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JIT prevents short overflow (and PeVerify doesn't catch it)
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using System;
namespace Owasp
{
class byteStackSize
{
public static void Main()
{
int iInt = 0xffff;
short sShort = 0xff;
byte bByte = 0x12;
short sShort2 = 0xBB;
sShort++;
Console.WriteLine(sShort.ToString());
sShort = 0xAAA;
Console.WriteLine("Values: " + iInt.ToString() + " - " + sShort.ToString() + " - " + bByte + " - " + sShort2);
}
}
}
I compiled and edited the IL so that I assigned 0xAAAAA to sShort:
before:
IL_0026: ldc.i4 0xaaa
IL_002b: stloc.1
after
IL_0026: ldc.i4 0xaaaaa
IL_002b: stloc.1
Reflector will confirm that the change was successfully ILASMed:
public static void Main()
{
int num1 = 0xffff;
short num2 = 0xff;
byte num3 = 0x12;
short num4 = 0xbb;
num2 = (short) (num2 + 1);
Console.WriteLine(num2.ToString());
num2 = 0xaaaaa;
object[] objArray1 = new object[] { "Values: ", num1.ToString(), " - ", num2.ToString(), " - ", num3, " - ", num4 } ;
Console.WriteLine(string.Concat(objArray1));
}
but if we look at the actual Assembly code that is created by the JIT we will see that it is clever and doesn't overflow the sShort with my value (note: a short's maximum value is 0xffff):
; end of prologue ; IL_0000: ldc.i4 0x0000FFFF ; IL_0005: stloc.0 0x6D5249A: C745FCFFFF0000 MOV DWORD PTR [EBP-0x4],0xFFFF ; VAR:0x4 ; IL_0006: ldc.i4 0x000000FF ; IL_000B: stloc.1 ; IL_000C: ldc.i4.s 0x12 ; IL_000E: stloc.2 ; IL_000F: ldc.i4 0x000000BB ; IL_0014: stloc.3 0x6D524A1: C745F8FF000000 MOV DWORD PTR [EBP-0x8],0xFF ; VAR:0x8 ; IL_0015: ldloc.1 ; IL_0016: ldc.i4.1 ; IL_0017: add ; IL_0018: conv.i2 ; IL_0019: stloc.1 0x6D524A8: 0FBF45F8 MOVSX EAX,WORD PTR [EBP-0x8] ; VAR:0x8 0x6D524AC: 40 INC EAX 0x6D524AD: 668945F8 MOV WORD PTR [EBP-0x8],AX ; VAR:0x8 ; IL_001A: ldloca.s 0x01 ; IL_001C: call System.Int16::ToString() ; IL_0021: call System.Console::WriteLine() 0x6D524B1: 6A00 PUSH 0x0 0x6D524B3: 8D4DF8 LEA ECX,DWORD PTR [EBP-0x8] ; VAR:0x8 0x6D524B6: 33D2 XOR EDX,EDX 0x6D524B8: FF15C42CD606 CALL DWORD PTR [0x6D62CC4] 0x6D524BE: 8B0D2C20B705 MOV ECX,DWORD PTR [0x5B7202C] 0x6D524C4: 8BD0 MOV EDX,EAX 0x6D524C6: 8B01 MOV EAX,DWORD PTR [ECX] 0x6D524C8: FF90D8000000 CALL DWORD PTR [EAX+0xD8] ; IL_0026: ldc.i4 0x000AAAAA ; IL_002B: stloc.1 0x6D524CE: C745F8AAAAFFFF MOV DWORD PTR [EBP-0x8],0xFFFFAAAA; VAR:0x8
What is very interesting is that PeVerify doesn't catch this, which I would expected it to do.
So here is a good example of the CLR and the JIT doing the right thing and enforcing type safety.
Note that in the real stack (i.e. the unmanaged one) the types byte, short and int are 32 bits long, where in the managed stack they are 1 element:
âÂÂ...It is important to realize that the stack of the execution engine is a logical stack. The slements of the stack are not any particular size, such as 32 bits, or 64 bits (On the JVM, stack elements are 32 bits, and pushing a long makes the stack deeper by 2!). Thus each element of the stack may be one of the primitive types, or a reference,or even the value of a whole stuctâÂÂ... page 26 of Compiling for the .Net Common Language Runtime (CLR)"
