[PATCH 0/4] macOS/arm64 support

[Martin Storsjö]

Hi,

On Sat, 15 Aug 2020, Stefan Dösinger wrote:

>
>       - Setting -pagezero_size to anything less than 4 GB seems to
>       make
>        macOS refuse to run the executable. So this makes it impossible
>       to
>        map anything into the lower 4 GB of the address space. For now,
>        I've worked it around by moving the address at which
>       user_shared_data
>        is allocated.
> 
> 
> Shouldn't you be able to forcefully remap parts in the zero page? To my
> knowledge the only thing you can't do is unmap pages that the dynamic loader
> thinks are allocated - if it gets them again when loading a different .dylib
> it will crash and burn. But you can map stuff there with MAP_FIXED yourself
> as long as you can be sure you don't overwrite anything important - which
> shouldn't be the case in the zero page.

Hmm, I'll have to experiment with this and see how it behaves.

>       - Memory mappings can't be writable and executable at the same
>       time.
>        If one mmap()s a page and request it to be both writable and
>        executable, writing to it fails, same if changing protection
>       with
>        mprotect().
> 
> 
> In Catalina you can bypass that with the right protected runtime
> entitlements. That means you'll have to sign your executable with a manifest
> that enables the protected runtime and requests the entitlements. Do you
> know if this still works in Big Sur x86_64 and/or big sur ARM?

Oh, good point. I'll have to look into this as well.

For the cases I've worked around regarding this so far, the solutions 
doesn't look too horrible, but I'm not sure if there's other cases that it 
breaks.

> - Darwin also treats x18 as reserved, just like windows, but IIRC
>  the system can spuriously(?) overwrite the register to zero at
>  some times. I haven't run into this in the context of wine on macOS
>  yet though.
> 
> 
> Yeah someone warned me that this happens on iOS. The Darwin kernel code
> should be accessible I think. Any way to find out under which conditions
> this happens?

Someone linked me this:

https://github.com/apple/darwin-xnu/blob/a449c6a3b8014d9406c2ddbdc81795da24aa7443/osfmk/arm64/cswitch.s#L120

So apparently it would be forcibly overwritten on context switch, just to 
make sure it's not used.

I wrote a quick test program that sets x18 and reads it back after a few 
sleep(1), and it does seem like this doesn't happen any longer in macOS 
11.0 though, so maybe this one isn't an issue for now.

// Martin