Virtual Address Space

[Arshad Hussain]

On 30-Sep-2016, at 10:10 pm, Román Martínez <rgmf at riseup.net> wrote:

> Hi,
> 
> I compile main.s and it generates main.o. With objdump I can see:
> 
> 0000000000000000 <_start>:
>    0:	b8 01 00 00 00       	mov    $0x1,%eax
>    5:	bb 00 00 00 00       	mov    $0x0,%ebx
>    a:	cd 80                	        int    $0x80
> 
> After link main.o it generates main. With objdump I now can see:
> 
> 0000000000400078 <_start>:
>   400078:	b8 01 00 00 00       	mov    $0x1,%eax
>   40007d:	bb 00 00 00 00       	mov    $0x0,%ebx
>   400082:	cd 80                	        int    $0x80
> 
> So, linker generates virtual address, doesn't it? But why it starts at 400078 and not in other any location? Is there any logic here? A virtual address can start at 0?
> 
The linker script would normally leave out some area and _not_ start from 0. (although this is not
an absolute necessity ). It will pick up and valid virtual address and assign start of .text,.bss and 
.stack. This is possible because we now have virtual memory. And every program thinks that it 
has _all_ the memory for itself.  So, 400078 is perfectly valid virtual address space and linker
has chosen this for this executable.

Also note that if the address space is getting shifted by little , this is because of the KASLR
(layout randomisation) - If this is disabled then the linker would generate the same virtual 
address for all the executable every time.



<snip>