Is it possible to modify a shared library (.so) in Linux without getting its source code???
I know about LD_PRELOAD, but is that useful for functions that are used IN the shared library itself???
Is there a way to decompile/disassemble and then recompile/reassemble binary ELF files?
Modifying applications is difficult to get right even with all the available documentation, code and support. Attempting to modify an application in binary form, (presumably) with no debug symbols, without documentation (judging by the fact you don't have the code) is therefore a much more arduous and risky undertaking.
Application reverse engineering is difficult, but can be done given enough resources, determination, tools and knowledge: all of this hinges on having a sufficiently valuable goal.
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I think a major design flaw in Linux is the shared object hell when it comes to distributing programs in binary instead of source code form.
Here is my specific problem: I want to publish a Linux program in ELF binary form that should run on as many distributions as possible so my mandatory dependencies are as low as it gets: The only libraries required under any circumstances are libpthread, libX11, librt and libm (and glibc of course). I'm linking dynamically against these libraries when I build my program using gcc.
Optionally, however, my program should also support ALSA (sound interface), the Xcursor, Xfixes, and Xxf86vm extensions as well as GTK. But these should only be used if they are available on the user's system, otherwise my program should still run but with limited functionality. For example, if GTK isn't there, my program will fall back to terminal mode. Because my program should still be able to run without ALSA, Xcursor, Xfixes, etc. I cannot link dynamically against these libraries because then the program won't start at all if one of the libraries isn't there.
So I need to manually check if the libraries are present and then open them one by one using dlopen() and import the necessary function symbols using dlsym(). This, however, leads to all kinds of problems:
1) Library naming conventions:
Shared objects often aren't simply called "libXcursor.so" but have some kind of version extension like "libXcursor.so.1" or even really funny things like "libXcursor.so.0.2000". These extensions seem to differ from system to system. So which one should I choose when calling dlopen()? Using a hardcoded name here seems like a very bad idea because the names differ from system to system. So the only workaround that comes to my mind is to scan the whole library path and look for filenames starting with a "libXcursor.so" prefix and then do some custom version matching. But how do I know that they are really compatible?
2) Library search paths: Where should I look for the *.so files after all? This is also different from system to system. There are some default paths like /usr/lib and /lib but *.so files could also be in lots of other paths. So I'd have to open /etc/ld.so.conf and parse this to find out all library search paths. That's not a trivial thing to do because /etc/ld.so.conf files can also use some kind of include directive which means that I have to parse even more .conf files, do some checks against possible infinite loops caused by circular include directives etc. Is there really no easier way to find out the search paths for *.so?
So, my actual question is this: Isn't there a more convenient, less hackish way of achieving what I want to do? Is it really so complicated to create a Linux program that has some optional dependencies like ALSA, GTK, libXcursor... but should also work without it! Is there some kind of standard for doing what I want to do? Or am I doomed to do it the hackish way?
Thanks for your comments/solutions!
I think a major design flaw in Linux is the shared object hell when it comes to distributing programs in binary instead of source code form.
This isn't a design flaw as far as creators of the system are concerned; it's an advantage -- it encourages you to distribute programs in source form. Oh, you wanted to sell your software? Sorry, that's not the use case Linux is optimized for.
Library naming conventions: Shared objects often aren't simply called "libXcursor.so" but have some kind of version extension like "libXcursor.so.1" or even really funny things like "libXcursor.so.0.2000".
Yes, this is called external library versioning. Read about it here. As should be clear from that description, if you compiled your binaries using headers on a system that would normally give you libXcursor.so.1 as a runtime reference, then the only shared library you are compatible with is libXcursor.so.1, and trying to dlopen libXcursor.so.0.2000 will lead to unpredictable crashes.
Any system that provides libXcursor.so but not libXcursor.so.1 is either a broken installation, or is also incompatible with your binaries.
Library search paths: Where should I look for the *.so files after all?
You shouldn't be trying to dlopen any of these libraries using their full path. Just call dlopen("libXcursor.so.1", RTLD_GLOBAL);, and the runtime loader will search for the library in system-appropriate locations.
I understand the benefits of dynamic linking (old code can take advantage of library upgrades automatically, it's more space efficient), but it definitely has downsides, especially in the heterogeneous Linux ecosystem. It makes it difficult to distribute a distribution-agnostic binary that "just works" and makes a previously working program more likely to break due to a system upgrade that breaks backwards compatibility or introduces regressions into a shared library.
Given these disadvantages, why does dynamic linking seem to be so universally preferred? Why is it so hard to find statically linked, distribution-agnostic Linux binaries, even for small applications?
There are three big reasons:
GNU libc doesn't support static linkage to itself, because it makes extensive internal use of dlopen. (This means that static linkage to anything else is less worthwhile, because you can't get a totally static binary without replacing the C library.)
Distributions don't support static linkage to anything else, because it increases the amount of work they have to do when a library has a security vulnerability.
Distributions have no interest whatsoever in distribution-agnostic binaries. They want to get the source and build it themselves.
You should also keep in mind that the Linux-not-Android software ecology is entirely source-based. If you are shipping binaries and you're not a distribution vendor you are Doing It Wrong.
There are several reasons we prefer dynamic linkage:
Licensing. This is a particular issue with the LGPL, though there are other licenses with similar strictures.
Basically, it's legal for me to send you a binary built against LGPL libfoo.so.*, and even to give you a binary for that library. I have a various responsibilities, such as responding to requests for the source for the LGPL'd library, but the important thing here is that I don't have to give you the source for my program, too. Since glibc is LGPL and almost every binary on a Linux box is linked to it, that alone will force dynamic linkage by default.
Bandwidth costs. People like to say bandwidth is free, but that's true only in principle. In many practical cases, bandwidth still matters.
My company's main C++-based system packs up into a ~4 MB RPM, which takes a few minutes to upload over the slow DSL uplinks at most of our customers' sites. We still have some customers only accessible via modem, too, and for those an upload is a matter of "start it, then go to lunch." If we were shipping static binaries, these packages would be much larger. Our system is composed of several cooperating programs, most of which are linked to the same set of dynamic libraries, so the RPM would contain redundant copies of the same shared code. Compression can squeeze some of that out, but why keep shipping it again and again for each upgrade?
Management. Many of the libraries we link against are part of the OS distro, so we get free updates to those libraries independent from our program. We don't have to manage it.
We do separately ship some libraries which aren't part of the OS, but they have to change much less often than our code does. Typically, these are installed on the system when we build the server, then never updated again. This is because we are most often more interested in stability than new features from these libraries. As long as they're working, we don't touch them.
Is there any way to protect a shared library (.so file) against reverse engineering ?
Is there any free tool to do that ?
The obvious first step is to strip the library of all symbols, except the ones required for the published API you provide.
The "standard" disassembly-prevention techniques (such as jumping into the middle of instruction, or encrypting some parts of code and decrypting them on-demand) apply to shared libraries.
Other techniques, e.g. detecting that you are running under debugger and failing, do not really apply (unless you want to drive your end-users completely insane).
Assuming you want your end-users to be able to debug the applications they are developing using your library, obfuscation is a mostly lost cause. Your efforts are really much better spent providing features and support.
Reverse engineering protection comes in many forms, here are just a few:
Detecting reversing environments, such as being run in a debugger or a virtual machine, and aborting. This prevents an analyst from figuring out what is going on. Usually used by malware. A common trick is to run undocumented instructions that behave differently in VMWare than on a real CPU.
Formatting the binary so that it is malformed, e.g. missing ELF sections. You're trying to prevent normal analysis tools from being able to open the file. On Linux, this means doing something that libbfd doesn't understand (but other libraries like capstone may still work).
Randomizing the binary's symbols and code blocks so that they don't look like what a compiler would produce. This makes decompiling (guessing at the original source code) more difficult. Some commercial programs (like games) are deployed with this kind of protection.
Polymorphic code that changes itself on the fly (e.g. decompresses into memory when loaded). The most sophisticated ones are designed for use by malware and are called packers (avoid these unless you want to get flagged by anti-malware tools). There are also academic ones like UPX http://upx.sourceforge.net/ (which provides a tool to undo the UPX'ing).
I seen several discussions here on the subject, but wanted to ask about my particular situation:
If I have some 3rd part libraries which my application is using, and I'd like to link them together in order to save myself the hassle in LD_LIBRARY, etc., is there any downside to it on Linux, other then larger file size?
Also, is it possible to statically link only some libraries, and other (standard Linux libraries) to link dynamically?
Thanks.
It is indeed possible to dynamically link against some libraries and statically link against others.
It sounds like what you really want to do is dynamically link against the system libraries, and statically link against the nonstandard ones that a user may not have installed (or that different users may have different installations of).
That's perfectly reasonable.
It's not generally a good idea to statically link against system libraries, especially libc.
It can often make sense to statically link against libraries that do not come with the OS and that will not be distributed with your application.
There are some bits of libc - those that use nsswitch - that need to load libraries dynamically. This can cause problems if you want to produce a completely static binary.
Statically linking your 3rd party libraries into your application should be completely fine.
The statically linked binary will be larger than if you had uses a shared library, but I find that disadvantage outweight the library path hassles, provided I control the distribution of all the libraries involved. If you are dependant on a particular distros shared libraries, then you have no choice but to use dynamic linking.
The main disadvantage I see is your application loses any automatic bugfixes that might be applied to a shared library. On the flip-side you don't get new bugs.
Static linking does not just affect the file size of the library, it also affects the memory footprint and start up time of the application. Dynamically linked libraries are loaded once no matter how many programs use them. Statically linked libraries must be loaded once per program that uses them (because they are now part of that program).
To answer your second question, yes, it is possible to have dynamic and static libraries linked to the same application. Just be careful to avoid interlibrary dependencies so you don't have a problem with library order. You should be able to list the libraries in any arbitrary order. Where I work, we prefer to list them alphabetically.
Edit: To link a static library, use the flag -lfoo. To add a directory to the library search path, use -L/path/to/libfoo.
Edit: You don't have to link a dynamic library. Your program can use a function provided by your compiler to open a dynamic library at run time, or you can link it at compile time and the compiler will resolve the symbols but not include them in the binary. See pjc50's comment below.
Statically linking will make your binary bulky, but you wont need to have a shared version of that library on the target runtime environment. This is especially the case while developing embedded apps.
How to create a shared object that is statically linked with pthreads and libstdc++ on Linux/gcc?
Before I go to answering your question as it was described, I will note that it is not exactly clear what you are trying to achieve in the end, and there is probably a better solution to your problem.
That said - there are two main problems with trying to do what you described:
One is, that you will need to decompose libpthread and libstdc++ to the object files they are made with. This is because ELF binaries (used on Linux) have two levels of "run time" library loading - even when an executable is statically linked, the loader has to load the statically linked libraries within the binary on execution, and map the right memory addresses. This is done before the shared linkage of libraries that are dynamically loaded (shared objects) and mapped to shared memory. Thus, a shared object cannot be statically linked with such libraries, as at the time the object is loaded, all static linked libraries were loaded already. This is one difference between linking with a static library and a plain object file - a static library is not merely glued like any object file into the executable, but still contains separate tables which are referred to on loading. (I believe that this is in contrast to the much simpler static libraries in MS-DOS and classic Windows, .LIB files, but there may be more to those than I remember).
Of course you do not actually have to decompose libpthread and libstdc++, you can just use the object files generated when building them. Collecting them may be a bit difficult though (look for the objects referred to by the final Makefile rule of those libraries). And you would have to use ld directly and not gcc/g++ to link, to avoid linking with the dynamic versions as well.
The second problem is consequential. If you do the above, you will sure have such a shared object / dynamic library as you asked to build. However, it will not be very useful, as once you try to link a regular executable that uses those libpthread/libstdc++ (the latter being any C++ program) with this shared object, it will fail with symbol conflicts - the symbols of the static libpthread/libstdc++ objects you linked your shared object against will clash with the symbols from the standard libpthread/libstdc++ used by that executable, no matter if it is dynamically or statically linked with the standard libraries.
You could of course then try to either hide all symbols in the static objects from libstdc++/libpthread used by your shared library, make them private in some way, or rename them automatically on linkage so that there will be no conflict. However, even if you get that to work, you will find some undesireable results in runtime, since both libstdc++/libpthread keep quite a bit of state in global variables and structures, which you would now have duplicate and each unaware of the other. This will lead to inconsistencies between these global data and the underlying operating system state such as file descriptors and memory bounds (and perhaps some values from the standard C library such as errno for libstdc++, and signal handlers and timers for libpthread.
To avoid over-broad interpretation, I will add a remark: at times there can be sensible grounds for wanting to statically link against even such basic libraries as libstdc++ and even libc, and even though it is becoming a bit more difficult with recent systems and versions of those libraries (due to a bit of coupling with the loader and special linker tricks used), it is definitely possible - I did it a few times, and know of other cases in which it is still done. However, in that case you need to link a whole executable statically. Static linkage with standard libraries combined with dynamic linkage with other objects is not normally feasible.
Edit: One issue which I forgot to mention but is important to take into account is C++ specific. C++ was unfortunately not designed to work well with the classic model of object linkage and loading (used on Unix and other systems). This makes shared libraries in C++ not really portable as they should be, because a lot of things such as type information and templates are not cleanly separated between objects (often being taken, together with a lot of actual library code at compile time from the headers). libstdc++ for that reason is tightly coupled with GCC, and code compiled with one version of g++ will in general only work with the libstdc++ from with this (or a very similar) version of g++. As you will surely notice if you ever try to build a program with GCC 4 with any non-trivial library on your system that was built with GCC 3, this is not just libstdc++. If your reason for wanting to do that is trying to ensure that your shared object is always linked with the specific versions of libstdc++ and libpthread that it was built against, this would not help because a program that uses a different/incompatible libstdc++ would also be built with an incompatible C++ compiler or version of g++, and would thus fail to link with your shared object anyway, aside from the actual libstdc++ conflicts.
If you wonder "why wasn't this done simpler?", a general rumination worth pondering: For C++ to work nicely with dynamic/shared libraries (meaning compatibility across compilers, and the ability to replace a dynamic library with another version with a compatible interface without rebuilding everything that uses it), not just compiler standartization is needed, but at the level of the operating system's loader, the structure and interface of object and library files and the work of the linker would need to be significantly extended beyond the relatively simple Unix classics used on common operating systems (Microsoft Windows, Mach based systems and NeXTStep relatives such as Mac OS, VMS relatives and some mainframe systems also included) for natively built code today. The linker and dynamic loader would need to be aware of such things as templates and typing, having to some extent functionality of a small compiler to actually adapt the library's code to the type given to it - and (personal subjective observation here) it seems that higher-level intermediate intermediate code (together with higher-level languages and just-in-time compilation) is catching ground faster and likely to be standardized sooner than such extensions to the native object formats and linkers.
You mentioned in a separate comment that you are trying to port a C++ library to an embedded device. (I am adding a new answer here instead of editing my original answer here because I think other StackOverflow users interested in this original question may still be interested in that answer in its context)
Obviously, depending on how stripped down your embedded system is (I have not much embedded Linux experience, so I am not sure what is most likely), you may of course be able to just install the shared libstdc++ on it and dynamically link everything as you would do otherwise.
If dynamically linking with libstdc++ would not be good for you or not work on your system (there are so many different levels of embedded systems that one cannot know), and you need to link against a static libstdc++, then as I said, your only real option is static linking the executable using the library with it and libstdc++. You mentioned porting a library to the embedded device, but if this is for the purpose of using it in some code you write or build on the device and you do not mind a static libstdc++, then linking everything statically (aside from perhaps libc) is probably OK.
If the size of libstdc++ is a problem, and you find that your library is actually only using a small part of its interfaces, then I would nonetheless suggest first trying to determine the actual space you would save by linking against only the parts you need. It may be significant or not, I never looked that deep into libstdc++ and I suspect that it has a lot of internal dependencies, so while you surely do not need some of the interfaces, you may or may not still depend on a big part of its internals - I do not know and did not try, but it may surprise you. You can get an idea by just linking a binary using the library against a static build of it and libstdc++ (not forgetting to strip the binary, of course), and comparing the size of the resulting executable that with the total size of a (stripped) executable dynamically linked together with the full (stripped) shared objects of the library and libstdc++.
If you find that the size difference is significant, but do not want to statically link everything, you try to reduce the size of libstdc++ by rebuilding it without some parts you know that you do not need (there are configure-time options for some parts of it, and you can also try to remove some independent objects at the final creation of libstdc++.so. There are some tools to optimize the size of libraries - search the web (I recall one from a company named MontaVista but do not see it on their web site now, there are some others too).
Other than the straightforward above, some ideas and suggestions to think of:
You mentioned that you use uClibc, which I never fiddled with myself (my experience with embedded programming is a lot more primitive, mostly involving assembly programming for the embedded processor and cross-compiling with minimal embedded libraries). I assume you checked this, and I know that uClibc is intended to be a lightweight but rather full standard C library, but do not forget that C++ code is hardly independent on the C library, and g++ and libstdc++ depend on quite some delicate things (I remember problems with libc on some proprietary Unix versions), so I would not just assume that g++ or the GNU libstdc++ actually works with uClibc without trying - I don't recall seeing it mentioned in the uClibc pages.
Also, if this is an embedded system, think of its performance, compute power, overall complexity, and timing/simplicity/solidity requirements. Take into consideration the complexity involved, and think whether using C++ and threads is appropriate in your embedded system, and if nothing else in the system uses those, whether it is worth introducing for that library. It may be, not knowing the library or system I cannot tell (again, embedded systems being such a wide range nowadays).
And in this case also, just a quick link I stumbled upon looking for uClibc -- if you are working on an embedded system, using uClibc, and want to use C++ code on it -- take a look at uClibc++. I do not know how much of the standard C++ stuff you need and it already supports, and it seems to be an ongoing project, so not clear if it is in a state good enough for you already, but assuming that your work is also under development still, it might be a good alternative to GCC's libstdc++ for your embedded work.
I think this guy explains quite well why that wouldn't make sense. C++ code that uses your shared object but a different libstdc++ would link alright, but wouldn't work.