Page-aligned executables (Was re: [9fans] Non-stack-based calling conventions)

["Joel C. Salomon" <joelcsalomon@gmail.com>]

On Feb 18, 2008 3:38 AM, Anant Narayanan <anant@kix.in> wrote:
> Just as an observation (may not be related to the thread), all the
> executable file formats that Linux currently supports, have page-
> aligned sections in the file! (Now I know why hexdump always shows me
> a bunch of zeros in-between).
>
> That means, all the loader really does is mmap the sections into the
> right memory locations. What next, maybe also put in the BSS in the
> file so you wouldn't have to allocate that either ;)

Ron mentioned that tidbit in his IWP9 talk about booting Plan 9 under
lguest.  Lguest (or was it another virtualizer?) uses mmap so it can
load any arbitrary number of client kernels; 9l-produced ELF files
break that model.

> Hence, writing the loader for Plan 9's a.out proved to be a challenge.
>
> I ended up writing a user-space program that padded out the required
> gap between TEXT and DATA before asking the kernel to execute it.
> Suboptimal, but it works.
>
> If anyone has any ideas as to how I can improve the situation, i'll be
> grateful.

What's the context that you're building this loader for?

--Joel