Re: Allow pie links to create PLT entries

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

On Fri, Jan 30, 2015 at 11:15 AM, Sriraman Tallam <tmsriram@google.com> wrote:
> On Thu, Jan 29, 2015 at 3:13 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Thu, Jan 29, 2015 at 2:17 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>>> On Thu, Jan 29, 2015 at 12:17 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Thu, Jan 29, 2015 at 12:08 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>>>>> On Thu, Jan 29, 2015 at 11:48 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>> On Thu, Jan 29, 2015 at 11:00 AM, Sriraman Tallam <tmsriram@google.com> wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>>     Here is a simple example that fails to link with -pie but which
>>>>>>> should work just fine without having to use -fPIE.
>>>>>>>
>>>>>>> foo.cc
>>>>>>> ======
>>>>>>> int extern_func();
>>>>>>> int main()
>>>>>>> {
>>>>>>>   extern_func();
>>>>>>>   return 0;
>>>>>>> }
>>>>>>>
>>>>>>> bar.cc
>>>>>>> =====
>>>>>>> int extern_func()
>>>>>>> {
>>>>>>>   return 1;
>>>>>>> }
>>>>>>>
>>>>>>> $ g++ -fPIC -shared bar.cc -o libbar.so
>>>>>>> $ g++ foo.cc -lbar -pie
>>>>>>>
>>>>>>> ld: error: foo.o: requires dynamic R_X86_64_PC32 reloc against
>>>>>>> '_Z11extern_funcv' which may overflow at runtime; recompile with -fPIC
>>>>>>>
>>>>>>> It fails because the linker disallows creating a PLT for
>>>>>>> R_X86_64_PC32 reloc when it is perfectly fine to do so.  Note that I
>>>>>>> could have recompiled foo.cc with -fPIE or -fPIC but I still think
>>>>>>> this can be allowed.  With support for copy relocations in pie in gold
>>>>>>> and with this support, the cases where we would need to use -fPIE to
>>>>>>> get working pie links is smaller.  This would help us link non-PIE
>>>>>>> objects into pie executables.
>>>>>>
>>>>>> You can't do it for x86 since EBX isn't setup for calling via PLT.
>>>>>> For x86-64, there should be little difference between PIE
>>>>>> and non-PIE code.
>>>>>
>>>>> True but that little difference is sometimes causing non-trivial
>>>>> performance penalties. With copyrelocations support for PIE added
>>>>> recently, one big difference causing non-trivial performance penalty
>>>>> went away.  However, there are still differences in the way global
>>>>> arrays are accessed.  For instance,
>>>>>
>>>>> uint32 a[] = {1, 2, 3, 4}
>>>>>
>>>>> a[i] can be accessed with one insn without -fPIE, whereas with -fPIE,
>>>>> we need two. One extra to get the 64-bit address of a.
>>>>>
>>>>> Without -fPIE:
>>>>>
>>>>> movslq   0x1655(%rip),%rax  # 401b80 <i>
>>>>> mov    0x401b30(,%rax,4),%esi # a[i]
>>
>> If you link it with -pie, you will have TEXTREL in executable.
>> Do you want relocations in text sections in PIE?
>
> I have been told TEXTRELs are not preferred though I never understood why.
>
> Just to make sure I understand, are you saying that the absolute
> address in the case of -pie will be a text relocation?   I think that

It is not about absolute address.  It is about symbol address.  Since
the address of symbol, a,  in

movl a(,%rdi,4), %eax

is unknown at link-time, linker has to generate relocation in text
section to resolve it at run-time.

> is not true because this mov instruction
>
> mov    0x401b30(,%rax,4),%esi
>
> does not allow a 64-bit absolute value which is needed for -pie.  What
> I was instead suggesting is to  make that PC-relative like:
>
> mov    0xabcd(%rip,%rax,4),%esi
>
> which would not need a text relocation.  However, I do not think such
> an insn is supported yet, thought it would be useful.
>

That will be useful.


-- 
H.J.