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NULL Data Pointer is a special version of the more general Invalid Pointer pattern like NULL Code Pointer. The effective address is below 0xFFFF and it is usually a register with 0 value and the small offset, for example:

0: kd> r
Last set context:
eax=8923b008 ebx=00000000 ecx=00000000 edx=8923b008 esi=891312d0 edi=89f0b300
eip=8081c7c4 esp=f1b5d7a4 ebp=f1b5d7a4 iopl=0 nv up ei ng nz ac pe nc
cs=0008 ss=0010 ds=0023 es=0023 fs=0030 gs=0000 efl=00010296
nt!IoIsOperationSynchronous+0xe:
8081c7c4 f6412c02  test    byte ptr [ecx+2Ch],2   ds:0023:0000002c=??

Here, after disassembling the function backwards, we see the succession of dereferences starting from [EBP+8] and this means that a pointer to a structure (an IRP here) was passed to the function and it had a data pointer in it, pointing to another structure and the latter contained an NULL pointer:  

0: kd> ub 8081c7c4
nt!IoIsOperationSynchronous:
8081c7b6 8bff            mov     edi,edi
8081c7b8 55              push    ebp
8081c7b9 8bec            mov     ebp,esp
8081c7bb 8b4508          mov     eax,dword ptr [ebp+8]
8081c7be 8b4860          mov     ecx,dword ptr [eax+60h]
8081c7c1 8b4918          mov     ecx,dword ptr [ecx+18h]

Next post will feature a full case study.

Note: pointers are discussed in great detail in my latest book: Windows Debugging: Practical Foundations

- Dmitry Vostokov @ DumpAnalysis.org -

           

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This entry was posted on Tuesday, April 14th, 2009 at 6:21 pm and is filed under Crash Dump Analysis, Crash Dump Patterns, Debugging. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.