Enable hidden visibility in libc.a compiled with PIE

  When building libc.a with PIE, enable hidden visibility to allow direct
access to definitions within libc.a without using GOT.

Size comparison of elf/ldconfig when the compiler defaults to -fPIE:

On x86-64:
        text	   data	    bss	    dec	    hex
Before: 619206	  20132	   5488	 644826	  9d6da
After : 619062	  20132	   5488	 644682	  9d64a
On i686:
        text	   data	    bss	    dec	    hex
Before: 556305	  10816	   3056	 570177	  8b341
After : 553688	  10756	   3056	 567500	  8a8cc

OK for master?

H.J.
---
	* include/libc-symbols.h (attribute_hidden): Enable hidden
	visibility in libc.a compiled with PIE.
---
 include/libc-symbols.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
  

On Mon, Aug 14, 2017 at 5:51 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Aug 8, 2017 at 9:40 AM, H.J. Lu <hongjiu.lu@intel.com> wrote:
>> When building libc.a with PIE, enable hidden visibility to allow direct
>> access to definitions within libc.a without using GOT.
>>
>> Size comparison of elf/ldconfig when the compiler defaults to -fPIE:
>>
>> On x86-64:
>>         text       data     bss     dec     hex
>> Before: 619206    20132    5488  644826   9d6da
>> After : 619062    20132    5488  644682   9d64a
>> On i686:
>>         text       data     bss     dec     hex
>> Before: 556305    10816    3056  570177   8b341
>> After : 553688    10756    3056  567500   8a8cc
>>
>> OK for master?
>>
>> H.J.
>> ---
>>         * include/libc-symbols.h (attribute_hidden): Enable hidden
>>         visibility in libc.a compiled with PIE.
>> ---
>>  include/libc-symbols.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/libc-symbols.h b/include/libc-symbols.h
>> index fe3ab81c51..d6a1c260f6 100644
>> --- a/include/libc-symbols.h
>> +++ b/include/libc-symbols.h
>> @@ -363,7 +363,8 @@ for linking")
>>    strong_alias(real, name)
>>  #endif
>>
>> -#if defined SHARED || defined LIBC_NONSHARED
>> +#if defined SHARED || defined LIBC_NONSHARED \
>> +  || (BUILD_PIE_DEFAULT && IS_IN (libc))
>>  # define attribute_hidden __attribute__ ((visibility ("hidden")))
>>  #else
>>  # define attribute_hidden
>> --
>> 2.13.4
>>
>
> Any comments or objections?
>

I will check it in tomorrow.
  

On 08/08/2017 06:40 PM, H.J. Lu wrote:
> -#if defined SHARED || defined LIBC_NONSHARED
> +#if defined SHARED || defined LIBC_NONSHARED \
> +  || (BUILD_PIE_DEFAULT && IS_IN (libc))
>  # define attribute_hidden __attribute__ ((visibility ("hidden")))
>  #else
>  # define attribute_hidden

I think this broke “make subdirs=benchtests bench-build” because
json-lib.c is erroneously compiled with -DMODULE_NAME=libc.

Thanks,
Florian
  

On 08/08/17 17:40, H.J. Lu wrote:
> When building libc.a with PIE, enable hidden visibility to allow direct
> access to definitions within libc.a without using GOT.
...
> -#if defined SHARED || defined LIBC_NONSHARED
> +#if defined SHARED || defined LIBC_NONSHARED \
> +  || (BUILD_PIE_DEFAULT && IS_IN (libc))
>  # define attribute_hidden __attribute__ ((visibility ("hidden")))

btw what happens if this is enabled for non-pie libc.a ?
i'd expect that to work too (it prevents some symbol
interpositions?)
  

On Mon, Aug 21, 2017 at 9:31 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> On 08/08/17 17:40, H.J. Lu wrote:
>> When building libc.a with PIE, enable hidden visibility to allow direct
>> access to definitions within libc.a without using GOT.
> ...
>> -#if defined SHARED || defined LIBC_NONSHARED
>> +#if defined SHARED || defined LIBC_NONSHARED \
>> +  || (BUILD_PIE_DEFAULT && IS_IN (libc))
>>  # define attribute_hidden __attribute__ ((visibility ("hidden")))
>
> btw what happens if this is enabled for non-pie libc.a ?

It makes those symbols in libc.a hidden.

> i'd expect that to work too (it prevents some symbol
> interpositions?)
>

You can't interpose any symbols in libc.a, PIE or non-PIE.
What my patch does is to avoid GOT/PLT in PIE libc.a when
accessing them within libc.a.
  

On 21/08/17 17:44, H.J. Lu wrote:
> On Mon, Aug 21, 2017 at 9:31 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>> On 08/08/17 17:40, H.J. Lu wrote:
>>> When building libc.a with PIE, enable hidden visibility to allow direct
>>> access to definitions within libc.a without using GOT.
>> ...
>>> -#if defined SHARED || defined LIBC_NONSHARED
>>> +#if defined SHARED || defined LIBC_NONSHARED \
>>> +  || (BUILD_PIE_DEFAULT && IS_IN (libc))
>>>  # define attribute_hidden __attribute__ ((visibility ("hidden")))
>>
>> btw what happens if this is enabled for non-pie libc.a ?
> 
> It makes those symbols in libc.a hidden.
> 
>> i'd expect that to work too (it prevents some symbol
>> interpositions?)
>>
> 
> You can't interpose any symbols in libc.a, PIE or non-PIE.
> What my patch does is to avoid GOT/PLT in PIE libc.a when
> accessing them within libc.a.
> 

yes, but my point is that the ifdef can be simplified
if it's valid for the non-pie case too.