Full path of build path is leaked in libnunchuk-android.so file

[emanuelb]

from strings diff of libnunchuk-android.so from nunchunk apk downloaded from GooglePlay and apk built with Cotnainerfile at: #1 (comment)

< /Users/khacthong.le/Documents/Personal/nunchuk-android-nativesdk/src/main/native/libnunchuk/embedded/../contrib/bitcoin/src/prevector.h
---
> /home/appuser/app/nunchuk/nunchuk-android-nativesdk/src/main/native/libnunchuk/embedded/../contrib/bitcoin/src/prevector.h
52985c52972
< /Users/khacthong.le/Documents/Personal/nunchuk-android-nativesdk/src/main/native/libnunchuk/src/nunchukimpl.cpp
---
> /home/appuser/app/nunchuk/nunchuk-android-nativesdk/src/main/native/libnunchuk/src/nunchukimpl.cpp

full build path leakage is a issue cause:

1. for local builds it might leak username (/home/username) or other information like khacthong.le and appuser in above examples.
2. it makes the build non reproducible when different build-path is used (build path variations break reproducibility)
3. to reproduce current build it require using a directory which only root user can create on linux /Users/ (in case it can be reproduced on linux)
4. fdroid.org builds are using /home/vagrant/ as path as they built using vagrant in regular user, and fdroid can use upstream APK directly only if it's reproducible in their build server.

the build process shouldn't write above full paths lines to output (no path at all or relative path to project root dir like src/main/native/libnunchuk/src/nunchukimpl.cpp)

this can be fixed by using -prefix-map options, details in: https://reproducible-builds.org/docs/build-path/

as current apk built with clang 9, it might require fixing first #4 (increase the ndk version used to latest supported) in case -fmacro-prefix-map should be used (which require clang > 10)

[JayceLuong]

I updated ndk version to latest, could you show me how to fix the issue related to leak the build path

[emanuelb]

Add -ffile-prefix-map to the clang invocations such as -ffile-prefix-map=${pwd}=. (instead of . a path like /src can be used too) and instead of pwd a variable that will point to local nunchuk-android-nativesdk (in case during the compilation command the current directory can be different) such as (examples of the content the variable should have in different setups)
/Users/khacthong.le/Documents/Personal/nunchuk-android-nativesdk/src/main/native
or
/home/appuser/app/nunchuk/nunchuk-android-nativesdk/src/main/native

for more details about the option: https://reproducible-builds.org/docs/build-path/