ARM hard-float linker path - consensus

[Steve McIntyre <steve dot mcintyre at linaro dot org>]

Hi folks,

As promised, here's minutes from the call we had this
afternoon. Spoiler: the result we've agreed is

  /lib/ld-linux-armhf.so.3

And here's a transcription of the minutes from

  https://wiki.linaro.org/OfficeofCTO/HardFloat/LinkerPathCallApr2012

========================================

Meeting: 13th April 2012, 15:00 UTC

Agenda
------

 * Debian/Ubuntu have so far built using /lib/arm-linux-gnueabihf/ld-linux.so.3
 * Some other distros (Fedora, OpenSUSE) are still using /lib/ld-linux.so.3 option which matches the older soft-float ABI
 * Some people are proposing /libhf/ld-linux.so.3 or /libhfp/ld-linux.so.3 (multilib)
 * Some people proposed /lib/ld-arm-linux-gnueabihf.so.3 (similar to x86_64, libs still in /lib, from Michael Hope)
 * What should we do as a community? 

Present
-------

Name               Affiliations

Steve McIntyre     ARM, Debian, Linaro
Wookey             ARM, Debian, Linaro
Richard Earnshaw   ARM, gcc
Jeff Law           Fedora, Red Hat, gcc, glibc
Jon Masters        Fedora, Red Hat
Andrew Haley       Fedora, Red Hat, gcc
Andreas Jaeger     SUSE, openSUSE, glibc
Carlos O'Donnell   Mentor, gcc
Steve Langasek     Canonical, Ubuntu, Debian
Dann Frazier       Canonical, Ubuntu, Debian
Adam Conrad        Canonical, Ubuntu, Debian
Matthias Klose     Canonical, Ubuntu, Debian
Mike Frysinger     Gentoo
Dennis Gilmore     Fedora, Red Hat

Discussion
----------

We started with a couple of questions up front to establish the
grounds for discussion:

 * We believed that decision makers were present for all the important
   parties, i.e. all the arm hard-float distros, plus toolchain
   developers. This meant that a decision taken at the meeting could
   be implemented without needing further arguments/negotiations.

 * All the people present understood the importance of cross-distro
   binary compatibility, and they all wanted it. This led to agreement
   that we needed to agree on a standard path for the runtime linker
   for ARM hard-float Linux binaries.

Debian and Ubuntu had so far been using the "multi-arch" path of
/lib/arm-linux-gnueabihf/ld-linux.so.3. Fedora and OpenSUSE were thus
far using /lib/ld-linux.so.3, the same as the soft-float ABI. Others
had proposed alternative paths such as /libhf/ld-linux.so.3 or
/libhfp/ld-linux.so.3 (multilib) or
/lib/ld-arm-linux-gnueabihf.so.3. Discussion showed that none of these
were found to be universally acceptable.

Two parties were likely to be soon affected by an agreement here:

1. Ubuntu 12.04 (releasing with armhf in ~2 weeks)

Adam/Steve L agreed that all efforts would be put in to switch the
compilers in Ubuntu to a new path before release. Default things like
gcc would be correct, but less common tools might still be targetting
the old path /lib/arm-linux-gnueabihf/ld-linux.so. at release. They
could be fixed in the longer term and would not stop progress here.

2. Mentor (Codebench due for release in ~1 week)

Carlos mentioned this - Codebench has been using /lib/ld-linux.so.3
for hard-float binaries for some time and it was too late to change
that for this upcoming release. Next release due in October. Suggested
and accepted that this should be mentioned in release notes: if people
want to use Codebench on some systems (Debian/Ubuntu and derivatives),
they'll need to tweak their system setup. He may be able to do the
linker change and rebuild in a point release in a few weeks.

It was briefly suggested that the soft-float linker should be renamed
away from /lib/ld-linux.so.3 as well at this time, but that idea was
quickly shot down.

Proposal #1: /lib/ld-armhf.so.3       (not generally liked)

Proposal #2: /lib/ld-linux-armhf.so.3 (not favourite, but considered
                                       an acceptable compromise by all)

No need to go any further.

Conclusion
----------

All the people in the meeting agreed: the new runtime linker path for
ARM hard-float Linux binaries was to be

/lib/ld-linux-armhf.so.3

ACTION: People at the meeting to present this decision to their
        companies / communities and get the appropriate changes made.

Further discussion
------------------

General unhappiness with the mess that led to this meeting. Future
planning needs to be better between the various groups for the next
time we have a new CPU/ABI/whatever.

ACTION: Jon Masters to talk to the Linux Foundation about setting up a
        forum for such discussions.

In the meantime, strong consensus to use the
cross-distro@lists.linaro.org mailing list for any more conversations
now we have people in contact.

ACTION: Steve McIntyre to write up the minutes and circulate. Include
        an updated linker path patch for gcc to match the decision
        made here.

More discussion about triplets and naming, but nothing came of it in
the end. Distro folks have already decided what they're using and have
patched various software to build appropriately. Richard wants to move
gcc's config.guess to use arm-linux-gnueabihf; no strong objections to
that.

Linker path patch for gcc
-------------------------

Adapted from earlier work by Dann Frazier <dann.frazier@canonical.com>
and Michael Hope <michael.hope@linaro.org>

2012-04-13 Steve McIntyre <steve.mcintyre@linaro.org>

       * config/arm/linux-eabi.h (GLIBC_DYNAMIC_LINKER_HARD_FLOAT): Define.
       (GLIBC_DYNAMIC_LINKER): Redefine to use the hard float path.

diff --git a/gcc/config/arm/linux-eabi.h b/gcc/config/arm/linux-eabi.h
index 80bd825..8c9d2e7 100644
--- a/gcc/config/arm/linux-eabi.h
+++ b/gcc/config/arm/linux-eabi.h
@@ -62,7 +62,11 @@
 /* Use ld-linux.so.3 so that it will be possible to run "classic"
    GNU/Linux binaries on an EABI system.  */
 #undef  GLIBC_DYNAMIC_LINKER
-#define GLIBC_DYNAMIC_LINKER "/lib/ld-linux.so.3"
+#define GLIBC_DYNAMIC_LINKER_SOFT_FLOAT "/lib/ld-linux.so.3"
+#define GLIBC_DYNAMIC_LINKER_HARD_FLOAT "/lib/ld-linux-armhf.so.3"
+#define GLIBC_DYNAMIC_LINKER \
+   "%{mfloat-abi=hard:" GLIBC_DYNAMIC_LINKER_HARD_FLOAT "} \
+    %{!mfloat-abi=hard:" GLIBC_DYNAMIC_LINKER_SOFT_FLOAT "}"

 /* At this point, bpabi.h will have clobbered LINK_SPEC.  We want to
    use the GNU/Linux version, not the generic BPABI version.  */


Post-meeting on IRC
-------------------

Suggested that Richard should push the change into gcc trunk
ASAP. Steve McIntyre agreed to work on that with Richard.

Also suggested that we want to get a patch into glibc too to change
the installation path for ARM hard-float. Andrew agreed to push glibc
upstream for that.

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