Syscall - interposer
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Dynamically loaded (DL) libraries are libraries that are loaded at times other than during the startup of a program. They're particularly useful for implementing plugins or modules, because they permit waiting to load the plugin until it's needed. For example, the Pluggable Authentication Modules (PAM) system uses DL libraries to permit administrators to configure and reconfigure authentication.
Shared libraries are libraries that are loaded by programs when they start. When a shared library is installed properly, all programs that start afterwards automatically use the new shared library. It's actually much more flexible and sophisticated than this, because the approach used by Linux permits you to:
Sometimes, you might want to determine what is happening in a shared library without modifying the library (have you tried to build glibc lately?). Other times, you might want to override only a few functions within a library and get them to do something else--force a process to a specific CPU, prevent a specific USB message from being sent and so on. All of these tasks are possible if you use the LD_PRELOAD environment variable and a small shim program placed between the application and the library. As an example, say you create a shared library object called shim.so and want it to be loaded before any other shared library.
I produced hereafter some information about shared libraries on different systems.
This article is intended to help C & C++ programmers understand the essentials of what the linker does. I've explained this to a number of colleagues over the years, so I decided it was time to write it down so that it's more widely available (and so that I don't have to explain it again). [Updated March 2009 to include more information on the pecularities of linking on Windows, plus some clarification on the one definition rule.] A typical example of what triggers this explanation is when I help someone who has a link error like: If your reaction to this is ' almost certainly missing extern "C" ' then you probably already know everything in this article.
From WikiContent The kernel is the heart of an operating system. It is responsible for such core functionality as memory management, process scheduling, TCP/IP networking, and so on. Linux Kernel Modules (LKMs) allow you to extend Linux kernel functionality on-the-fly.
7.2. Intercepting System Calls Processes run in two modes: user and kernel.
NOTE: click here if you get an empty page. LD.SO(8) Linux Programmer's Manual LD.SO(8) ld.so, ld-linux.so* - dynamic linker/loader The programs ld.so and ld-linux.so* find and load the shared libraries needed by a program, prepare the program to run, and then run it. Linux binaries require dynamic linking (linking at run time) unless the -static option was given to ld during compilation.
Recently I have used Dtrace to change the output of uname() syscall. But if one wants a more permanent and selective approach it is easier to write a small library which would interpose the uname() syscall (well, actually uname() libC function and not a syscall itself). I slightly modified the malloc_interposer example . After you compiled the library all you have to do is to LD_PRELOAD it in your script so everything started by that script will use it or you can LD_PRELOAD it only for a given binary as shown below. Additionally you have to set a variable uname_release to whatever string you like otherwise the library won't do anything.
April 11, 2001, 1:50 PM — Summary: Library interposition is a useful technique for tuning performance, collecting runtime statistics, or debugging applications. This article offers helpful tips and tools for working with the technique and gets you started on your own interposer. Most of today's applications use shared libraries and dynamic linking, especially for such system libraries as the standard C library (libc), or the X Window or OpenGL libraries. Operating system vendors encourage this method because it provides many advantages. With dynamic linking, you can intercept any function call that an application makes to any shared library.
Late last week, I cracked open the Commodore 64 emulator code once again, in preparation to post it. However, I had to have a change made to the source control on CodePlex, so I had a few days to make some changes. So far, it's shaping up quite nicely: