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The bytes corresponding to the AddressOfEntryPoint are shown in bold and, at a high level, translate to the following:
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JMP 402000
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The big question here is what does 402000 symbolize The answer is that 0x402000 references another part of the PE image file, namely the import section, which lists all the modules on which the PE file is dependent At load time, the actual address of the dependent import is fixed and the correct calls can be made To find out which import 0x402000 refers to, we look at the import section of the PE file and find the following:
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mscoreedll 402000 40243C 0 0 Import Address Table Import Name Table time date stamp Index of first forwarder reference
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0 _CorExeMain
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As you can see, 0x402000 refers to the mscoreedll (Microsoft Object Runtime Execution Engine), which has one export named _CorExeMain The previous JMP instruction can then be translated into the following pseudo code:
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JMP _CorExeMain
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As we have seen, _CorExeMain is part of mscoreedll and is the first function called when loading a NET assembly The primary purpose of mscoreedll (and _CorExeMain) is to bootstrap the CLR For mscoreedll to bootstrap the CLR, a number of tasks have to be performed: 1 Find the CLR version that the NET assembly is built against by checking the metadata in the PE file 2 Find the path to the correct version of the CLR on the system 3 Load and initialize the CLR After the CLR has been initialized, the entry point of the assembly (such as
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Main()) is found in the CLR header portion of the PE image The entry point is JIT
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compiled and execution begins The CLR has so far been referred to as a logical component without mentioning which image actually serves up its functionality A large part of the CLR is implemented in mscorwksdll Furthermore, mscorwksdll can have multiple versions present on any given machine For example, if you have NET 11 and NET 20 installed, you will have the following CLR DLL versions installed on your machine:
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c:\Windows\MicrosoftNET\Framework\v114322\mscorwksdll c:\Windows\MicrosoftNET\Framework\v2050727\mscorwksdll
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The basic idea behind multiple installs is that NET applications target specific versions of the CLR An application written for the NET 10 CLR will correctly load the 10 CLR even in the presence of the NET 20 CLR This mechanism, in essence, supports the side-by-side execution model present in the NET world The job of mscoreedll is to find out which version of the CLR the assembly is targeting by looking at the CLR header in the PE image file More specifically, mscoreedll looks at the MajorRuntimeVersion and MinorRuntimeVersion of the header and loads the appropriate version of the CLR So far, we have illustrated how a NET assembly (in the form of an EXE) is launched Much in the same way that native code Windows applications support the notion of an executable as well as a dynamic link library, so does NET In the case of a NET library, as far as the loader is concerned, the only difference is that instead of
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Application Domains
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the PE image importing the _CorExeMain imported function, it imports the _CorDllMain function One interesting question surrounding the loading of mscoreedll is why is the unmanaged stub function that calls _CorExeMain required Because the PE image file contains a NET header, is it possible for the Windows loader to identify that the PE image is a NET assembly and automatically load mscoreedll The answer is yes In Windows XP and later, the Windows loader was updated to recognize a NET assembly PE image and automatically load the CLR To summarize the NET assembly loading algorithm, the following occurs: 1 The user executes a NET assembly 2 The Windows loader looks at the AddressOfEntryPoint field and references the text section of the PE image file 3 The bytes located at the AddressOfEntryPoint location are simply a JMP instruction to an imported function in mscoreedll 4 Control is transferred to the _CorExeMain function in mscoreedll to bootstrap the CLR and transfer execution to the assembly s entry point The PE file format is a very versatile format (as can be witnessed by the relative ease of supportingNET assemblies) and contains a ton of information related to the PE image being loaded and executed This section focused on how the PE file format was extended to enable native execution of NET assemblies Next, we will delve into other critical parts of the CLR starting with the concept of an application domain
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