> > i'd expect p[N-1] to be the end token and p[N-2] to be the cap
> > token,
> > not p[PAGE_ALIGN(N*8)/8-2].
> 
> Yes, that probably would be more helpful.

HJ made a similar request on the x86 side. He wanted an unaligned size
passed in to result in unaligned token placement.

> 
> > if we allow misalligned size here (and in munmap) then i think it's
> > better to not page align.  size%8!=0 || size<16 can be an error.
> 
> Honestly I'd be a lot happier to just not allow misalignment but that
> raises the issue with binaries randomly not working when moved to a
> kernel with a different page size.  I'll have a think but possibly
> the
> safest thing would be requiring a multiple of 4K then rounding up to
> our
> actual page size.


Someday when the x86 side is finally upstream I have a manpage for
map_shadow_stack. Any differences on the arm side would need to be
documented, but I'm not sure why there should be any differences. Like,
why not use the same flags? Or have a new flag for token+end marker
that x86 can use as well?