busybox tar ignores fakeroot on Ubuntu but it works on OS X

Question

On my mac machine this just works as expected:

#!/bin/sh -euf
touch test.sh
chown 888:888 test.sh
busybox tar -czvf out.tar.gz test.sh

Invocation:

$ fakeroot -- ./generateArchive.sh
$ busybox tar -tzvf out.tar.gz
-rw-r--r-- 888/888         0 2017-08-02 20:52:50 test.sh

But on my virtual ubuntu machine I get:

$ fakeroot -- ./generateArchive.sh
$ busybox tar -tzvf out.tar.gz
-rwxrwxr-x marco/marco       215 2017-08-02 20:53:32 test.sh

Why does busybox on ubuntu not "fall" for the faked ownership?

It works on ubuntu when I use tar instead of busybox tar.

Additional information:

Mac (10.12.5): 
    fakeroot version 1.20.2
    BusyBox v1.20.0.git (2017-05-17 10:01:40 CEST) multi-call binary.
Ubuntu (14.04.5 LTS):
    fakeroot version 1.20
    BusyBox v1.21.1 (Ubuntu 1:1.21.0-1ubuntu1) multi-call binary.

Answer 1

Most likely reason is that your busybox was statically linked. Fakeroot uses LD_PRELOAD, which intercepts the calls as they pass from the program to the run time library (glibc). Statically linked executables are invisible to LD_PRELOAD, and thus, to fakeroot.

Fakeroot-ng uses the ptrace mechanism, which intercepts system calls when they pass from the program to the kernel. As such, fakeroot-ng, can catch system calls executed by statically linked programs (which is one of the reasons I created fakeroot-ng in the first place).

On Mac, statically linking of the run time library is not allowed. They do not even provide a static version of the run time library. As such, fakeroot has no problem intercepting the system calls there (which is a good thing, because fakeroot-ng does not have an OS-X version, partially for the reason stated above).

Answer 2

I have a guess why it is not working:

Maybe busybox on Ubuntu uses open() which fakeroot cannot intercept.

Workaround:

I installed fakeroot-ng on my Ubuntu machine and now it works as expected.