Why won't LD_PRELOAD work with Python?

Question

Using function interposition for open() with Python doesn't seem to work after the first few calls. I suspect Python is doing some kind of initialization, or something is temporarily bypassing my function.

Here the open call is clearly hooked:

$ cat a
hi
$ LD_PRELOAD=./libinterpose_python.so cat a
sandbox_init()
open()
hi

Here it happens once during Python initialization:

$ LD_PRELOAD=./libinterpose_python.so python
sandbox_init()
Python 2.7.2 (default, Jun 12 2011, 20:20:34) 
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
open()
>>> 
sandbox_fini()

Here it doesn't happen at all, and there's no error to indicate the file handle had write privileges removed:

$ LD_PRELOAD=./libinterpose_python.so python3 -c 'b = open("a", "w"); b.write("hi\n"); b.flush()'
sandbox_init()
sandbox_fini()

The code is here. Build with make -f Makefile.interpose_python.

A full solution is given here.

Answer 1

There are open() and open64() functions, you might need to redefine both.