Fwd: writing a jump table

[Hendrik Visage <hvjunk@xxxxxxxxx>]

---------- Forwarded message ----------
From: Hendrik Visage <hvjunk@xxxxxxxxx>
Date: Thu, Mar 17, 2011 at 9:49 AM
Subject: Re: writing a jump table
To: Nicolas Bock <nicolasbock@xxxxxxxxx>
Cc: Brian Raiter <breadbox@xxxxxxxxxxxxxx>, linux-assembly@xxxxxxxxxxxxxxx




On Thu, Mar 17, 2011 at 4:26 AM, Nicolas Bock <nicolasbock@xxxxxxxxx> wrote:
>
> But how does non-PIC code where it is in memory? When I disassemble the
> non-PIC version, the address space starts at 0 which must mean that the
> linker relocates the code.

non-PIC code would typically start at the start of the address space
for that platform.

>
> Also, labels represent offsets if I
> understand this correctly, which is also a relative address and not an
> absolute. That's what I meant with that I don't fully understand why
> these extra hoops are necessary.

PIC code can be put anywhere in address space. So let's assume a few things:

START_ADDR: The starting point of the platform's address space for
code/data/etc.

LABEL_OFFSET: The offset for that LABEL inside the code/data relative
from 0 for the code/data when just normally assembled.

Now in the non-PIC case, it is "easy" to find the actual place in
address space: START_ADDR + LABEL_OFFSET, and this could be hard coded
during the static linking phase.

But in the PIC case, we also have:

PIC_OFFSET_CODE_SEG_A: The offset from START_ADDR where the RUNTIME
linker put the code for CODE_SEG_A. This is unknown till the code
actually gets executed. Okay, in *nost* cases you would find it is the
same place everytime, for a given piece of compiled code, but never
bargain on that, especially when you have hardened kernels that would
randomly assign address space offsets to frustrate crackers.

Now instead of a fixed calculated START_ADDR+PIC_OFFSET_CODE_SEG_A +
LABEL_OFFSET or compile/link time, we now have to calculate that value
at RUNTIME, as PIC_OFFSET_CODE_SEG_A could be different for each
execution.

That is a layman's description as I understand it, and I definitely
miss some other implementation details, but that is the gist of why
you need those extra hoops for PIC code.



>
> On 03/16/11 20:23, Brian Raiter wrote:
> >> I think I figured it out now. I used gcc to compile PIC for the C
> >> switch statement and checked what it does. I don't fully understand
> >> it to be honest, but it seems to do the job also for non PIC code.
> >
> > Of course -- PIC code just means that the code doesn't assume it knows
> > where it's located in memory, which for a shared-object library is a
> > necessary thing. It does mean the code has to jump through a few more
> > hoops, which is why the compiler doesn't make it the default.
> >
> > b
>
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