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The last two instructions in the current chunk perform another check on that same parameter, except that this time the code is using EBX, which as you might recall is the incremented version of EDI. Here EBX is compared against EDX, and the program jumps to ntdll.7C962559 if EBX is greater. Notice that the jump target address, ntdll.7C962559, is the same as the address of the previous conditional jump. This is a strong indication that the two jumps are part of what was a single compound conditional statement in the source code. They are just two conditions tested within a single conditional statement. Another interesting and informative hint you find here is the fact that the conditional jump instruction used is JA (jump if above), which uses the carry flag (CF). This indicates that EBX and EDX are both treated as unsigned values. If they were signed, the compiler would have used JG, which is the signed version of the instruction. For more information on signed and unsigned conditional codes refer to Appendix A. If you try to put the pieces together, you ll discover that this last condition actually reveals an interesting piece of information about the second parameter passed to this function. Recall that EDX was loaded from offset +14 in the structure, and that this is the member that stores the total number of elements in the table. This indicates that the second parameter passed to RtlGetElement GenericTable is an index into the table. These last two instructions simply confirm that it is a valid index by comparing it against the total number of elements. This also sheds some light on why the index was incremented. It was done in order to properly compare the two, because the index is probably zerobased, and the total element count is certainly not. Now that you understand these two conditions and know that they both originated in the same conditional statement, you can safely assume that the validation done on the index parameter was done in one line and that the source code was probably something like the following:
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ULONG AdjustedElementToGet = ElementToGet + 1; if (ElementToGet == 0xffffffff || AdjustedElementToGet > Table->TotalElements) return 0;
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How can you tell whether ElementToGet + 1 was calculated within the if statement or if it was calculated into a variable first You don t really know for sure, but when you look at all the references to EBX in Listing 5.2 you can see that the value ElementToGet + 1 is being used repeatedly throughout the function. This suggests that the value was calculated once into a local variable and that this variable was used in later references to the incremented value. The compiler has apparently assigned EBX to store this particular local variable rather than place it on the stack. On the other hand, it is also possible that the source code contained multiple copies of the statement ElementToGet + 1, and that the compiler simply
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optimized the code by automatically declaring a temporary variable to store the value instead of computing it each time it is needed. This is another case where you just don t know this information was lost during the compilation process. Let s proceed to the next code sequence:
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7C962501 7C962503 7C962505 7C962507 7C962509 7C96250B 7C96250D 7C96250F 7C962511 CMP ESI,EBX JE SHORT ntdll.7C962554 JBE SHORT ntdll.7C96252B MOV EDX,ESI SHR EDX,1 CMP EBX,EDX JBE SHORT ntdll.7C96251B SUB ESI,EBX JE SHORT ntdll.7C96254E
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This section starts out by comparing ESI (which was taken earlier from offset +10 at the table structure) against EBX. This exposes the fact that offset +10 also points to some kind of an index into the table (because it is compared against EBX, which you know is an index into the table), but you don t know exactly what that index is. If ESI == EBX, the code jumps to ntdll.7C962554, and if ESI <= EBX, it goes to ntdll.7C96252B. It is not clear at this point why the second jump uses JBE even though the operands couldn t be equal at this point or the first jump would have been taken. Let s first explore what happens in ntdll.7C962554:
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