CODEWITHSUNDEEP
pythonclinuxfunction-interpositioninterposing
Matt Joiner
Matt Joiner

Reputation: 118550

Why won't LD_PRELOAD work with Python?

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.

Upvotes: 9

Views: 7432

Answers (3)

Matt Joiner
Matt Joiner

Reputation: 118550

It turns out there is an open64() function:

$ objdump -T /lib32/libc.so.6  | grep '\bopen'
00064f10 g    DF .text  000000fc  GLIBC_2.4   open_wmemstream
000cc010 g    DF .text  0000007b  GLIBC_2.0   openlog
000bf6d0  w   DF .text  000000b6  GLIBC_2.1   open64
00094460  w   DF .text  00000055  GLIBC_2.0   opendir
0005f9b0 g    DF .text  000000d9  GLIBC_2.0   open_memstream
000bf650  w   DF .text  0000007a  GLIBC_2.0   open
000bf980  w   DF .text  00000081  GLIBC_2.4   openat
000bfb90  w   DF .text  00000081  GLIBC_2.4   openat64

The open64() function is a part of the large file extensions, and is equivalent to calling open() with the O_LARGEFILE flag.

Running the example code with the open64 section uncommented gives:

$ LD_PRELOAD=./libinterpose_python.so python3 -c 'b = open("a", "w"); b.write("hi\n"); b.flush()'
sandbox_init()
open64()
open64()
open64()
Traceback (most recent call last):
  File "<string>", line 1, in <module>
open64()
open64()
open64()
open64()
open64()
open64()
open64()
IOError: [Errno 9] Bad file descriptor
sandbox_fini()

Which clearly shows all of Python's open calls, and several propagated errors due to the write flag being stripped from the calls.

Upvotes: 1

Andrew Aylett
Andrew Aylett

Reputation: 40720

You should be able to find out what your python process is actually doing by running it under strace (probably without your pre-load).

My python3.1 (on AMD64) does appear to use open:

axa@ares:~$ strace python3.1 -c 'open("a","r+")'
...
open("a", O_RDWR)                       = -1 ENOENT (No such file or directory)

Upvotes: 2

zvrba
zvrba

Reputation: 24546

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

Upvotes: 8

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