Memory Leaks in Software

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Memory Leaks
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0003a5d0: 0003a5e0: 0003a608: 0003a618: 0003a640: 0003a650: 0003a678: 0003a688: 0003a6b0: 0003a6c0: 0003a6e8: 0003a6f8: 0003a720: 0003a730: 0003a758: 0003a768: 0003a790: 0003a7a0: 0003a7c8: 0003a7d8: 0003a800: 0003a810: 0003a838: 0003a848:
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There appears to be tons and tons of blocks allocated of user sizes 8 and 1c As a matter of fact, sampling random blocks in the log file yields a fairly large number of these allocated blocks Considering that the execution is over and the application is about to terminate, chances are good that we have discovered a memory leak At this point, we are halfway there The next step is to find out what these blocks actually contain If we were leaking memory, it would be reasonable to expect data related to our application contained within those blocks The tricky and sometimes lengthy part is finding out what the blocks contain Let s look at the memory of one of these blocks
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0:001> dd 0003a7d0 0003a7e0 0003a7f0 0003a800 0003a810 0003a820 0003a830 0003a840
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0003a7c8+0x8 010012bc 0003a7a8 00000501 05000000 125f5219 2b3be507 00050002 0008011f 00020005 000c011d 00000015 42f831d9 000003ec 00000000 010012bc 0003a818
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9 RESOURCE LEAKS
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Resource Leaks
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Do the first two DWORDs seem to resemble anything From this point on, it is a matter of trying to recognize something in the data that can be applicable to your application Try to use a variety of the different dump command flavors to see if you can find anything that makes sense For example, using the da or du commands allows you to dump a particular pointer as a string If that doesn t work, you can try resolving the contents of the allocation For example, by using the ln command on the first DWORD, you get the following result
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0:001> ln 010012bc (010012bc) 09memleak!CSID::`vftable 09memleak!CIdentity::`vftable Exact matches: | (010012c8)
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Now that is too good to be a coincidence Our test application definitely works with classes of type CIdentity And as we already know, CSID is a class derived from CIdentity Because virtual function tables typically come first in the object layout, we can hypothesize
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Judging from the pattern of allocations in the !heap extension command output, chances are good one of these heap blocks is leaked Furthermore, by looking around at the heap block contents, we can see that it contains virtual function tables of objects that we are working with
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It can sometimes be a daunting task trying to recognize the contents of leaked heap blocks Fortunately, after looking at memory leaks for some time, you will learn to recognize certain categories of data by simply using the dd command
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Heap Searching
Before we come to the conclusion that this is in fact a leak (remember caching can cause objects to stay around even after they are done being used), we should verify the theory If these potentially leaked blocks were being used (perhaps cached), there would also need to be a reference somewhere in memory that points to that heap block If there are no references, it means that we definitely have a leak Once again, the !heap extension command provides us the means of finding this out Using the x and v switches, we can ask the !heap extension command to search the entire memory space of the process for the presence of a specified address In our example, searching for address 0003a7d0 (remember, block address + 0x8 gives the user mode allocation) yields the following: