Method to intercept child process filesystem activity

Question

I have a small command-line application written in C that acts as a wrapper/launcher for other programs (think: xargs). The application is written to compile on FreeBSD/Linux (via fork()/exec()) and Windows (CreateProcess()). In addition to being able to intercept, inject, or otherwise manipulate the command-line arguments for the child application, I was wondering if there is an easy way to intercept the child program's filesystem activity (or it's children, and so forth). I'm mainly interested in just the filenames accessed for either read or write, but not the contents of said files, and would like the interception to be as lightweight as possible.

In googling some relevant keywords from above, it seems there are many ways to do this in Win32. From file system filter drivers to tampering with PE import table headers. None of these seem trivial or something I could self-contain within my wrapper program's executable (e.g. most would require extra DLLs or driver files alongside the main executable.) Also, I'd like this to work on Windows XP through 7, if possible, without having to hack around UAC or other platform deltas. It's my child process, so I figure I should be able to securely monitor its activity :)

On Linux, there is inotify(), but it monitors general filesystem access without regard to ONLY my child process/es. Same thing goes for FreeBSD's kqueue(). These also break down in SMP cases where multiple instances of the wrapper might be running different programs, and each needs to disambiguate their own child's filesystem activity from one another.

I'd certainly appreciate any suggestions that the SO community might have to offer.

Answer 1

The only suggestion I would make is to use strace (trace system calls and signals), though this is more of a debugging tool, and does affect the performance of the process being traced

strace -f -e trace=file -o <output-file> <cmd-line>

-f - follow forks
-e trace=file - will output system calls related to the file-system
-o <output-file>

IMHO - if you get familiar with strace, it is always a useful tool to have in the armory.

Answer 2

Write a "interposer" library that hooks your fopen and set LD_PRELOAD environment variable for all child processes. This works with dynamically linked libraries thou.

Example of how to do this can be found here: http://developers.sun.com/solaris/articles/lib_interposers.html and http://lists.debian.org/debian-powerpc/2004/11/msg00039.html shows a partial implementation for fopen() interposer..