Fwd: Preventing preemption of 'protected' symbols in GNU ld 2.26

[Joe Groff <jgroff@apple.com>]

On Mar 29, 2016, at 8:44 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> 
> On Mon, Mar 28, 2016 at 4:21 PM, Alan Modra <amodra@gmail.com> wrote:
>> On Mon, Mar 28, 2016 at 03:38:01PM -0700, Cary Coutant wrote:
>>>>>> Did you look at what the costs were in startup time and dirty pages by using
>>>>>> copy relocations? What do you do if the size of the definition changes in a
>>>>>> new version of the library?
>>>>> 
>>>>> There wouldn't be a measurable cost in dirty pages; the copied objects
>>>>> are simply allocated in bss in the executable.
>>>> 
>>>> Wouldn't references to the symbol from within the .so need to be relocated to reference the now-canonical copy in the executable?
>>> 
>>> No, references from within the .so would have always used the GOT.
>>> Non-protected global symbols in a shared library are still
>>> pre-emptible, so they are always indirect, and there's always a
>>> dynamic relocation for the GOT entry. Whether the prevailing
>>> definition winds up in the executable or the shared library, the
>>> dynamic loader still has to bind the symbol and apply the relocation.
>> 
>> HJ's changes to protected visibility meant compiler changes so that
>> protected visibility in shared libraries is no longer seen as local.
>> So yes, protected visibility symbols in shared libraries now go
>> through the GOT.  Prior to his changes, they were optimized to a
>> pc-relative access.  Joe is correct in pointing out that shared
>> libraries needed a change.  Bad luck if you're using an older
>> compiler.  Also bad luck if you want to use protected visibility to
>> optimize your shared library.
>> 
>> HJ also made glibc ld.so changes to ensure the semantics of protected
>> visibility symbols remain unchanged when multiple shared libraries
>> define the same protected visibility symbol.
>> 
>> Apparently most people in the gcc and glibc communities saw these
>> toolchain modifications as fiendishly clever.
>> 
> 
> As I said before, copy relocation and protected symbol are fundamentally
> incompatible.  Since copy relocation is the part of x86 psABIs, I updated
> GCC, glibc and ld to make protected symbol to work with copy relocation.
> That is protected symbol may be external, but won't be preempted.  The
> price I paid is that protected symbol won't be accessed via PC-relative
> relocation within the shared object.  To access protected symbol via
> PC-relative relocation within the shared object, we need to disable copy
> relocation in executable, which is a psABI change.  That is why I proposed
> to mark the object as such so that we won't get surprise at run-time.

I think what Cary's arguing (and I honestly would expect) is that copying the protected symbol *is* for all intents and purposes a preemption. I'd expect copy relocations against protected symbols to be linker errors. I guess what's missing for gcc's intended optimization is an indication to the compiler that a symbol is protected in its home library, to suppress emitting PC-relative references to a copy relocation.

-Joe