Debugging Managed Heap Corruptions in VS .NET

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Debugging Managed Heap Corruptions
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while rooted, buffer overruns are trapped and surfaced as an exception, and dangling pointers are not easily achieved Although the CLR very effectively eliminates a lot of the heap corruption culprits, it does so only when the code runs within the confines of the managed execution environment Often, it is necessary for a managed code application to call into native code and pass data to the native API The second that the code transitions into the native world, the data that reside on the managed heap and are passed to the native code are no longer under the protection of the CLR and can cause all sorts of problems unless carefully managed before making the transition For example, buffer overruns are no longer trapped and the compacting nature of the GC can cause pointers to become stale The managed to native code interaction is one of the biggest heap corruption culprits in the managed world
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CAN THERE BE MANAGED HEAP CORRUPTIONS WITHOUT NATIVE CODE INVOLVEMENT Although it is possible for a managed heap to become corrupted without any native code interactions, it is a very rare occurrence and usually indicates a bug in the CLR itself
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5 MANAGED HEAP AND GARBAGE COLLECTION
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In this part of the chapter, we will look at an example of an application that suffers from a heap corruption Listing 5-7 illustrates the application s source code Listing 5-7 Example of an application that suffers from a heap corruption
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using System; using SystemText; using SystemRuntimeInteropServices; namespace AdvancedNETDebugging5 { class Heap { static void Main(string[] args) { Heap h = new Heap(); hRun(); } public void Run() {
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(continues)
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5
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Managed Heap and Garbage Collection
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Listing 5-7 Example of an application that suffers from a heap corruption (continued)
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byte[] b = new byte[50]; for (int i = 0; i < 50; i++) b[i] = 15;
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ConsoleWriteLine("Press any key to invoke native method"); ConsoleReadKey(); InitBuffer(b, 50); ConsoleWriteLine("Press any key to exit"); ConsoleReadKey(); } [DllImport("05Nativedll")] static extern void InitBuffer(byte[] buffer, int size); } }
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The source code and binary for Listing 5-7 can be found in the following folders:
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Source code: C:\ADND\5\Heap Binary: C:\ADNDBin\05Heapexe and C:\ADNDBin\05Nativedll
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Note that to better illustrate the debug session, the native source code is not shown The application in Listing 5-6 allocates a byte array (50 elements) and calls into a native API to initialize the memory by passing in the byte array as well as the size of the array If we run the application under the debugger, we can very quickly see that an access violation occurs:
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Press any key to invoke native method ModLoad: 71190000 711ab000 C:\ADNDBin\05Nativedll ModLoad: 63f70000 64093000 C:\Windows\WinSxS\x86_microsoftvc90debugcrt _1fc8b3b9a1e18e3b_90210228_none_96748342450f6aa2\MSVCR90Ddll (1b0026e4): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling This exception may be expected and handled eax=77767574 ebx=00000001 ecx=01c659a4 edx=01c66ad8 esi=01c66868 edi=00000017 eip=7936ab16 esp=0031edac ebp=00000017 iopl=0 nv up ei pl nz na pe nc
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Debugging Managed Heap Corruptions
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cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010206 *** WARNING: Unable to verify checksum for C:\Windows\assembly\NativeImages_v2050727_32\ mscorlib\5b3e3b0551bcaa722c27dbb089c431e4\mscorlibnidll mscorlib_ni+0x2aab16: 7936ab16 ff90a4000000 call dword ptr [eax+0A4h] ds:0023:77767618= 0:000> !ClrStack OS Thread Id: 0x26e4 (0) ESP EIP 0031edac 7936ab16 SystemIOStreamWriterFlush(Boolean, Boolean) 0031edcc 7936b287 SystemIOStreamWriterWrite(Char[], Int32, Int32) 0031edec 7936b121 SystemIOTextWriterWriteLine(SystemString) 0031ee04 7936b036 SystemIOTextWriter+SyncTextWriterWriteLine(SystemString) 0031ee10 793e9d86 SystemConsoleWriteLine(SystemString) 0031ee1c 00810171 AdvancedNETDebugging5HeapRun() 0031ee48 008100a7 AdvancedNETDebugging5HeapMain(SystemString[]) 0031f068 79e7c74b [GCFrame: 0031f068]
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What is interesting about the access violation is the stack trace of the offending thread It looks like the access violation occurred while making our second call to the ConsoleWriteLine method (right after our call to the native InitBuffer API) Even if we assume that a heap corruption is taking place, why is it failing in some seemingly random place in the code base Again, it is important to remember that a heap corruption rarely breaks at the point of the corruption; rather, it breaks at some seemingly random place later in the execution flow This would certainly qualify as random because we certainly do not expect a call to ConsoleWriteLine to ever fail with an access violation Armed with the knowledge that an access violation has occurred and that the access violation occurred in a rather strange part of the execution flow, we can now theorize that we have a possible heap corruption on our hands The big question is, how do we verify our theory Remember our earlier definition of a heap corruption: a violation of the integrity of the heap If we can walk all objects on the heap, and verify the validity of each object, we can say for sure whether the integrity has been violated Although it s possible to walk the entire managed heap by hand, it is a time-consuming process to say the least Fortunately, the SOS VerifyHeap command automates this process for us The VerifyHeap command walks the entire managed heap, validating each object along the way, and reports the results of the validation If we run the command in our debug session, we can see the following:
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0:000> !VerifyHeap -verify will only produce output if there are errors in the heap object 01c65968: does not have valid MT curr_object : 01c65968 Last good object: 01c65928
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