Writing Custom Debugger Extensions in Software

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proceed with filling in the name and returning If we do not have enough room, we put the required length into the BufferSize parameter and return S_FALSE, which indicates that the debug engine should allocate a bigger buffer of the size specified and try calling again If we supported custom formatting of multiple data types, we would put each name into the buffer separated by NULL terminators and ending the entire string with another null terminator Now we move on to implementing the evaluator The debug engine in this case passes us a flag that contains the DEBUG_KNOWN_STRUCT_GET_SINGLE_LINE_ OUTPUT value and an address to the data structure The address can be passed to our helper function GetNodeValues defined earlier, which reads the node contents at the specified address and returns the node values After we have successfully read the node values, we can format the output and return to the debug engine Note that we take extra care not to exceed the buffer size by explicitly checking to make sure that the data will fit:
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else if(Flag==DEBUG_KNOWN_STRUCT_GET_SINGLE_LINE_OUTPUT) { ULONG ulData=0; ULONG ulLeft=0; ULONG ulRight=0; if(FAILED(hRes=GetNodeValues(Address, &ulData, &ulLeft, &ulRight))) { return hRes; } DWORD dwLen=_scprintf(SYSEXTS_KNOWNSTRUCT_OUT, ulData, ulLeft, ulRight)+1; if(dwLen>(*BufferSize)) { dprintf( KnownStructOutput unable to fit return data into buffer\n ); hRes=E_FAIL; return hRes; } hRes=StringCchPrintfA(Buffer, dwLen, SYSEXTS_KNOWNSTRUCT_OUT, ulData,
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ulLeft, ulRight); if(FAILED(hRes)) { dprintf ( KnownStructOutput unable to write data into buffer\n ); } }
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We have now implemented an augmentation to the standard dt command that allows us to see the root node contents in addition to the data output by the dt command Let s try it out Run bstreeexe under the debugger, set a breakpoint in the traverse method, and issue the dt command against the CBinaryTree root node instance:
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0:000> dt CBinaryTree::_TreeNode 0x000369c8 Binary tree node contents Data: 10 Left Child Pointer: 0x36fb0 Right Child Pointer: 0x36fd8 +0x000 data : 10 +0x004 pLeftChild : 0x00036fb0 Binary tree node contents Data: 5 Left Child Pointer: 0x0 Right Child Pointer: 0x37000 +0x008 pRightChild : 0x00036fd8 Binary tree node contents Data: 15 Left Child Pointer: 0x37028 Right Child Pointer: 0x0
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Implementing Command Cancellations
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As with any potentially long-running task, the capability to cancel a command comes into play Imagine that we tried to dump a CBinaryTtree instance with thousands and thousands of nodes and wanted to cancel the command partway through As it stands now, our dumptree command is not cancelable, which can cause frustration to users of this debugger extension How can you add cancellation support The answer is actually quite simple An API (as part of the WDbgExts model) allows debugger extensions to query for a CTRL+C by the user:
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ULONG CheckControlC (VOID);
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By calling this API at regular intervals in long-running commands, we can make sure to respect the user s wish to cancel In addition to dutifully respecting the CTRL+C requests by the user, this has the value add of allowing a user to cancel a dumptree command that would potentially never finish (with the exception of a blown stack because of the recursive nature of the implementation) If, for some reason, a bug were to surface in the CBinaryTree implementation that causes the tree to point back to itself, we could potentially recurse endlessly In our case, the dumptree command calls a recursive function that keeps printing nodes until the tree is fully exhausted If we put this check in prior to starting each recursive call, we can simply end the recursion and return to the user:
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VOID InOrderTraversal(CBinaryTree::_TreeNode* pNode,IDebugDataSpaces* pDataSpaces) { HRESULT hRes; if(CheckControlC()==TRUE) { dprintf( Control C hit, canceling command\n ); return; } // // Rest of the InOrderTraversal function code // }
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If we were to modify the client code of the CBinaryTree slightly to add 1,000 nodes to the tree instance and run it under the debugger, followed by a dumptree command, we could press CTRL+C to cancel the command:
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0:000> !sysextsdumptree 0x262D48 ** Node ** Data: 0 Left child pointer: 0x0 Right child pointer: 0x262d70 ** Node ** Data: 1 Left child pointer: 0x0 Right child pointer: 0x262d98 ** Node ** Data: 2 Left child pointer: 0x0
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