Basic Debugger Tasks in Software

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Listing 233 Use of dt command
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0:000> * dt interprets this object type when displaying the memory block 0:000> dt this Local var @ 0x6fee4 Type KBTest* 0x77c146f0 +0x000 __VFN_table : +0x004 m_lastN : Memory read error 0x77c146f0 0:000> * dt uses the data type passed in when displaying the memory block 0:000> dt KBTest 0x0006fee4 02sample!KBTest +0x000 __VFN_table : +0x004 m_lastN : 0:000> * dt interpret the object type when displaying the memory block 0:000> dt 02sample!gGlobal gGlobal +0x000 m_ref : 1
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2 INTRODUCTION TO THE DEBUGGERS
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If you are arbitrarily inspecting a heap block, it is very possible to find in the first few positions a v-table symbol, indicating the type of C++ object located (or previously located) at that address You can then use the type information to display the object, as shown in the following listing captured at the same break as Listing 233
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0:000> dc @ecx l4 0006fee4 00401504 ffffffff 0006ff90 01002b28 (+ 0:000> ln 00401504 (00401504) 02sample!KBTest::`vftable | (00401508) 02sample!`string Exact matches: 0:000> dt KBTest @ecx 02sample!KBTest +0x000 __VFN_table : 0x00401504 +0x004 m_lastN : -1
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In Listing 232, the value displayed for the this pointer variable does not look right, as that value is usually reserved for system binary code segments By looking at the code, you can see that the object is allocated on the stack and should have a value close to the current stack pointer Let us examine the output from the dv /V this command:
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0006fee4 @ebp-0x08 this = 0x77c146f0
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2
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Introduction to the Debuggers
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The this pointer is stored at the stack location 0006fee4 and is accessed by the function code by using the frame-based register @ebp-0x08 The value stored at that address is, in fact, wrong How can that be The member function call follows the __thiscall convention, meaning that the ecx register contains the this pointer value The register value is later saved in the function stack frame at the location @ebp-0x08, meaning that the value becomes accurate after the function executes the following statement:
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00401878 894df8 mov dword ptr [ebp-8],ecx
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The question now becomes this: Why doesn t the compiler generate better symbols for tracking the local variable locations Try to imagine what will happen in code highly optimized with many variables: The registers are reused and the writes to the function stack frame are minimized, meaning that the compiler will have to generate a new symbol reference for each assembly instruction touching the variables This means that the symbol files will be larger This larger file must be moved around and loaded by debuggers at debug time, as well as examined much more often, resulting in poor user experience with minimal benefits Until a better solution is found to this problem, you must make sure that the variable value is correct before continuing the investigation You can then inspect it using the dt command, as in the next listing:
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0:000> dt kbTest @ecx 02sample!KBTest +0x000 __VFN_table : 0x00401504 +0x000 m_lastN : -1
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LOCAL VARIABLE VERSUS INPUT PARAMETERS Generally, most of the input parameters can be found on the stack and are addressed using the frame-based parameters with a positive offset, such as @ebp+8, whereas the local parameters are accessed using negative offsets, such as @ebp-8 At times, the compiler reuses the variable storage, which can cause difficulties when debugging
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How Do You Inspect Memory
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When investigating a problem in a debugger, we often have to examine different memory blocks to understand the reason behind the problem and to later prove that
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Basic Debugger Tasks
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the scenario is indeed valid Because the state of various objects persists in memory, the memory content is equivalent to the object s state The display command takes an address or a range of addresses and displays the content stored at those addresses according to the command arguments The most common form of display command simply reads formats and displays the data based on the types stored at the address The debugger does not attempt to guess what data is stored in that location because it will more than likely be wrong in most cases The user determines the format in which the data should be interpreted display has the following syntax:
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d[type] [AddressRange]
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To illustrate various forms of this command, we use the same 02sampleexe, but we start it with multiple command-line arguments Even if the arguments are ignored, they are still passed to the main function The function signature is the standard main declaration, as follows: