Distinguishing SF/HF ABI binaries, take two

[Steve McIntyre <steve.mcintyre@linaro.org>]

Hi folks,

We're currently carrying patches in glibc in Debian (and Ubuntu) that
I wrote which are used to work out whether an ELF binary is hard-float
or soft-float. We're using these to allow us to do the right thing on
a multi-arch system, which is to pick a consistent set of binaries
(programs and libraries) at runtime; if you try to mix binaries using
different ABIs, you're prone to all kinds of weird and wonderful
results but generally badness occurs.

Upstream glibc have generally not been welcoming of these patches, and
I understand this; the approach taken (reading ARM-specific build
attributes) is far from clean and doesn't fit well in the design of
ld.so in particular. So, I've been looking into alternative methods
for achieving the goal of identifying ABI. After a couple of false
starts and discussion with some of the helpful toolchain and ABI folks
in ARM, I think we have a solution that will work well in the long
term. I just wish we'd thought about this *way* back when we first
started the armhf port, as it would have been much easier to work on
and standardise this back then. Modulo availability of time machines,
there's not much we can do on that front... :-)

What I'm proposing is to use two new values in the OSABI field in the
ELF header:

  #define ELFOSABI_LINUX_ARM_AEABI_SF 65
  #define ELFOSABI_LINUX_ARM_AEABI_HF 66

and use these values in the future for soft- and hard-float binaries
so that can unambiguously identify them.

There's already precedent for binaries using different values in this
field, with support in glibc for parsing and understanding
them. Adding more possible values is quite easy, assuming that the
maintainers are amenable. I'm about to post a similar message there.

I have a plan of attack for how to make a staged switch over,
deliberately to minimise any potential compatibility problems. See the
attached doc for that. It's deliberately not very specific in terms of
timeline, as that's something I'm hoping to get feedback
about. Comments very welcome; please point out if you think there are
problems with this approach, or if there are any more implementations
of toolchain / linker that will need to be addressed.

Cheers,
-- 
Steve McIntyre                                steve.mcintyre@linaro.org
<http://www.linaro.org/> Linaro.org | Open source software for ARM SoCs

Step 1
======

Add new value for ARM into the allowed space for the OSABI
field. Names/values *not* finalised yet, but proposing:

#define ELFOSABI_LINUX_ARM_AEABI_SF 65
#define ELFOSABI_LINUX_ARM_AEABI_HF 66

These will go into a new version of the "Linux for The ARM
Architecture" ARM ABI supplement
(https://sourcery.mentor.com/GNUToolchain/kbattach142/arm_gnu_linux_abi.pdf)

Step 2
======

Add support for those into all the runtime linkers:

 * (e)glibc
 * uclibc    (not needed AFAICS, currently ignores OSABI)
 * klibc     (not needed AFAICS, too simple to care)
 * bionic    (not needed AFAICS, too simple to care)
 * others...?

For now, support/recognition of these new values will be a no-op,
matching and working in just the same way as existing ARM
entries. Interoperability is key for now; we don't want to break
anything that currently exists.

Step 3
======

After all of the linkers have released new versions, tweak binutils
and other toolchains to generate the new values(s) for SF/HF ABI
binaries as appropriate:

 * GNU binutils
 * ARM toolchains
 * others?

Step 4
======

Add code into the linkers to differentiate based on the new OSABI
values.

For objects still using the old values, continue working as
previously; assume nothing about them, they'll work by default. In
Debian/Ubuntu we'll keep the attributes-checking code to distinguish
so we can have multi-arch working.

For objects using the new values, we will (finally!) be able to
unambiguously determine which variant of the ABI they are using.

Step 5
======

(Potentially) after a few years, remove the Debian/Ubuntu
hacks. Maybe. TBD later after review...

History
=======
2012-08-01  0.1   Initial draft
2012-08-02  0.2   Trivial wording tweaks after first discussion,
                  investigated bionic

Steve McIntyre
steve.mcintyre@linaro.org