Cisco Command Summary. Dynamically Loaded (DL) Libraries. 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. They're also useful for implementing interpreters that wish to occasionally compile their code into machine code and use the compiled version for efficiency purposes, all without stopping. For example, this approach can be useful in implementing a just-in-time compiler or multi-user dungeon (MUD).
In Linux, DL libraries aren't actually special from the point-of-view of their format; they are built as standard object files or standard shared libraries as discussed above. The interface used by Linux is essentially the same as that used in Solaris, which I'll call the ``dlopen()'' API. 4.3. dlsym() Shared Libraries. 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: update libraries and still support programs that want to use older, non-backward-compatible versions of those libraries;override specific libraries or even specific functions in a library when executing a particular program.do all this while programs are running using existing libraries. 3.1.
For shared libraries to support all of these desired properties, a number of conventions and guidelines must be followed. 3.1.1. Every shared library has a special name called the ``soname''. Every shared library also has a ``real name'', which is the filename containing the actual library code.
The key to managing shared libraries is the separation of these names. 3.1.2. 3.2. 3.3. 3.3.3. 3.4. Modifying a Dynamic Library Without Changing the Source Code. 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. Say you also want to run the program "test". These things can be done on the command line with LD_PRELOAD: LD_PRELOAD=/home/greg/test/shim.so . This command tells glibc to load the library shim.so from the directory /home/greg/test first, before it loads any other shared libraries the test program might need. To compile this code, run GCC with the following options: Using static and shared libraries across platforms.
I produced hereafter some information about shared libraries on different systems. However, this information is probably very sparse on details and may even not be up-to-date. Actually, keeping track of changes is nearly impossible. The definitive information usually comes from the operating system docs. (e.g. HP's "HP-UX Linker and Libraries User's Guide", Sun's "Linker and Libraries Guide", SGI's "MIPSpro Compiling and Performance Tuning Guide", IBM's "AIX Linking and Loading Mechanisms", etc.), where there is often advice on the best options for performance. GNU libtool offers an abstraction layer to shared library management. Feedback is welcome. Adapted from: Cross platform development, Using Shared Libraries across Platforms, Shah, Amal ; Xiao, Hong, C/C++ Users Journal, May 1998, Volume 16 Number 5 Notes: cells with xxxx implies that the OS does not support that particular feature. ??
Juan Altmayer Pizzorno, Wolfgang Haefelinger, Per Losenborg, Anil E. Beginner's Guide to Linkers. 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.
Table of Contents Naming of Parts: What's in a C File This section is a quick reminder of the different parts of a C file. The first division to understand is between declarations and definitions. A definition of a variable induces the compiler to reserve some space for that variable, and possibly fill that space with a particular value. System Call Table. 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. Because it is easy to insert and remove LKMs using command-line tools, malicious users prefer to install LKM-based rootkits and backdoors on a compromised system to maintain access to the host.
This chapter will show you how to write your own LKMs and teach you how authors of malicious rootkits and backdoors leverage the power of LKMs to perform various types of tricks, such as process and file hiding as well as system call interception. Warning Do not run the examples presented in this chapter on mission-critical or production hosts. Hello World To learn the basics of writing LKMs, first we'll attempt to write a simple module that prints Hello World! Next, define hello( ), which simply prints the string Hello World! Tip. Section 7.2. Intercepting System Calls. 7.2. Intercepting System Calls Processes run in two modes: user and kernel. Most of the time processes run under the user mode when they have access to limited resources. When a process needs to perform a service offered by the kernel, it invokes a system call . 7.2.1. The Linux kernel maintains a system call table , which is simply a set of pointers to functions that implement the system calls. 7.2.2. strace Is Your Friend Often it is necessary to hook into programs to understand what system calls they invoke.
#include <stdio.h> int main(void) { FILE *myfile; char tempstring[1024]; if(! Assuming you have compiled the preceding code with the gcc compiler to produce an executable called a.out , run the following strace command: [notroot]$ strace -o strace.out . Now the output from strace is stored in strace.out . [notroot]$ grep "/etc/passwd" strace.out open("/etc/passwd", O_RDONLY) = 3 7.2.3.
Original_sys_open =(void * )xchg(&sys_call_table[_ _NR_open], our_fake_open_function); set_fs(fs); Interrupt and exception handling. Minou/ldx-box. Ld.so () 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.
The program ld.so handles a.out binaries, a format used long ago; ld- linux.so* handles ELF (/lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2 for glibc2), which everybody has been using for years now. Otherwise both have the same behaviour, and use the same support files and pro- grams ldd(1), ldconfig(8) and /etc/ld.so.conf. --list List all dependencies and how they are resolved. There are four important environment variables. The ld.so functionality is available for executables compiled using libc version 4.4.3 or greater. What is the debian way to update Library path? Library Interposer. Intercept file open system call. Tuning with Library Interposers. Oracle Technology Network > Java Software Downloads View All Downloads Top Downloads New Downloads What's New Java in the Cloud: Rapidly develop and deploy Java business applications in the cloud. Essential Links Developer Spotlight Java EE—the Most Lightweight Enterprise Framework?
Blogs Technologies Contact Us About Oracle Cloud Events Top Actions News Key Topics Oracle Integrated Cloud Applications & Platform Services. Building library interposers for fun and profit. 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.
Once you intercept it, you can do whatever you want in that function, as well as call the real function that the application originally intended to call. Performance tuning is one use of this technology. Building and running your first interposer Here's the source for this interposer: malloc_interposer.c. HELP! Problem getting interposer/LD_PRELOAD working - comp.unix.programmer | Computer Group. Intercepting system calls on Linux. The Commodore 64 Emulator: Emulating Pointers in a sandbox when the real thing is not allowed. 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: I went back to the latest version of the Frodo C64 emulator source code and decided to port some of their changes over to this version.
Frodo is written in C and C++, and makes very heavy use of pointers (and not always safe use of them, as there was at least one logical overrun). Silverlight doesn't support pointers or unsafe code. So, I decided to finish what I started and build out some decent almost-pointerish safe analogs in Silverlight. How Pointers Work If you've never written any C/C++ code, there's a better than zero chance that you haven't done any explicit pointer manipulation in your own code.
It's only when you have to do a lot of memory walking in a performance-critical application that you run into this. Converting Strings. Using Assembly Language in Linux. By Phillip phillip@ussrback.com Last updated: Monday 8th January 2001 Contents: Introduction. This article will describe assembly language programming under Linux. Contained within the bounds of the article is a comparison between Intel and AT&T syntax asm, a guide to using syscalls and a introductory guide to using inline asm in gcc.
This article was written due to the lack of (good) info on this field of programming (inline asm section in particular), in which case i should remind thee that this is not a shellcode writing tutorial because there is no lack of info in this field. Various parts of this text I have learnt about through experimentation and hence may be prone to error. There is only one prerequisite for reading this article, and thats obviously a basic knowledge of x86 assembly language and C. Intel and AT&T Syntax. Prefixes. In Intel syntax there are no register prefixes or immed prefixes. Example: Direction of Operands. Memory Operands. Example: As you can see, AT&T is very obscure.
How to create a lightweight C code sandbox. Secure C Programming.