[Feature]: Support secondary users via binary patching and extra slashes

[chriswoope]

Feature description

A better way to support secondary users than the ones proposed so far could be to set the prefix to something like /data/////////data/ or /data/././././data, with enough slashes or /.s to be able to binary patch all instances of it in files of the package upon package installation to /data/user/NN/ for all practical values of NN without changing the string length (looking into any gzip/bz2/xz/zip archives if any, with special handling of checksummed files if any).

While somewhat inelegant, it seems to me this is likely to work, with the exception of any non-standard compressed files or files with checksums (both of these are rare or nonexistent in common Linux packages and can be handled with ad-hoc code), or anything that doesn't like the paths not being canonical (also probably rare or non-existent, since due to the /usr merge everything is accessible via both / and /usr in common Linux distributions).

The few applications that might not work, if any, can be patched.

Unlike other solutions, it doesn't have any performance or functionality downside, although it might have some slight aesthetic downside and there's a small risk of breaking compatibility in existing installation (but it's possible to build all packages twice, once like now and once with this scheme).

Additional information

Yes for secondary users support. None known for binary patching.

[segfault-bilibili]

To summarize my thoughts in #2765 :

1. According to AOSP source code, the (currently) max allowed userId is up to 21473.
2. Adding redundant eliminate-able part to make longer equivalent path might break some simple string comparisons like [[ "$path1" == "/data/user/0/./././com.termux" ]] where path1 went through realpath but /data/user/0/./././com.termux didn't.
3. There are references pointing to somewhere outside Termux's data directory, like /data/data/com.wireguard.android, which still needs to be handled.

Personally I suggest something like /data/user/0/com.termux/xxxxx/usr/ as the prefix. The number of x could be increased to be somewhat more future-proof.

In case it's installed as a secondary Android user (userId=10), the path should then be modified into /data/user/10/com.termux/xxxx/usr (only 4 x characters here, because 10 is one character longer than 0)

[romanovj]

I installed all required packages for home assistant (python, clang, rust, etc ...)

Moved termux to /data/local/tmp/termu

Hexpatched /data/data/com.termux -> /data/local/tmp/termu (with magiskboot hexpatch)

Then I installed home assistant without problems.

[basilgello]

As a long-term everyday Termux user and Debian maintainer, I came across this topic and decided to share some thoughts about the idea:

• Hexpatching must be done in dpkg trigger, so that packages themselves are intact.
• The idea with fixed length prefix full path is the only v8able solution since Google already merged https://android-review.googlesource.com/q/bug:156305599 and especially https://android-review.googlesource.com/c/platform/system/vold/+/2095483
• Hexpatching must touch text files, too. Maybe we can generate some mapping for every package (via debhelper addon) what to patch and which files to look for.
  This will make patching reproducible.