For context, GMSCompat is Google Mobile Services Compatibility. GrapheneOS installed the google play store and services as normal apps, and worked backwards to make it behave. There is no google specific sandbox, rather it uses the standard android user app sandbox. This means google is bound by the same rules, as special casing anything creates more maintenance burden and attack surface. GMSCompat is fully open source.
> "Thanks, but there's no way..."
Its reposted because the information is accurate, and misinformation regarding it is very prevalent.
> "Yes, I knew it's in a sandbox..."
Relative to MicroG, sandboxed google play is much more private, secure, and usable. I would not describe it as a privacy paradise, but MicroG does not improve upon this, and instead makes these aspects worse.
> "The sandbox still needs internet access..."
Most google libraries operate independently of google services and do not depend on them to function. FCM is an exception due to how push notifications are optimized (by using one app for the connection). MicroG does not avoid this.
> "For example, Signal will actively reach..."
You do not need to provide an identity to google. This can also be avoided with a VPN, and is not specific to google. There is the concern of metadata but Signal sends empty notifications without any identifying info. They are only used to wake the app up to fetch its own notifications.
> "So while the sandbox is definitely very useful..."
It confines google services to the same rules and restrictions as all other apps. MicroG does not. MicroG also does not avoid running unwanted software, referring to the google libraries in apps and the google code MicroG downloads.
> "Do you know what privileged context means..."
MicroG violates the security model by necessitating signature spoofing, which puts it in a position to receive data it was not intended to receive, there is also attack surface exposed by having access forbidden by the app sandbox. Sandboxed google play is bound by the same app sandbox as all other apps, and would not be any more or less capable of exploiting the device than any other app. The idea that google would try to exploit the device is nonsensical though. But granting both google and a 3rd party privileged access is still unacceptable.
> "Rather than running the unwanted proprietary (but necessary) software..."
Google play services runs in the android user app sandbox. It is not an "attempt", it is successful at doing this. MicroG being open source does not matter in regards to privacy or security. It did not change how MicroG has leaked location to apps without location permissions, it does not change how it downloads and runs google code both privileged and outside of its own APK, and it does not change how other apps are running google libraries anyway. Note that the proprietary code it downloads is not confined to the app sandbox.
> "For example, microG will replace Gmaps..."
Im unsure if you are referring to the app Google Maps, or google maps integration. GrapheneOS reroutes googlefusedlocation requests to the OS, rather than google services. You can use an app other than google maps, and apps with google map integration can simply send your location to google directly, independent of google services or MicroG.
> "It seems fairly obvious to me that less data sharing..."
Googles access to data is not limited by using MicroG, relative to sandboxed google play. And the size of proprietary code is irrelevant, that code can be anything. It can be malicious with 2 lines, or benign with 2 million. Access is what is vital, not size. Google is not permitted to "run wild", and is granted no additional access compared to any other app. Im unsure what you mean by self updating functionality, but for apps from the playstore, nearly all of them are signed with a key that google holds, and MicroG can do nothing about this. GrapheneOSs App Store is responsible for updating google play and google services, it cannot update itself.
> "What threat would sandboxed microG pose that sandboxed GMS doesn't?..."
Using MicroG necessitates GrapheneOS violate the android security model, trust a 3rd party unnecessarily, cripple 99% perfect compatibility, use code that is not near as battletested as play services, run google code as privileged, and run a software that has had serious privacy violations in the past. Not only is the base insufficient, but any finished product based on it still would not compare to GMSCompat. The logic is that GrapheneOS wants the best compatibility, the least changes to the android app sandbox, 0 privileged google components, no violations to the android security model, and no need to maintain a reimplementation when google services and store are already maintained by a huge organization.