Re: [Automated-testing] Call for nommu LTP maintainer [was: Re: [PATCH 00/36] Remove UCLINUX from LTP]

Geert Uytterhoeven
2024-01-11T13:11:58Z

Hi Rob,

On Wed, Jan 10, 2024 at 8:17 PM Rob Landley <rob@landley.net> wrote:
> You can't fork() on nommu because copies of the mappings have different
> addresses, meaning any pointers in the copied mappings would point into the OLD
> mappings (belonging to the parent process), and fixing them up is 100%
> equivalent to the "garbage collection in C" problem. (It's AI-complete. Of the
> C3PO kind, not the "autocorrect with syntax checking" kind.) People get hung up
> on the "it would be very inefficient to do that because no copy-on-write"
> problem and miss the "the child couldn't FUNCTION because its pointer variables
> all contain parent addresses" problem.

Actually you can implement fork(), if you teach the compiler to use
separate stacks for return addresses and data:
- The first stack would contain only absolute addresses, to be
relocated after copying,
- The second stack would contain integers and relative pointers
(see FDPIC below), which do not need relocation after copying.

> The OTHER fun thing about nommu is you can't run conventional ELF binaries,
> because everything is linked at fixed address. So you might be able to run ONE
> instance of the program as your init task, assuming those addresses were
> available even then, but as soon as you try to run a second one it's a conflict.
>
> The quick and dirty work around is to make PIE binaries, which can relocate
> everything into available space, which works but doesn't scale. The problem with
> ELF PIE is that everything is linked contiguously from a single base pointer,
> meaning your text, rodata, data, and bss segments are all one linear blob. So if
> you run two instances of bash, you've loaded two copies of the test and the
> rodoata. This fills up your memory fast.
>
> AND PIE requires contiguous memory, which nommu is bad at providing because it
> has no page tables to remap stuff. With an mmu it can coalesce scattered
> physical pages into a virtually contiguous range, but without an mmu you can
> have plenty of memory free but in tiny chunks, none big enough to satisfy an
> allocation request.
>
> So they invented FDPIC, which is ELF with FOUR base pointers. Each major section
> (rodata, text, data, and bss) has its own base pointer, so you need to find
> smaller chunks of memory to load them into (and thus it can work on a more
> fragmented system), AND it means that two instances of the same program can
> share the read-only sections (rodata and text) so you only need new copies of
> the writeable segments (data and bss. And the heap. And the stack.)

Or Amiga LoadSeg() relocatable binaries and shared libraries ;-)
As this supported splitting code, data, and bss in lots of smaller
hunks, it could counter fragmented memory quite well.

BTW, can't you run and thus test nommu-binaries under normal Linux, too?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds