flat assembler - test bit always returns true

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aq83326

Joined: 25 Jun 2011
Posts: 21

aq83326 03 May 2012, 18:51

Figured it out, I put "uses eax" when using stdcall, but as we (I) should know, "uses" get restored but eax is where the return value is located.

Code:

proc testBit stdcall uses eax ebx,input,position
     mov ebx, [position]
     mov eax, 0
     bt [input],ebx
     setc al
     ret
endp

Code:

proc testBit stdcall uses eax ebx,input,position
     mov ebx, [position]
     mov eax, [input]
     bt eax,ebx
     mov eax,0
     setc al
     ret
endp

Both of the above always return true.

03 May 2012, 18:51

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19251
Location: In your JS exploiting you and your system

revolution 03 May 2012, 22:43

aq83326 wrote:

Figured it out, I put "uses eax" when using stdcall, but as we (I) should know, "uses" get restored but eax is where the return value is located.
Code:
proc testBit stdcall uses eax ebx,input,position
     mov ebx, [position]
     mov eax, 0
     bt [input],ebx
     setc al
     ret
endp    
Code:
proc testBit stdcall uses eax ebx,input,position
     mov ebx, [position]
     mov eax, [input]
     bt eax,ebx
     mov eax,0
     setc al
     ret
endp    
Both of the above always return true.

Actually, both of the above always return eax untouched back to the caller. If the caller sets eax=0 then they both return false.

03 May 2012, 22:43

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