Re: [PATCH v3 21/36] arm64/mm: Implement map_shadow_stack()

[Mark Brown <broonie@kernel.org>]

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On Tue, Aug 08, 2023 at 09:21:03AM +0100, Szabolcs Nagy wrote:
> The 08/07/2023 14:00, Mark Brown wrote:

> > That's not what the manual page or a quick check of the code suggest
> > that mmap() does, they say that the kernel just takes it as a hint and

> i should have said that i expect MAP_FIXED_NOREPLACE semantics
> (since the x86 code seemed to use that) and then the mapped
> address must match exactly thus page aligned.

Ah, I see.  We do pass MAP_FIXED_NOREPLACE when allocating the stack if
an address was specified but currently leave it up to the VM subsystem
to figure out what to do with the address.  I don't immediately see
where mmap() enforces this requirement, but I have to admit I didn't
look overly hard.  I don't see a problem with enforcing a PAGE_SIZE
constraint here.

> > > > +	if (size == 16 || size % 16)
> > > > +		return -EINVAL;

> > > why %16 and not %8 ?

> > I don't think that's needed any more - there was some stuff in an
> > earlier version of the code but no longer.

> it's kind of important to know the exact logic so the cap token
> location can be computed in userspace for arbitrary size.

> (this is why i wanted to see the map_shadow_stack man page first
> but i was told that comes separately on linux..)

Right, I'd already changed it to % 8 in the version I posted yesterday.

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