Decompile Vb6 Exe To Source Code
Cristoforo Kanoy
When you debug a .NET application, you might find that you want to view source code that you don't have. For example, breaking on an exception or using the call stack to navigate to a source location.
Starting in Visual Studio 2022 version 17.7, the Visual Studio Debugger supports autodecompilation of external .NET code. You can autodecompile when stepping into external code or when using the Call Stack window.
If you step into code that has been implemented externally, the debugger automatically decompiles it and displays the current point of execution. If you want to step into external code, Just My Code must be disabled.
To disable the automatic decompilation of external code, go to Tools > Options > Debugging > General and deselect Automatically decompile to source when needed (managed only).
In addition to generating source code for a specific location, you can generate all the source code for a given .NET assembly. To do this task, go to the Modules window and from the context menu of a .NET assembly, and then select the Decompile Source to Symbol File command. Visual Studio generates a symbol file for the assembly and then embeds the source into the symbol file. In a later step, you can extract the embedded source code.
The extracted source files are added to the solution as miscellaneous files. The miscellaneous files feature is off by default in Visual Studio. You can enable this feature from the Tools > Options > Environment > Documents > Show Miscellaneous files in Solution Explorer checkbox. If this feature isn't enabled, you can't open the extracted source code.
Generating source code using decompilation is only possible when the debugger is in break mode and the application is paused. For example, Visual Studio enters break mode when it hits a breakpoint or an exception. You can easily trigger Visual Studio to break the next time your code runs by using the Break All command ().
Generating source code from the intermediate format (IL) that is used in .NET assemblies has some inherent limitations. As such, the generated source code doesn't look like the original source code. Most of the differences are in places where the information in the original source code isn't needed at runtime. For example, information such as whitespace, comments, and the names of local variables aren't needed at runtime. We recommend that you use the generated source to understand how the program is executing and not as a replacement for the original source code.
A relatively small percentage of decompilation attempts can result in failure. This behavior is due to a sequence point null-reference error in ILSpy. We have mitigated the failure by catching these issues and gracefully failing the decompilation attempt.
The Just My Code (JMC) setting allows Visual Studio to step over system, framework, library, and other nonuser calls. During a debugging session, the Modules window shows which code modules the debugger is treating as My Code (user code).
A decompiler is a computer program that translates an executable file to high-level source code. It does therefore the opposite of a typical compiler, which translates a high-level language to a low-level language. While disassemblers translate an executable into assembly language, decompilers go a step further and translate the code into a higher level language such as C or Java, requiring more sophisticated techniques. Decompilers are usually unable to perfectly reconstruct the original source code, thus will frequently produce obfuscated code. Nonetheless, they remain an important tool in the reverse engineering of computer software.
The term decompiler is most commonly applied to a program which translates executable programs (the output from a compiler) into source code in a (relatively) high level language which, when compiled, will produce an executable whose behavior is the same as the original executable program. By comparison, a disassembler translates an executable program into assembly language (and an assembler could be used for assembling it back into an executable program).
Decompilation is the act of using a decompiler, although the term can also refer to the output of a decompiler. It can be used for the recovery of lost source code, and is also useful in some cases for computer security, interoperability and error correction.[1] The success of decompilation depends on the amount of information present in the code being decompiled and the sophistication of the analysis performed on it. The bytecode formats used by many virtual machines (such as the Java Virtual Machine or the .NET Framework Common Language Runtime) often include extensive metadata and high-level features that make decompilation quite feasible. The application of debug data, i.e. debug-symbols, may enable to reproduce the original names of variables and structures and even the line numbers. Machine language without such metadata or debug data is much harder to decompile.[2]
Some compilers and post-compilation tools produce obfuscated code (that is, they attempt to produce output that is very difficult to decompile, or that decompiles to confusing output). This is done to make it more difficult to reverse engineer the executable.
The success level achieved by decompilers can be impacted by various factors. These include the abstraction level of the source language, if the object code contains explicit class structure information, it aids the decompilation process. Descriptive information, especially with naming details, also accelerates the compiler's work. Moreover, less optimized code is quicker to decompile since optimization can cause greater deviation from the original code.[5]
The first decompilation phase loads and parses the input machine code or intermediate language program's binary file format. It should be able to discover basic facts about the input program, such as the architecture (Pentium, PowerPC, etc.) and the entry point. In many cases, it should be able to find the equivalent of the main function of a C program, which is the start of the user written code. This excludes the runtime initialization code, which should not be decompiled if possible. If available the symbol tables and debug data are also loaded. The front end may be able to identify the libraries used even if they are linked with the code, this will provide library interfaces. If it can determine the compiler or compilers used it may provide useful information in identifying code idioms.[6]
Idiomatic machine code sequences are sequences of code whose combined semantics are not immediately apparent from the instructions' individual semantics. Either as part of the disassembly phase, or as part of later analyses, these idiomatic sequences need to be translated into known equivalent IR. For example, the x86 assembly code:
Some idiomatic sequences are machine independent; some involve only one instruction. For example, xor eax, eax clears the eax register (sets it to zero). This can be implemented with a machine independent simplification rule, such as a = 0.
In general, it is best to delay detection of idiomatic sequences if possible, to later stages that are less affected by instruction ordering. For example, the instruction scheduling phase of a compiler may insert other instructions into an idiomatic sequence, or change the ordering of instructions in the sequence. A pattern matching process in the disassembly phase would probably not recognize the altered pattern. Later phases group instruction expressions into more complex expressions, and modify them into a canonical (standardized) form, making it more likely that even the altered idiom will match a higher level pattern later in the decompilation.
The places where register contents are defined and used must be traced using data flow analysis. The same analysis can be applied to locations that are used for temporaries and local data. A different name can then be formed for each such connected set of value definitions and uses. It is possible that the same local variable location was used for more than one variable in different parts of the original program. Even worse it is possible for the data flow analysis to identify a path whereby a value may flow between two such uses even though it would never actually happen or matter in reality. This may in bad cases lead to needing to define a location as a union of types. The decompiler may allow the user to explicitly break such unnatural dependencies which will lead to clearer code. This of course means a variable is potentially used without being initialized and so indicates a problem in the original program.[citation needed]
A good machine code decompiler will perform type analysis. Here, the way registers or memory locations are used result in constraints on the possible type of the location. For example, an and instruction implies that the operand is an integer; programs do not use such an operation on floating point values (except in special library code) or on pointers. An add instruction results in three constraints, since the operands may be both integer, or one integer and one pointer (with integer and pointer results respectively; the third constraint comes from the ordering of the two operands when the types are different).[7]
Various high level expressions can be recognized which trigger recognition of structures or arrays. However, it is difficult to distinguish many of the possibilities, because of the freedom that machine code or even some high level languages such as C allow with casts and pointer arithmetic.
The final phase is the generation of the high level code in the back end of the decompiler. Just as a compiler may have several back ends for generating machine code for different architectures, a decompiler may have several back ends for generating high level code in different high level languages.
ff7609af8f