PNaCl changed it by making it portable, and WebAssembly is effectively the same thing but from scratch with complete portability / avoidance of undefined behavior as a core part of it rather than based on a frozen LLVM IR subset like PNaCl with an attempt to purge those things.
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ARC is dead though. They gave up on using the strong renderer sandbox + NaCl within that, because they would have had to reimplement the entirety of the Android system APIs in Chrome. They made really good progress but at some point they just decided to take a shortcut instead.
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It's a whole lot harder to achieve 100% compatibility when you actually have to reimplement the APIs for a strong sandbox vs. just running the *entirety* of Android natively on top of a kernel with Android IPC support and separating it with namespaces and some bridging to host.
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Also, it didn't have any direct Linux kernel access, since it was not just in the OS sandbox directly. Apps had to target it as a separate arch (i.e. compile their native code for it). Now, they could use gVisor for the Linux kernel ABI without providing direct kernel access.
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There were a decent number of apps targeting it, including apps like VLC which is the kind of app that would be most difficult to support it. Apps without native code could trivially run on it. Apps with native code had to build for it and cope with having a subset of Linux APIs.
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I think it's unfortunate that they gave up, instead of extending with Linux kernel ABI compatibility and finishing up the Android API implementation via the Chromium broker process. It was a much stronger way to sandbox apps. It could have even been dropped into Android itself.
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One of my early plans for my work was to drop in ARC to Android and use it to provide a stronger app sandbox for apps compatible with it, which could have been most apps and eventually every single app if they had kept at it for a few more years with a properly resourced team...
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i.e. eventually fully replacing the Android application layer for all non-base-system apps with the ARC implementation using the Chromium broker process + Chromium renderer sandbox, without the web parts of Chromium (Blink, v8, etc.) which weren't part of how it was implemented.
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It made apps as untrusted and as sandboxed as a web page. It has definitely bitrotted way too much by now for it to be any use, but the model it used is exactly what I want to do: take gVisor and run apps inside that, and implement a broker outside it providing the Android APIs.
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So the client side part is simply the existing AOSP code that runs inside the app, but the server side is a reimplementation rather than it talking to system_server, etc. directly. It can proxy to the actual AOSP system_server, it just enforces better security policy / isolation.
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I'll write up a page on how I want to reimplement the Android application layer / app sandbox and profiles to have it sit inside another OS without direct kernel or IPC with other apps or the base system. It's way too much information to try to keep tweeting about it anyway.
The host OS would initially still use the Linux kernel and Android, but everything outside the base system (all apps) would run non-natively via gVisor + similar project as gVisor reimplementing the Android IPC APIs and proxying everything. In the long term host OS gets replaced.