I have a C-based DLL that I wrote years ago for a project and it exports a set of functions that define an API. Now I need to re-write this DLL's internals but keep the API exactly the same.
The user of the DLL used static linking and they do not want to or are unable to recompile their executable.
I've noticed that the RVAs of the exported functions are different. My understanding is that means the executable won't be able to find the functions unless it is re-linked with the updated lib file.
Is there a way in VS2017 to force an exported function to use a specific RVA? I checked the Microsoft LINK DEF file format and I didn't see an option in there.
Even if it is possible, is fixing the RVAs enough to ensure the old executable will be able to use the updated DLL or are there additional complications that make this a non-starter?
Thanks.
When you statically link an EXE module against a DLL, you do indeed link against the the DLL's import library (a .LIB) created alongside the DLL when the DLL was built. This is not the same thing as linking against a static library which is confusing because those are also .LIB files.
The first thing you should do is figure out if your EXE module has an import entry for said DLL using a tool like Dependency Walker, Dumpbin, pelook or your favorite PE analyzer tool. If there is no DLL import entry, you have have probably linked the EXE against a static library as described by #HAL9000 's answer. Short of reverse-engineering the EXE, your best bet would be to rebuild the module as suggested if possible.
Otherwise, if you do find an import for said DLL, then yes you can swap out a newly-built DLL provided you have the same export (function) names and/or ordinal values as the original. DLLs find function by export names and/or ordinal values, not RVAs which in this case are only an internal detail. Whether the DLL is implicitly loaded (from being statically-linked) during process (EXE) initialization (before the EXE's entry point is called) or explicitly loaded (via code using LoadLibrary, etc.) the whole point of being a DLL is that it is a module is designed to be dynamically replaced and Windows was designed around this concept. The internal RVAs both within the EXE (referencing the DLL) and the DLL itself do not need to match an old DLL's values; this bookkeeping is automatically handled by the Windows loader during a process also known as runtime linking.
In the event the EXE is linked against said DLL and ALSO specifies hard-coded addresses (RVAs) for the DLL's exported functions (a process known as static binding), Windows will still verify the addresses still internally reflect the correct values in the DLL that is actually loaded which may be a different, updated DLL. This is done via a timestamp check in the import section for the DLL. If there is a mismatch, the Windows loader tosses-out all of the static RVAs and updates them with the current values incurring a slight performance penalty, but the program will still load. FWIW the bind.exe tool to do this static binding no longer ships with the Visual C++ toolset as the performance gain in modern versions of Windows is minimal. This optimization used to be be common practice to speed up load times, especially in OS-supplied system DLLs, but shouldn't affect what you are trying to do one way or the other.
If the user has statically linked in your library, then it is not a DLL, and making a drop in replacement without relinking is not possible. At least not without some ugly hacks. The old library functions have been copied into the executable, so there are no way around editing the executable. If you can't recompile or relink, then it is probably easier to rewrite the executable from scratch.
Mucking around with adresses of functions in your new DLL, if possible, can't have any effect if the executable doesn't have any code to load a DLL at all.
Related
How exactly do DLL files work? There seems to be an awful lot of them, but I don't know what they are or how they work.
So, what's the deal with them?
What is a DLL?
Dynamic Link Libraries (DLL)s are like EXEs but they are not directly executable. They are similar to .so files in Linux/Unix. That is to say, DLLs are MS's implementation of shared libraries.
DLLs are so much like an EXE that the file format itself is the same. Both EXE and DLLs are based on the Portable Executable (PE) file format. DLLs can also contain COM components and .NET libraries.
What does a DLL contain?
A DLL contains functions, classes, variables, UIs and resources (such as icons, images, files, ...) that an EXE, or other DLL uses.
Types of libraries:
On virtually all operating systems, there are 2 types of libraries. Static libraries and dynamic libraries. In windows the file extensions are as follows: Static libraries (.lib) and dynamic libraries (.dll). The main difference is that static libraries are linked to the executable at compile time; whereas dynamic linked libraries are not linked until run-time.
More on static and dynamic libraries:
You don't normally see static libraries though on your computer, because a static library is embedded directly inside of a module (EXE or DLL). A dynamic library is a stand-alone file.
A DLL can be changed at any time and is only loaded at runtime when an EXE explicitly loads the DLL. A static library cannot be changed once it is compiled within the EXE.
A DLL can be updated individually without updating the EXE itself.
Loading a DLL:
A program loads a DLL at startup, via the Win32 API LoadLibrary, or when it is a dependency of another DLL. A program uses the GetProcAddress to load a function or LoadResource to load a resource.
Further reading:
Please check MSDN or Wikipedia for further reading. Also the sources of this answer.
What is a DLL?
DLL files are binary files that can contain executable code and resources like images, etc. Unlike applications, these cannot be directly executed, but an application will load them as and when they are required (or all at once during startup).
Are they important?
Most applications will load the DLL files they require at startup. If any of these are not found the system will not be able to start the process at all.
DLL files might require other DLL files
In the same way that an application requires a DLL file, a DLL file might be dependent on other DLL files itself. If one of these DLL files in the chain of dependency is not found, the application will not load. This is debugged easily using any dependency walker tools, like Dependency Walker.
There are so many of them in the system folders
Most of the system functionality is exposed to a user program in the form of DLL files as they are a standard form of sharing code / resources. Each functionality is kept separately in different DLL files so that only the required DLL files will be loaded and thus reduce the memory constraints on the system.
Installed applications also use DLL files
DLL files also becomes a form of separating functionalities physically as explained above. Good applications also try to not load the DLL files until they are absolutely required, which reduces the memory requirements. This too causes applications to ship with a lot of DLL files.
DLL Hell
However, at times system upgrades often breaks other programs when there is a version mismatch between the shared DLL files and the program that requires them. System checkpoints and DLL cache, etc. have been the initiatives from M$ to solve this problem. The .NET platform might not face this issue at all.
How do we know what's inside a DLL file?
You have to use an external tool like DUMPBIN or Dependency Walker which will not only show what publicly visible functions (known as exports) are contained inside the DLL files and also what other DLL files it requires and which exports from those DLL files this DLL file is dependent upon.
How do we create / use them?
Refer the programming documentation from your vendor. For C++, refer to LoadLibrary in MSDN.
Let’s say you are making an executable that uses some functions found in a library.
If the library you are using is static, the linker will copy the object code for these functions directly from the library and insert them into the executable.
Now if this executable is run it has every thing it needs, so the executable loader just loads it into memory and runs it.
If the library is dynamic the linker will not insert object code but rather it will insert a stub which basically says this function is located in this DLL at this location.
Now if this executable is run, bits of the executable are missing (i.e the stubs) so the loader goes through the executable fixing up the missing stubs. Only after all the stubs have been resolved will the executable be allowed to run.
To see this in action delete or rename the DLL and watch how the loader will report a missing DLL error when you try to run the executable.
Hence the name Dynamic Link Library, parts of the linking process is being done dynamically at run time by the executable loader.
One a final note, if you don't link to the DLL then no stubs will be inserted by the linker, but Windows still provides the GetProcAddress API that allows you to load an execute the DLL function entry point long after the executable has started.
DLLs (dynamic link libraries) and SLs (shared libraries, equivalent under UNIX) are just libraries of executable code which can be dynamically linked into an executable at load time.
Static libraries are inserted into an executable at compile time and are fixed from that point. They increase the size of the executable and cannot be shared.
Dynamic libraries have the following advantages:
1/ They are loaded at run time rather than compile time so they can be updated independently of the executable (all those fancy windows and dialog boxes you see in Windows come from DLLs so the look-and-feel of your application can change without you having to rewrite it).
2/ Because they're independent, the code can be shared across multiple executables - this saves memory since, if you're running 100 apps with a single DLL, there may only be one copy of the DLL in memory.
Their main disadvantage is advantage #1 - having DLLs change independent your application may cause your application to stop working or start behaving in a bizarre manner. DLL versioning tend not to be managed very well under Windows and this leads to the quaintly-named "DLL Hell".
DLL files contain an Export Table which is a list of symbols which can be looked up by the calling program. The symbols are typically functions with the C calling convention (__stcall). The export table also contains the address of the function.
With this information, the calling program can then call the functions within the DLL even though it did not have access to the DLL at compile time.
Introducing Dynamic Link Libraries has some more information.
http://support.microsoft.com/kb/815065
A DLL is a library that contains code
and data that can be used by more than
one program at the same time. For
example, in Windows operating systems,
the Comdlg32 DLL performs common
dialog box related functions.
Therefore, each program can use the
functionality that is contained in
this DLL to implement an Open dialog
box. This helps promote code reuse and
efficient memory usage.
By using a DLL, a program can be
modularized into separate components.
For example, an accounting program may
be sold by module. Each module can be
loaded into the main program at run
time if that module is installed.
Because the modules are separate, the
load time of the program is faster,
and a module is only loaded when that
functionality is requested.
Additionally, updates are easier to
apply to each module without affecting
other parts of the program. For
example, you may have a payroll
program, and the tax rates change each
year. When these changes are isolated
to a DLL, you can apply an update
without needing to build or install
the whole program again.
http://en.wikipedia.org/wiki/Dynamic-link_library
DLL is a File Extension & Known As “dynamic link library” file format used for holding multiple codes and procedures for Windows programs. Software & Games runs on the bases of DLL Files; DLL files was created so that multiple applications could use their information at the same time.
IF you want to get more information about DLL Files or facing any error read the following post.
https://www.bouncegeek.com/fix-dll-errors-windows-586985/
DLLs (Dynamic Link Libraries) contain resources used by one or more applications or services. They can contain classes, icons, strings, objects, interfaces, and pretty much anything a developer would need to store except a UI.
According to Microsoft
(DLL) Dynamic link libraries are files that contain data, code, or resources needed for the running of applications. These are files that are created by the windows ecosystem and can be shared between two or more applications.
When a program or software runs on Windows, much of how the application works depends on the DLL files of the program. For instance, if a particular application had several modules, then how each module interacts with each other is determined by the Windows DLL files.
If you want detailed explanation, check these useful resources
What are dll files , About Dll files
I have an executable (that I created using Visual C++ 10), and I need to use its capabilities from another program I wrote (same environment). Due to complex deployment requirements which I won't go into, building a DLL from the required functionality and loading it in both programs is not something I can do.
So I thought that I can __declspec(dllexport) some functions in the EXE, and then LoadLibrary() will let me GetProcAddress() them.
Obviously this can't be done, though when I started looking at it - it looked feasible.
Specifically, when you __declspec(dllexport) functions in an EXE project, Visual C++ also generates a lib file for dynamic linking - so you don't even need to use LoadLibrary() - just link against the resulting lib and call the functions.
Unfortunately, the main problem is that when you declare the resulting file as an EXE, Visual C++ adds the "CRTmain" entry point into the resulting file, instead of the "CRTDLLmain" that a DLL gets. When Windows (automatically) LoadLibrary() the EXE from your main program, it doesn't call the the "CRTDLLmain" entry point (because it doesn't exist), the C runtime for the module doesn't get initialized, and as a result all interesting work (such as memory allocation) fails with interesting(*) runtime exceptions.
So as follows, my question is: is there a way to cause Visual C++ to build into the resulting file both the "CRTmain" entry point and the "CRTDLLmain" entry point?
(*) "Interesting" as in an old Chinese curse.
Yes it is possible.
http://www.codeproject.com/Articles/1045674/Load-EXE-as-DLL-Mission-Possible
The idea is a) to patch the IAT and b) to call the CRT before calling your exports.
Simply no!
The Problem is that the CRT and in the EXE you want load, uses some globle variables. You main EXE does the same. So how should Memory allocation work?
If you want to use such a structure you must use a DLL to be Aware of meulti threading, CRT initialization ans all this other stuff. You need this!
But what about COM Automation? Wouldn't tis be an easy solution to use your code in one EXE from another?
The short answer, is "no". After looking far and wide, there is no way to get VC++ to do what I want, and quite likely not any other compiler.
The main issue is that the main() entry point most people know and love is not the real entry point in C++ executables: the compiler needs to do a lot of initialization work to get the "C++ Run Time library" to a usable state, as well as initialize globals, statics and the likes. This initialization uses different code in shared libraries than in executables and there is no way to one to behave like another.
One thing that possibly can be done, is to build the shared functionality into a DLL, and for the primary executable to embed the DLL as a resource, and load it from a memory mapped region of the executable file (there are several code samples how to do this with VC++ on stackoverflow and elsewhere on the web). Now another program can do the same thing by loading the DLL from the bundling executable.
I have learned that you can:
Convert a .DLL file into a .DEF file, which includes its exports
(Edit: This doesn't work with many conventions)
Convert a .DEF file into a .LIB file, which you can use to link to the DLL
Why can't (most) linkers link to a DLL given only a .DEF file, instead of a .LIB file?
Ultimately, the answer here is 'because noone wanted it badly enough and it doesn't really help anything'.
The DEF file is an input file that creates an import lib for the DLL. And then, later, when the DLL is being consumed by another link, the importlib is itself an input. The importlib looks like something special on the outside, but when you look at the inside it's really just a slightly special lib with objects in it.
It totally would be possible to modify the linker to take a def file (or a DLL, for that matter) directly.
But the design centre of the linker is that it takes objects as inputs and outputs a PE executable. So taking a DEF or DLL as an input goes outside the design pattern.
Beyond that it'd be rather pointless - allowing the linker to take a DEF file or DLL as input would neither enable any important new scenarios, nor does leaving this feature out block anything. Converting a DEF file you have (even without the actual DLL) into a usable importlib is the work of a few moments (simply create fake empty function for each DEF entry and link that). So there's no reason to add the ability to link a DEF file directly.
Martyn
In terms of MSVC, .lib files are always static libraries. They get linked in as a compilation unit along with all your compiled .c/.cpp files, so all of the library's code is included in your final executable.
Some .lib files, however, (in particular most of the Windows system ones) merely contain stubs which tell the OS to load the desired DLL at loadtime and then the stubs route function calls to the DLL. But, those stubs are statically linked into your executable. Your program will then use DLLs (and gain all the advantages and disadvantages thereof), but since the named DLL functions it requires are happily located in the .lib (and thus actually located in the executable itself), your code doesn't have to know it's using a DLL (specifically using declspec(dllimport)).
A .def file is merely used as a sort of "settings" or "configuration" file during the creation of a .dll to specify what functions the file should export. It cannot be linked to, as it doesn't really describe anything that the linker understands.
You do not convert a dll to a DEF file. The DEF just indicates which dll functions will be accessible from the outside, exported.
From the docs:
A DLL file has a layout very similar to an .exe file, with one
important difference — a DLL file contains an exports table. The
exports table contains the name of every function that the DLL exports
to other executables. These functions are the entry points into the
DLL; only the functions in the exports table can be accessed by other
executables. Any other functions in the DLL are private to the DLL.
The exports table of a DLL can be viewed by using the DUMPBIN tool
with the /EXPORTS option.
You can export functions from a DLL using two methods:
Create a module definition (.def) file and use the .def file when
building the DLL. Use this approach if you want to export functions
from your DLL by ordinal rather than by name.
Use the keyword __declspec(dllexport) in the function's definition.
When exporting functions with either method, make sure to use the
__stdcall calling convention.
Use the provided link to learn more about exporting from your dll's.
I think you got down voted because your point is not really clear, at least not to me. Also check this. It explains how to choose the export method.
Mehrdad, this isn't always a question of how to LINK to a DLL, as I, personally have NEVER linked a DLL using a .DEF file. What I HAVE done is take someone else's DLL, and very painstakingly constructed a header file, or rather, function prototypes that I could use with
LoadLibrary() in C, Declare Function ... Lib "Foo.dll" Alias "OrdinalName" in VB, and
[DllImport()] in C#.
Of course, this is RARELY done, as if you are using a DLL for something, normally you have permission to do so, and the authors provide the .lib's, and the headers to go with the binary DLL file.
I've never done the exact techniques you speak of, by converting a .DEF info a .LIB, etc... But, I suppose it would be easy to take a lib, or the DLL itself and export .DEF from it. Now, THAT I actually HAVE done, in a project where the DLL code was built with a vbScript that took code from the main project, and created an API out of all the existing, compiled, and tested code. This level of complication was only done because I had no idea what functions were going to BE in the DLL, as the main project could change at any time, so a static .DEF file would have never worked. So, I had to build the DLL once, capture the dimpbin /exports, undecorate the functions, and then build the .DEF file, and re-link the DLL.
If you find yourself in that type of situation, perhaps you need to re-think your original designs, and fix the problem from there...
As for .LIB files, USUALLY you'd only NEED those for static linkage, but they are also used when the .H file is available, often making debugging a just a little nicer...
I have a heavily obfuscated native win32 dll with unknown functionality and it does not seem to call any function from other dlls. My question is that theoretically is this code able to modify its code to do any harm. Sub questions:
- is the code part of a dynamically loaded dll read only by default or is it writable?
- if a code does not use any other dll it is still able to
do something that requires some code from a system dll?
How could it load another dll and find a function if the function for loading dlls is not loaded by default?
Thanks in advance.
Is this code able to modify its code to do any harm.
Yes. And indeed it may do harm without even needing to modify itself.
Is the code part of a dynamically loaded dll read only by default or is it writable?
By default, DLLs are capable of modifying themselves. They need to call VirtualProtect in order to do so, but that's perfectly possible.
If a code does not use any other dll it is still able to do something that requires some code from a system dll?
The DLL can just load any system DLL and call whatever functions it chooses.
How could it load another dll and find a function if the function for loading dlls is not loaded by default?
By calling LoadLibrary and GetProcAddress. If the DLL really imports nothing at all then it's tricky to get hold of GetProcAddress. But not impossible. If Windows manages to do it, then surely the DLL can as well. One thing it could do is read the in-memory contents of the kernel32 DLL which is loaded into every process. It could parse the PE export table and use that to find the address of GetProcAddress.
I have a dll which I'd like to use in a c program,
Do you think is efficient to have a dll (lots of common functions) and then create a program that will eventually use them, or have all the source code?
To include the dll, What syntax must be followed?
Do you think is efficient to have a dll (lots of common functions) and then create a program that will eventually use them,or have all the source code.
For memory and disk space, it is more efficient to use a shared library (a DLL is the Windows implementation of shared libraries), assuming that at least two programs use this component. If only one program will ever use this component, then there is no memory or disk space savings to be had.
Shared libraries can be slightly slower than statically linking the code; however, this is likely to be incredibly minor, and shared libraries carry a number of benefits that make it more than worthwhile (such as the ability to load and handle symbols dynamically, which allows for plugin-like architectures). That said, there are also some disadvantages (if you are not careful about where your DLLs live, how they are versioned, and who can update them, then you can get into DLL hell).
To include the dll, What syntax must be followed?
This depends. There are two ways that shared libraries can be used. In the first way, you tell the linker to reference the shared library, and the shared library will automatically be loaded on program startup, and you would basically reference the code like normal (include the various headers and just use the name of the symbol when you want to reference it). The second way is to dynamically load the shared library (on Windows this is done via LoadLibrary while it is done on UNIX with dlopen). This second way makes it possible to change the behavior of the program based on the presence or absence of symbols in the shared library and to inspect the available set of symbols. For the second way, you would use GetProcAddress (Windows) or dlsym (UNIX) to obtain a pointer to a function defined in the library, and you would pass around function pointers to reference the functions that were loaded.
You can put your functions into either a static library ( a .lib) which is merged into your application at compile time and is basically the same as putting the .c files in the project.
Or you can use a dll where the functions are included at run time. the advantage of a dll is that two programs which use the same functions can use the same dll (saving disk space) and you can upgrade the dll without changing the program - neither of these probably matters for you.
The dll is automatically loaded when your program runs there is nothing special you need to do to include it ( you can load a dll specifically in your code - there are sometimes special reasons to do this)
Edit - if you need to create a stub lib for an existing dll see http://support.microsoft.com/kb/131313