Memory Corruption Detection Process in Software

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Memory Corruption Detection Process
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It should be quite clear that, when faced with a memory corruption, we want to be notified as soon as the offending thread writes to memory that it does not own rather than having to backtrack from a strange application behavior that might surface days after the invalid memory write took place Short of getting lucky that the pointer points to truly invalid memory (causing an access violation right away), most of the memory corruptions surface in the form of strange application behaviors or crashes after the memory has already been altered Fortunately, with the right strategy and a powerful tool set, we can maximize our efficiency when analyzing a potential memory corruption and force the strategy of crash immediately to make it easier to figure out the source of the memory corruption
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Memory Corruption Detection Process
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This section outlines the memory corruption detection process It includes a graphical representation of the process, as well as a brief discussion of each step It is important to understand that figuring out the root cause of a memory corruption might include several iterations of the process illustrated in Figure 51, depending on the nature of the memory corruption
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5 MEMORY CORRUPTION PART I STACKS
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State Analysis
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Source Code Analysis
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Use Memory Corruption Detection Tools
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Step 1: State Analysis
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The very first step in investigating a memory corruption is to assure yourself that the failure you are looking at is indeed because of a memory corruption This step can be further broken down, as seen in Figure 52
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Memory Corruption Part I Stacks
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Identify Memory and State
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Figure 52 As we mentioned earlier, memory corruption symptoms fall into two categories: crashes and noncrashing and unpredictable behavior This first step calls for an initial analysis of the behavior seen by means of analyzing the state of the corrupted memory How do we know which state to analyze With crashes, finding the starting point is pretty simple The code that crashed did so because of some unexpected state, and the code is well-known at crash time By looking at the state of the memory when the crash occurred in conjunction with focused code reviewing, we can make sound judgment calls on the origins of the state Valid, albeit buggy, code paths can lead to the state If that is the case, you are not experiencing a memory corruption, per se, but rather an unexpected code path that erroneously wrote to the memory If, however, no code paths allow for the memory to get into that state, the only plausible explanation is that someone overwrote that memory, and hence a memory corruption has occurred If you are not experiencing a crash, but instead are seeing periodic strange behaviors in the application, finding which memory had its state potentially corrupted is not as clear as with crashes Typically, when unexpected behavior occurs, you would break into the debugger and start with some initial analysis For example, if clients are experiencing error after error when trying to authorize credit cards, you might start by investigating the thread pool state (which services all credit card authorizations) and see why they are failing If you notice that the thread pool is not accepting requests due to being shut down, you would proceed to step 2 and the source code analysis to identify a valid code path or (if one does not exist) conclude that a memory corruption has occurred
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Step 2: Source Code Analysis
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After you have identified (in step 1) that you are faced with a possible memory corruption bug, the next step is to do some source code analysis to see if the root cause can be identified A memory corruption might occur when a thread writes to a memory location that it does not own A very important observation can be made from this statement The thread writes data to the memory block Presumably, the data being written is of interest to that particular thread, and, as such, if we could analyze the data and make sense out of it, we could further narrow down the scope of possible
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