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

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

On Tue, Mar 29, 2016 at 12:36 PM, Joe Groff <jgroff@apple.com> wrote:
>
>> On Mar 29, 2016, at 12:33 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Tue, Mar 29, 2016 at 12:31 PM, Joe Groff <jgroff@apple.com> wrote:
>>> 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.
>>
>> That is what I meant by "That is why I proposed to mark the object as such so
>> that we won't get surprise at run-time."
>
> I don't see why it would be a runtime failure. ld could refuse to resolve the relocation at static link time, couldn't it?
>

The link-time shared library may be very different from the run-time
shared library.  Since copy relocation works correctly against protected
symbol with the current gcc, binutils and glibc, we can't simply disallow
copy relocation against protected symbol at link-time.  It should be done
with a link command-line as well as markers on the input files.


-- 
H.J.