Absolute memory offset in 64 bit code (bug maybe?)

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revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20338
Location: In your JS exploiting you and your system

revolution 24 Apr 2006, 12:15

I am having some trouble with absolute memory offset in 64 bit code

Code:

        use64

        org     0

        mov     [0],eax         ;<-- Generates: mov [RIP-6],eax
        mov     [1 shl 32],eax  ;<-- Error: value out of range.

        org     1 shl 32
        mov     [0],eax         ;<-- Error: value out of range.
        mov     [1 shl 32],eax  ;<-- Error: value out of range.

I think the encodings below are valid ways to do the above

Code:


        db      0a3h,8 dup 0            ;mov [0],eax
        db      089h,4,25h,0,0,0        ;mov [0],eax
        db      0a3h,4 dup 0,1,0,0,0    ;mov [2^32],eax

24 Apr 2006, 12:15

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 24 Apr 2006, 12:42

revolution: shouldn't you use 1 shl 64 instead of 32? If i understand right what you wanted

24 Apr 2006, 12:42

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20338
Location: In your JS exploiting you and your system

revolution 24 Apr 2006, 13:12

Quote:

shouldn't you use 1 shl 64 instead of 32?

1 shl 64 is too big! 1 shl 32 is 4294967296 which is what I intended.

24 Apr 2006, 13:12

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20338
Location: In your JS exploiting you and your system

revolution 24 Apr 2006, 13:16

I just saw another thread concerning the same thing.

http://board.flatassembler.net/topic.php?t=5154

I will reply there.

24 Apr 2006, 13:16

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 24 Apr 2006, 13:27

sorry then

24 Apr 2006, 13:27

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8353
Location: Kraków, Poland

Tomasz Grysztar 24 Apr 2006, 14:21

fasm's manual, section 2.1.19 wrote:

The long mode uses also the instruction pointer based addresses, you can specify it manually with the special RIP register symbol, but such addressing is also automatically generated by flat assembler, since there is no 64-bit absolute addressing in long mode. You can still force the assembler to use the 32-bit absolute addressing by putting the dword size override for address inside the square brackets. There is also one exception, where the 64-bit absolute addressing is possible, it's the mov instruction with one of the operand being accumulator register, and second being the memory operand. To force the assembler to use the 64-bit absolute addressing there, use the qword size operator for address inside the square brackets. When no size operator is applied to address, assembler generates the optimal form automatically.

So the correct way is:

Code:

        use64

        org     0

        mov     [0],eax         ; Generates: mov [RIP-6],eax
        mov     [dword 0],eax   ; Generates: mov [0],eax
        mov     [dword 1 shl 32],eax

        org     1 shl 32
        mov     [dword 0],eax
        mov     [dword 1 shl 32],eax

24 Apr 2006, 14:21

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20338
Location: In your JS exploiting you and your system

revolution 24 Apr 2006, 15:10

Code:

mov [dword 1 shl 32],eax

I think this is a bug, 1 shl 32 cannot fit into a dword.

24 Apr 2006, 15:10

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8353
Location: Kraków, Poland

Tomasz Grysztar 24 Apr 2006, 15:58

what a mess! Right, I thought it was 31 here (and that you wanted absolute dword addressing, not absolute qword). There's some slip in error checking, I'll find it out.

24 Apr 2006, 15:58

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8353
Location: Kraków, Poland

Tomasz Grysztar 25 Apr 2006, 20:13

It's fixed in 1.65.20.

25 Apr 2006, 20:13

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20338
Location: In your JS exploiting you and your system

revolution 26 Apr 2006, 00:31

Yep, it it working great.

26 Apr 2006, 00:31

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