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Re: [PATCH] PR ld/19636: [x86] Resolve undefweak and defined symbols in executable

From: Jiong Wang <jiong dot wang at foss dot arm dot com>
To: "H.J. Lu" <hjl dot tools at gmail dot com>
Cc: Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>, Michael Matz <matz at suse dot de>, Cary Coutant <ccoutant at gmail dot com>, Binutils <binutils at sourceware dot org>
Date: Wed, 2 Mar 2016 12:28:01 +0000
Subject: Re: [PATCH] PR ld/19636: [x86] Resolve undefweak and defined symbols in executable
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On 02/03/16 12:22, H.J. Lu wrote:

On Wed, Mar 2, 2016 at 4:03 AM, Jiong Wang <jiong.wang@foss.arm.com> wrote:


On 01/03/16 14:37, H.J. Lu wrote:

On Tue, Mar 1, 2016 at 6:02 AM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:

Hi HJ,

On 26/02/16 12:51, H.J. Lu wrote:

On Thu, Feb 25, 2016 at 10:59 AM, H.J. Lu <hjl.tools@gmail.com> wrote:

Here is the updated patch I am testing.  The linker behavior is changed
in 2 cases when creating executable:

1. When there are mixed PIC and non-PIC references to undefined
weak symbols, undefined weak symbols are resolved to 0 at link-time.
2. If all references to undefined weak symbols are PIC, dynamic
relocations against undefined weak symbols will be generated unless
-z nodynamic-undefined-weak is passed to linker.

BTW,  We have to resolve R_X86_64_32/R_X86_64_PC32 relocations
against undefined weak symbols to zero.  Otherwise, we will get
run-time
relocation overflow for dynamic R_X86_64_32/R_X86_64_PC32 relocations.

This is what I am checking in.

I'm seeing:

NA->FAIL: Mixing PIC and non-PIC
on aarch64-none-linux-gnu.

You can either fix aarch64 backend or skip the test for aarch64.


H.J,

   For your testcase, AArch64 is not generating dynamic relocation for
   weak undefined symbol referenced from non-pic code when linking
   exectuable, instead, it's resolved to zero during static linking stage.
   As far as I know, this behavior is exactly what's described here at

     https://sourceware.org/ml/binutils/2008-04/msg00269.html

   And reading those historical discussions,

    https://sourceware.org/ml/binutils/2008-04/msg00032.html
    https://sourceware.org/ml/binutils/2008-02/msg00264.html

   Looks to me the ld behavior changes introduced by your patch is quite
   sensitive and there still be lack of consensus.

What linker change were you referring to?  I only added a testcase.


I mean those linker changes added together with this testcase.

commit aec6b87e0b66d707ead62ca40d220ee78b4cf5a5
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Feb 26 04:16:15 2016 -0800

    [x86] Resolve non-PIC undefweak symbols in executable

   Therefore, instead of skipping this testcase on AArch64 (I guess a few
   other targets will fail on native tests as well), can you please only
   enable this testcase on x86 arches firstly?

These backends are also broken.

Follow-Ups:
- Re: [PATCH] PR ld/19636: [x86] Resolve undefweak and defined symbols in executable
  - From: H.J. Lu

References:
- Re: [PATCH] PR ld/19636: [x86] Resolve undefweak and defined symbols in executable
  - From: Kyrill Tkachov
- Re: [PATCH] PR ld/19636: [x86] Resolve undefweak and defined symbols in executable
  - From: H.J. Lu
- Re: [PATCH] PR ld/19636: [x86] Resolve undefweak and defined symbols in executable
  - From: Jiong Wang
- Re: [PATCH] PR ld/19636: [x86] Resolve undefweak and defined symbols in executable
  - From: H.J. Lu

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