Linux-Fsdevel Archive on
help / color / mirror / Atom feed
From: Rob Landley <>
To: Greg Ungerer <>, Rich Felker <>
Cc: Linus Torvalds <>,
	Russell King - ARM Linux admin <>,
	Jann Horn <>, Nicolas Pitre <>,
	Andrew Morton <>,
	Christoph Hellwig <>,
	Linux Kernel Mailing List <>,
	Linux-MM <>,
	linux-fsdevel <>,
	Alexander Viro <>,
	"Eric W . Biederman" <>,
	Oleg Nesterov <>,
	Linux ARM <>,
	Mark Salter <>,
	Aurelien Jacquiot <>,,
	Yoshinori Sato <>,
	Linux-sh list <>
Subject: Re: [PATCH v2 0/5] Fix ELF / FDPIC ELF core dumping, and use mmap_sem properly in there
Date: Fri, 1 May 2020 14:09:56 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 5/1/20 1:00 AM, Greg Ungerer wrote:
>> This sounds correct. My understanding of FLAT shared library support
>> is that it's really bad and based on having preassigned slot indices
>> for each library on the system, and a global array per-process to give
>> to data base address for each library. Libraries are compiled to know
>> their own slot numbers so that they just load from fixed_reg[slot_id]
>> to get what's effectively their GOT pointer.

fdpic is to elf what binflt is to a.out, and a.out shared libraries were never
pretty. Or easy.

>> I'm not sure if anybody has actually used this in over a decade. Last
>> time I looked the tooling appeared broken, but in this domain lots of
>> users have forked private tooling that's not publicly available or at
>> least not publicly indexed, so it's hard to say for sure.
> Be at least 12 or 13 years since I last had a working shared library
> build for m68knommu. I have not bothered with it since then, not that I
> even used it much when it worked. Seemed more pain than it was worth.

Shared libraries worked fine with fdpic on sh2 last I checked, it's basically
just ELF PIC with the ability to move the 4 segments (text/rodata/bss/data)
independently of each other. (4 base pointers, no waiting.)

I don't think I've _ever_ used shared binflt libraries. I left myself
breadcrumbs back when I was wrestling with that stuff:

But it looks like that last time I touched anything using elf2flt was:

And that was just because arm's fdpic support stayed out of tree for years so I
dug up binflt and gave it another go. (It sucked so much I wound up building
static pie for cortex-m, taking the efficiency hit, and moving on. Running pie
binaries on nommu _works_, it's just incredibly inefficient. Since the writeable
and readable segments of the ELF are all relative to the same single base
pointer, you can't share the read-only parts of the binaries without address
remapping, so if you launch 4 instances of PIE bash on nommu you've loaded 4
instances of the bash text and rodata, and of course none of it can even be
demand faulted. In theory shared libraries _do_ help there but I hit some
bug and didn't want to debug a half-assed solution, so big hammer and moved on
until arm fdpic got merged and fixed it _properly_...)


P.S. The reason for binflt is bare metal hardware engineers who are conceptually
uncomfortable with software love them, because it's as close to "objcopy -O
binary" as they can get. Meanwhile on j-core we've had an 8k ROM boot loader
that loads vmlinux images and does the ELF relocations for 5 years now, and ever
since the switch to device tree that's our _only_ way to feed a dtb to the
kernel without statically linking it in, so it's ELF all the way down for us.

  reply	other threads:[~2020-05-01 19:03 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-29 21:49 Jann Horn
2020-04-29 21:49 ` [PATCH v2 1/5] binfmt_elf_fdpic: Stop using dump_emit() on user pointers on !MMU Jann Horn
2020-05-05 10:48   ` Christoph Hellwig
2020-05-05 11:42     ` Jann Horn
2020-05-05 12:15       ` Christoph Hellwig
2020-08-11  3:05         ` Jann Horn
2020-04-29 21:49 ` [PATCH v2 2/5] coredump: Let dump_emit() bail out on short writes Jann Horn
2020-04-29 21:49 ` [PATCH v2 3/5] coredump: Refactor page range dumping into common helper Jann Horn
2020-05-05 10:50   ` Christoph Hellwig
2020-05-05 11:44     ` Jann Horn
2020-04-29 21:49 ` [PATCH v2 4/5] binfmt_elf, binfmt_elf_fdpic: Use a VMA list snapshot Jann Horn
2020-05-05 11:03   ` Christoph Hellwig
2020-05-05 12:11     ` Jann Horn
2020-04-29 21:49 ` [PATCH v2 5/5] mm/gup: Take mmap_sem in get_dump_page() Jann Horn
2020-04-29 21:56 ` [PATCH v2 0/5] Fix ELF / FDPIC ELF core dumping, and use mmap_sem properly in there Russell King - ARM Linux admin
2020-04-29 23:03   ` Linus Torvalds
2020-04-30  1:27     ` Nicolas Pitre
2020-04-30 14:10     ` Greg Ungerer
2020-04-30 14:51       ` Rich Felker
2020-04-30 21:13         ` Rob Landley
2020-05-01  6:00         ` Greg Ungerer
2020-05-01 19:09           ` Rob Landley [this message]
2020-04-30 16:54       ` Linus Torvalds
2020-04-30 19:07         ` Eric W. Biederman
2020-05-01  5:44           ` Greg Ungerer
2020-05-01 11:13             ` Eric W. Biederman
2020-05-01  7:14         ` Greg Ungerer
2020-04-30  1:59   ` Nicolas Pitre

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: [PATCH v2 0/5] Fix ELF / FDPIC ELF core dumping, and use mmap_sem properly in there' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).