GAS/GCC Assembler Code Modification

GAS/GCC Assembler Code Modification

[Martin]

Hi folks,

I have a question regarding GAS and GCC. 
Does the GAS modify the Assembler Code? If I try to assemble (for example) the following line:

popl %esi 

the gas changes that to

pop %esi 

Ok I guess this is ok, because the ESI Register is 'only' 32 Bit. So movw (= mov) would be ok. 

Next example:

If I try to assemble

movl $0xb,%eax 

the gas changes that to

mov $0xb,%eax

How can I tell gas that I dont want him to optimize the code?

Best regards,
Martin

PS: Sry for my poor english.

Re: GAS/GCC Assembler Code Modification

[sandeep]

Martin wrote:
> popl %esi 
> the gas changes that to
> pop %esi 
> Ok I guess this is ok, because the ESI Register is 'only' 32 Bit. So movw (= mov) would be ok. 
> 
> Next example:
> 
> If I try to assemble
> movl $0xb,%eax 
> the gas changes that to
> mov $0xb,%eax
There is no optimisation happening here. it's only that gas is using only mov as 
mnemonic, irrespective of whether you write movw or movl. I suggest you have a 
look at the generated instruction bytecodes.

while on linux, the default it assumes is 32 bit mode. so, though your movw also 
it will show as mov, but the generated instruction bytecode will also be 
prefixed with a mode byte, compared to had same movw been used in 16 bit mode.

> How can I tell gas that I dont want him to optimize the code?
Do you mean anything other than above as 'optimize the code' ?

I don't know if I cleared some things for you or increased the mess.

--
sandeep
--------------------------------------------------------------------------
I predict that today will be remembered until tomorrow!
--------------------------------------------------------------------------

Re: GAS/GCC Assembler Code Modification

[peter willy krause]

Am Samstag, 15. Mai 2004 10:28 schrieb Martin:
> Hi folks,
>
> I have a question regarding GAS and GCC.
> Does the GAS modify the Assembler Code? If I try to assemble (for
> example) the following line:
>
> popl %esi
>
> the gas changes that to
>
> pop %esi

though gas 'optimizes', which cannot be configured(!), in your example gas 
doesn't, re listing by gas, itself! (my version ex binutils 2.14.90) - 
apparently, your disassembler translates differently. compare the code...

the only 'optimization' by gas, which I'm aware of, is branches distance 
coding and selection of alignment fill code. branches coding can be 
forced to always 'long' (4 bytes) by declaring the destn label '.global'.

while assembling gas tries to deduce the oprands' formats by mnemonics and 
register nameing and, tries some correction if the proper size specs were 
missing, plus emanating a concerning warning (not an error!) message.


best,
	hp
-- 
mail to 'hp': lx at lxhp : in-berlin : de

Re: GAS/GCC Assembler Code Modification

[Martin]

peter willy krause <wklux@yahoo.co.uk> wrote:

> Am Samstag, 15. Mai 2004 10:28 schrieb Martin:
> > I have a question regarding GAS and GCC.
> > Does the GAS modify the Assembler Code? If I try to assemble (for
> > example) the following line:
> >
> > popl %esi
> >
> > the gas changes that to
> >
> > pop %esi
> 
> though gas 'optimizes', which cannot be configured(!), in your example gas 
> doesn't, re listing by gas, itself! (my version ex binutils 2.14.90) - 
> apparently, your disassembler translates differently. compare the code...
> 
> the only 'optimization' by gas, which I'm aware of, is branches distance 
> coding and selection of alignment fill code. branches coding can be 
> forced to always 'long' (4 bytes) by declaring the destn label '.global'.
Alright.
 
> while assembling gas tries to deduce the oprands' formats by mnemonics and 
> register nameing and, tries some correction if the proper size specs were 
> missing, plus emanating a concerning warning (not an error!) message.
Ok. I guess here is my error in reasoning. I just wondered because in AlephOnes
Paper about Stacksmashing he printed the following code:

[...]
        popl   %esi         
        movl   %esi,0x8(%esi)
        movb   $0x0,0x7(%esi)
        movl   $0x0,0xc(%esi)
        movl   $0xb,%eax
        movl   %esi,%ebx              
        leal   0x8(%esi),%ecx         
        leal   0xc(%esi),%edx         
        int    $0x80                
        movl   $0x1, %eax		
        movl   $0x0, %ebx
[...]

After assembling with gas and disassembling with gdb I got the following code:
[...]
        pop   %esi         
        mov   %esi,0x8(%esi)
        movb   $0x0,0x7(%esi)
        movl   $0x0,0xc(%esi)
        mov   $0xb,%eax      
        mov   %esi,%ebx      
        lea   0x8(%esi),%ecx 
        lea   0xc(%esi),%edx 
        int    $0x80         
        mov   $0x1, %eax	
        mov   $0x0, %ebx	
[...]

But if you are right, the code above should be the same. 

But I still have one question. Wouldn't be the code from AlephOne bigger (bytesize)
than the code I got?

Best regards,
thanks for your help.
Martin

Re: GAS/GCC Assembler Code Modification

[peter willy krause]

Am Samstag, 15. Mai 2004 21:08 schrieb Martin:
>
> But I still have one question. Wouldn't be the code from AlephOne
										^^^^^^^^^^ 
I've no idea of what that is. got a link?

best
	hp
-- 
mail to 'hp': lx at lxhp : in-berlin : de

Re: GAS/GCC Assembler Code Modification

[Martin]

peter willy krause <wklux@yahoo.co.uk> wrote:

> Am Samstag, 15. Mai 2004 21:08 schrieb Martin:
> >
> > But I still have one question. Wouldn't be the code from AlephOne
> 										^^^^^^^^^^ 
> I've no idea of what that is. got a link?

..my fault. sry.
http://www.phrack.org/show.php?p=49&a=14 
(etwa in der Mitte des Textes befindet sich der Code)