From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Rob Landley <rob@landley.net>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
Niklas Cassel <niklas.cassel@wdc.com>,
Jonathan Corbet <corbet@lwn.net>,
Linux-sh list <linux-sh@vger.kernel.org>,
Randy Dunlap <rdunlap@infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-m68k@lists.linux-m68k.org"
<linux-m68k@lists.linux-m68k.org>,
Christophe Lyon <christophe.lyon@linaro.org>,
John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
linux-riscv <linux-riscv@lists.infradead.org>,
"buildroot@buildroot.org" <buildroot@buildroot.org>,
Greg Ungerer <gerg@linux-m68k.org>,
"ltp@lists.linux.it" <ltp@lists.linux.it>,
"automated-testing@lists.yoctoproject.org"
<automated-testing@lists.yoctoproject.org>
Subject: Re: [LTP] [Automated-testing] Call for nommu LTP maintainer [was: Re: [PATCH 00/36] Remove UCLINUX from LTP]
Date: Thu, 11 Jan 2024 14:11:43 +0100 [thread overview]
Message-ID: <CAMuHMdX5ACKVBQvEwMi7KHZkSVGZPJoocEC1wosfB7zc0u2mbA@mail.gmail.com> (raw)
In-Reply-To: <40996ea1-3417-1c2f-ddd2-e6ed45cb6f4b@landley.net>
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
--
Mailing list info: https://lists.linux.it/listinfo/ltp
next prev parent reply other threads:[~2024-01-11 13:12 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-03 1:52 [LTP] [PATCH 00/36] Remove UCLINUX from LTP Petr Vorel
2024-01-03 7:58 ` Cyril Hrubis
2024-01-03 8:04 ` Cyril Hrubis
2024-01-03 8:39 ` Petr Vorel
2024-01-03 9:46 ` Geert Uytterhoeven
2024-01-03 11:49 ` Petr Vorel
2024-01-03 11:54 ` Geert Uytterhoeven
2024-01-03 12:09 ` Cyril Hrubis
2024-01-03 12:40 ` Petr Vorel
2024-01-05 3:52 ` Rob Landley
2024-01-05 13:11 ` [LTP] Call for nommu LTP maintainer [was: Re: [PATCH 00/36] Remove UCLINUX from LTP] Petr Vorel
2024-01-06 3:58 ` Rob Landley
2024-01-08 9:03 ` Petr Vorel
2024-01-08 10:07 ` Cyril Hrubis
2024-01-09 22:37 ` [LTP] [Automated-testing] " Bird, Tim
2024-01-10 5:01 ` Rob Landley
2024-01-10 14:14 ` Petr Vorel
2024-01-10 19:23 ` Rob Landley
2024-01-10 21:17 ` Niklas Cassel via ltp
2024-01-11 0:00 ` Greg Ungerer
2024-01-11 9:21 ` Niklas Cassel via ltp
2024-01-12 20:18 ` Rob Landley
2024-01-11 2:25 ` Greg Ungerer
2024-01-12 20:16 ` Rob Landley
2024-01-14 13:01 ` Greg Ungerer
2024-01-15 13:41 ` [LTP] [Buildroot] " Waldemar Brodkorb
2024-01-15 14:22 ` Cyril Hrubis
2024-01-11 13:11 ` Geert Uytterhoeven [this message]
2024-01-11 13:19 ` [LTP] " Greg Ungerer
2024-01-09 20:24 ` [LTP] " Rob Landley
2024-01-09 23:17 ` Greg Ungerer
2024-01-10 5:47 ` Rob Landley
2024-01-10 14:46 ` Greg Ungerer
2024-01-10 13:33 ` Petr Vorel
2024-01-10 18:23 ` Rob Landley
2024-01-10 22:33 ` Petr Vorel
2024-01-08 8:33 ` [LTP] [PATCH 00/36] Remove UCLINUX from LTP Andrea Cervesato via ltp
2024-01-08 8:34 ` Andrea Cervesato via ltp
2024-01-05 3:50 ` Rob Landley
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