Re: Specify how undefined weak symbol should be resolved in executable

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Feb 22, 2016 at 2:47 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Feb 22, 2016 at 2:43 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Feb 22, 2016 at 10:28 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Mon, Feb 22, 2016 at 10:17 AM, Michael Matz <matz@suse.de> wrote:
>>>> Hi,
>>>>
>>>> On Mon, 22 Feb 2016, H.J. Lu wrote:
>>>>
>>>>> > Typo?  What is the meaning of "dynamic relocation available at
>>>>> > runtime". Are you talking about a relocation entry or the relocation
>>>>> > process?
>>>>>
>>>>> Dynamic relocation isn't available with static linking.
>>>>
>>>> So you _are_ talking about the process.  I think it's customary to call
>>>> such executables "dynamic", or perhaps non-static, isn't it?  (Why I'm
>>>> confused with your wording: a dynamic executable which happens to have no
>>>> dynamic reloc entries, has "no dynamic relocation available at runtime",
>>>> and so we couldn't add one :))
>>>>
>>>>> > How about "When creating a dynamic executable the link editor should
>>>>> > ..."?
>>>>
>>>> So, I still can understand my version better (possibly with
>>>> s/dynamic/non-static/) :)
>>>>
>>>
>>> Let's go with:
>>>
>>> When creating dynamic executable, the link editor should generate dynamic
>>> relocations against unresolved weak symbols so that their values will be
>>> resolved at run-time.
>>>
>>
>> Second thought.  To generate dynamic relocation for R_X86_64_32
>> against fun in:
>>
>> [hjl@gnu-6 pr19704]$ objdump -dwr foo.o
>>
>> foo.o:     file format elf64-x86-64
>>
>>
>> Disassembly of section .text.startup:
>>
>> 0000000000000000 <main>:
>>    0: b8 00 00 00 00       mov    $0x0,%eax 1: R_X86_64_32 fun
>>    5: 48 85 c0             test   %rax,%rax
>>    8: 74 10                 je     1a <main+0x1a>
>>    a: 48 83 ec 08           sub    $0x8,%rsp
>>    e: e8 00 00 00 00       callq  13 <main+0x13> f: R_X86_64_PC32 fun-0x4
>>   13: 31 c0                 xor    %eax,%eax
>>   15: 48 83 c4 08           add    $0x8,%rsp
>>   19: c3                   retq
>>   1a: 31 c0                 xor    %eax,%eax
>>   1c: c3                   retq
>> [hjl@gnu-6 pr19704]$
>>
>> we create executable with dynamic R_X86_64_32 relocation against
>> fun .text section, which leads to DT_TEXTREL.  Is this what we want?
>>
>
> I believe the intention of undefined weak symbol is it should be resolved
> at link-time when creating executable, dynamic or static.

There is discrepancy between PIC input and non-PIC input on x86.
There may be a discrepancy between x86, where PIC is off by default,
and the target, where PIC is always on.  If we can't get an agreement
among Linux targets, we should address it in x86 psABI.

-- 
H.J.