cmp rax, 0xffffffff

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RIxRIpt

Joined: 18 Apr 2013
Posts: 50

RIxRIpt 10 Jun 2015, 20:40

Code:

flat assembler  version 1.71.39  (1048576 kilobytes memory)
myfilename.asm [321]:
                                cmp rax, 0xffffffff
error: value out of range.

Why?
It's possible to assemble this in x64 dbg and the hexcode is:

Code:

48 3D FF FF FF FF        | cmp rax,FFFFFFFF

Bug?

Update: stepping the instruction above in the debugger results in ZF=0 when RAX=00000000FFFFFFFF.
What's wrong with it?

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10 Jun 2015, 20:40

l_inc

Joined: 23 Oct 2009
Posts: 881

l_inc 10 Jun 2015, 23:39

RIxRIpt

Quote:

It's possible to assemble this in x64 dbg and the hexcode is

x64dbg assembles this incorrectly. The immediate is sign-extended and the comparison is therefore made with 0xffffffffffffffff, not with 0xffffffff.

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10 Jun 2015, 23:39

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 11 Jun 2015, 00:56

Values between -2^31 (0xffffffff80000000, stored as 0x80000000) and +2^31-1 (0x000000007fffffff, stored as 0x7fffffff) are encodable. Your number is 2^32-1 (0x00000000ffffffff) and cannot be encoded, because, as l_inc says, sign extending a stored value of 0xffffffff gives an absolute value 2^64-1 (0xffffffffffffffff).

11 Jun 2015, 00:56

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 11 Jun 2015, 08:44

When you use explicitly declaration you will get a more detailed error message. Wink

Code:

cmp rax, qword 0xffffffff

throws

"Error: not encodable with long immediate"

instead of just

"Error: value out of range"

It is possible to use

Code:

mov rbx,0xffffffff
cmp rax,rbx

instead.

11 Jun 2015, 08:44

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 11 Jun 2015, 08:48

shutdownall wrote:

When you use explicitly declaration you will get a more detailed error message.
Code:
cmp rax, qword 0xffffffff    
throws

"Error: not encodable with long immediate"

instead of just

"Error: value out of range".

Well actually that error is for all values:

Code:

cmp rax,qword 0
error: not encodable with long immediate.

11 Jun 2015, 08:48

shutdownall

Joined: 02 Apr 2010
Posts: 517
Location: Munich

shutdownall 11 Jun 2015, 08:50

revolution wrote:

Well actually that error is for all values:
Code:
cmp rax,qword 0
error: not encodable with long immediate.    

Yes, but if you omit the manual type conversion it is just "value out of range" for all values.
I think the "not encodable" message is more clear. Cool

11 Jun 2015, 08:50

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 11 Jun 2015, 08:53

shutdownall wrote:

Yes, but if you omit the manual type conversion it is just "value out of range" for all values.

Not quite. It is only "value out of range" for anything outside -2^31 to +2^31-1.

11 Jun 2015, 08:53

RIxRIpt

Joined: 18 Apr 2013
Posts: 50

RIxRIpt 11 Jun 2015, 10:24

Thanks for the answers, I think I got it.
That's what I've found:

Code:

;page 622 of Intel's manual
Opcode           Instruction       Op/  64-Bit Compat/   Description
                                   En   Mode   Leg Mode
REX.W + 3D id    CMP RAX, imm32    I    Valid    N.E.    Compare imm32 sign-extended to 64-bits with RAX.

and there's no cmp instruction with imm64 operand Confused

11 Jun 2015, 10:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 11 Jun 2015, 10:42

RIxRIpt wrote:

and there's no cmp instruction with imm64 operand .

The only imm64 available is MOV reg,imm64.

11 Jun 2015, 10:42

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