dot net qr code library IL and Verification in .NET

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IL and Verification
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IL is stack-based, which means that all of its instructions push operands onto an execution stack and pop results off the stack . Because IL offers no instructions to manipulate registers, it is easy for people to create new languages and compilers that produce code targeting the CLR . IL instructions are also typeless . For example, IL offers an add instruction that adds the last two operands pushed on the stack . There are no separate 32-bit and 64-bit versions of the add instruction . When the add instruction executes, it determines the types of the operands on the stack and performs the appropriate operation . In my opinion, the biggest benefit of IL isn t that it abstracts away the underlying CPU . The biggest benefit IL provides is application robustness and security . While compiling IL into native CPU instructions, the CLR performs a process called verification . Verification examines the high-level IL code and ensures that everything the code does is safe . For example, verification checks that every method is called with the correct number of parameters, that each parameter passed to every method is of the correct type, that every method s return value is used properly, that every method has a return statement, and so on . The managed module s metadata includes all of the method and type information used by the verification process . In Windows, each process has its own virtual address space . Separate address spaces are necessary because you can t trust an application s code . It is entirely possible (and unfortunately, all too common) that an application will read from or write to an invalid memory address . By placing each Windows process in a separate address space, you gain robustness and stability; one process can t adversely affect another process . By verifying the managed code, however, you know that the code doesn t improperly access memory and can t adversely affect another application s code . This means that you can run multiple managed applications in a single Windows virtual address space .
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Part I CLR Basics
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Because Windows processes require a lot of operating system resources, having many of them can hurt performance and limit available resources . Reducing the number of processes by running multiple applications in a single OS process can improve performance, require fewer resources, and be just as robust as if each application had its own process . This is another benefit of managed code as compared to unmanaged code . The CLR does, in fact, offer the ability to execute multiple managed applications in a single OS process . Each managed application executes in an AppDomain . By default, every managed EXE file will run in its own separate address space that has just the one AppDomain . However, a process hosting the CLR (such as Internet Information Services [IIS] or Microsoft SQL Server) can decide to run AppDomains in a single OS process . I ll devote part of 22, CLR Hosting and AppDomains, to a discussion of AppDomains .
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By default, Microsoft s C# compiler produces safe code . Safe code is code that is verifiably safe . However, Microsoft s C# compiler allows developers to write unsafe code . Unsafe code is allowed to work directly with memory addresses and can manipulate bytes at these addresses . This is a very powerful feature and is typically useful when interoperating with unmanaged code or when you want to improve the performance of a time-critical algorithm . However, using unsafe code introduces a significant risk: unsafe code can corrupt data structures and exploit or even open up security vulnerabilities . For this reason, the C# compiler requires that all methods that contain unsafe code be marked with the unsafe keyword . In addition, the C# compiler requires you to compile the source code by using the /unsafe compiler switch . When the JIT compiler attempts to compile an unsafe method, it checks to see if the assembly containing the method has been granted the System.Security.Permissions.Security Permission with the System.Security.Permissions.SecurityPermissionFlag s SkipVerification flag set . If this flag is set, the JIT compiler will compile the unsafe code and allow it to execute . The CLR is trusting this code and is hoping the direct address and byte manipulations do not cause any harm . If the flag is not set, the JIT compiler throws either a System.InvalidProgramException or a System.Security.VerificationException, preventing the method from executing . In fact, the whole application will probably terminate at this point, but at least no harm can be done . Note By default, assemblies that load from the local machine or via network shares are granted
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full trust, meaning that they can do anything, which includes executing unsafe code . However, by default, assemblies executed via the Internet are not granted the permission to execute unsafe code . If they contain unsafe code, one of the aforementioned exceptions is thrown . An administrator/end user can change these defaults; however, the administrator is taking full responsibility for the code s behavior .
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