Re: [Qemu-devel] Re: [RFC/PATCH] elfload: add FDPIC support

[Mike Frysinger]

On Mon, Jan 24, 2011 at 16:06, Stefano Bonifazi wrote:
> I don't understand.. what is the difference between pie binary for pcc and
> for your architecture?

as i said, i think this is all irrelevant to what you want to do.  but
since you asked and i feel like writing ...

i have no idea what "pcc" is.  there are really two stark differences
between FDPIC ELF and a PIE ELF on say x86.  since the data/text
sections can be relocated independently of each other, the PIC cannot
assume a fixed offset between its text and GOT.  so every PLT entry is
actually two sets of addresses -- the function address and the
function's GOT address.  the other big difference is that an FDPIC ELF
app must first do a little relocation processing of itself as soon as
it starts ... in order to do so, the kernel provides a loadmap (made
up of multiple loadsegs) which describes the executable's PT_LOADs
(the addr encoded in the program header and the addr the chunk was
actually relocated to) as well as the (optional) executable's
interpreter's PT_LOADs.

> As far as I understood pie code is independent from addresses, so it should
> not care if the OS running them would have an mmu at all.. it should be just
> the task of the dynamic linker to relocate it properly, am I wrong?

but the PIE code still has fixed offsets between its text and its
data.  so FDPIC ELF is even more flexible than a PIE ELF.
-mike