/proc/$$/exec with termux-exec

[abdirahmandev6]

Hi, I built the busybox package for my termux fork but when I asked it to create applets using "busybox --install -s", all the symlinks it created pointed to linker64 but when I did the same in the termux app(both the version from the play store and the one on github) the symlinks points correctly to busybox.

Termux uses linker64 to bypass Android 10's restrictions on executing code so /proc/self/exec will point to linker64 and not whatever command is actually running so Termux uses TERMUX_EXEC__PROC_SELF_EXE to tell packages what command is actually running.

But somehow calling "file /proc/self/exe" or "file /proc/$$/exe" inside the termux app currently displays the correct command and not linker64.
Something is intercepting reads of /proc/$$/exe and rewritting its content as it still in reality points to linker64 which can be verified by using "toybox file /proc/$$/exe" which says linker64.
Toybox is part of Android so whatever intercepts termux commands doesn't apply to it.

My question is what does this interception?
termux-exec?

I couldn't find any info on this in the termux docs and I've been trying to figure this out for a couple of days now.
Isn't TERMUX_EXEC__PROC_SELF_EXE now useless if /proc/self/exe now works correctly again?

[robertkirkman]

Hi, please show the source code of your Termux fork.

• It seems that you have a fork of Google Play Termux rather than F-Droid Termux, but the problem is that the original Google Play Termux is more accurately applying its termux-exec in some way than your fork. I might know what is wrong if I see the source code and can compare differences between it and the real Google Play Termux.

• Since it is a fork of Google Play Termux, it might be better to open this discussion in the Google Play Termux forum instead of here https://github.com/termux-play-store/termux-issues/discussions

• And you might know, F-Droid Termux (which is github.com/termux, where this discussion is) is not affected by this problem and does not need to use linker64 the same way that Google Play Termux does, so one workaround option is to try forking F-Droid Termux instead of Google Play Termux and the downside of that would just be that your app would lose the capabilities that are unique to Google Play Termux such as the ability to target the newest Android API level at build-time (which does not mean it wouldn't run on the newest Android 15 and 16, F-Droid Termux runs fine on the newest Android 15 and 16)

[agnostic-apollo]

Termux uses linker64 to bypass Android 10's restrictions on executing code so /proc/self/exec will point to linker64

That only applies to google play version of termux, not fdroid/github supplied by us.

Busybox in playstore could be patched to use TERMUX_EXEC__PROC_SELF_EXE.

It depends on which repo you are using to build packages. It might not be best to use this repo to build packages for linker, as all patches may not have been ported yet for linker.

As Robert said, this is more relevant to be reported in other org.

[abdirahmandev6]

My fork isn't actually a fork of the Termux app itself but I do use the Termux packages after building them with another package name and I then create apps that use those packages.

When Android 10 originally came out I thought it would be a death sentence for those apps but after reading through Termux discussions on workarounds, I started packaging the packages as fake shared libraries and then symlinking them at startup to their correct locations(I have a custom Gradle plugin that automates this) but I recently switched to the linker64 solution by using termux-exec and emulating Termux's environment as closely as possible(providing the same environment variables).

Here are some screenshots.
In the first I used F-droid Termux which verifies /proc/$$/exe is linked to Bash since linker64 isn't used.
In the second I used Play store Termux so /proc/$$/exe is linked to linker64 but the file command from Termux falsely states its actually linked to Bash(maybe its patched).

In the third I used F-droid Termux again and forced it to use linker64 and now Termux's file command states /proc/$$/exe is linked to linker64 so it seems like the play store version is patched so that any access to /proc/$$/exe leads to the actual command running and not linker64.
I wonder where this patch is located?
Where all packages patched in the play store version or is there a compatibility layer somewhere that somehow fixes /proc/$$ on the play store version?

[agnostic-apollo]

libtermux-exec.so provided by our termux-exec package is different from one by playstore. Playstore one has patches for readlink that don't exist in ours.

https://github.com/termux-play-store/termux-exec/commits/main/src