Patchwork Enable hidden visibility in libc.a compiled with PIE

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Submitter H.J. Lu
Date Aug. 8, 2017, 4:40 p.m.
Message ID <20170808164040.GA31184@gmail.com>
Download mbox | patch
Permalink /patch/21984/
State New
Headers show

Comments

H.J. Lu - Aug. 8, 2017, 4:40 p.m.
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(-)
H.J. Lu - Aug. 20, 2017, 6 p.m.
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.
Florian Weimer - Aug. 21, 2017, 3:48 p.m.
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
Szabolcs Nagy - Aug. 21, 2017, 4:31 p.m.
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?)
H.J. Lu - Aug. 21, 2017, 4:44 p.m.
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.
Szabolcs Nagy - Aug. 21, 2017, 4:50 p.m.
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.

Patch

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