Preventing preemption of 'protected' symbols in GNU ld 2.26

[Jeff Law]

On 04/15/2016 10:16 AM, H.J. Lu wrote:
> On Fri, Apr 15, 2016 at 9:09 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>> On 31/03/16 14:26, Ramana Radhakrishnan wrote:
>>> On Thu, Mar 31, 2016 at 1:52 AM, Jeff Law <law@redhat.com> wrote:
>>>> On 03/30/2016 06:40 PM, Cary Coutant wrote:
>>>>>>
>>>>>> It would help me immensely on the GCC side if things if you and Alan
>>>>>> could
>>>>>> easily summarize correct behavior and the impact if we were to just
>>>>>> revert
>>>>>> HJ's change.  A testcase would be amazingly helpful too.
>>>>>
>>>>>
>>>>> It looks like it's not just the one change. There's this patch:
>>>>>
>>>>>      https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01871.html
>>>>>
>>>>> which took the idea that protected can still be pre-empted by a COPY
>>>>> relocation and extended it to three more targets that use COPY
>>>>> relocations.
>>>>>
>>>>> I wonder how many other patches have been based on the same
>>>>> misunderstanding?
>>
>> (sorry i missed this thread)
>>
>> this was not a misunderstanding.
>>
>> that patch is necessary for correctness (odr) in
>> the presence of copy relocations as described in
>> https://gcc.gnu.org/ml/gcc-patches/2015-09/msg02365.html
>> and
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55012
>>
>> this was a long standing code gen bug in gcc and was
>> about time to fix it (it was also broken in glibc's
>> dynamic linker, but e.g. not in musl libc).
>>
>> (i don't see what is the issue with using the copy in
>> the main executable from a shared library, performance
>> is not a correctness issue, nor how it is possible to
>> avoid the copy relocs.)
>>
>
> Here is my understanding:
>
> Copy relocation and protected visibility are fundamentally incompatible.
> On on hand, copy relocation is the part of the psABI and is used to
> access global data defined in a shared object from the executable.  It
> moves the definition of global data, which is defined in a share object,
> to the executable at run-time.  On the other hand, protected visibility
> indicates that a symbol is defined locally in the shared object at
> run-time.  Both can't be true at the same time.  The current solution
> is to make protected symbol more or less like normal symbol, which
> prevents optimizing local access to protected symbol within the shared
> object.
>
> I propose to add GNU_PROPERTY_NO_COPY_ON_PROTECTED:
>
> https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI
>
> GNU_PROPERTY_NO_COPY_ON_PROTECTED This indicates that there
> should be no copy relocations against protected data symbols. If a relocat-
> able object contains this property, linker should treat protected data symbol
> as defined locally at run-time and copy this property to the output share
> object. Linker should add this property to the output share object if any pro-
> tected symbol is expected to be defined locally at run-time. Run-time loader
> should disallow copy relocations against protected data symbols defined in
> share objects with GNU_PROPERTY_NO_COPY_ON_PROTECTED prop-
> erty. Its PR_DATASZ should be 0.
I'd strongly suggest discussing directly with Carlos, Cary and Alan.  My 
worry here is this just adding another layer of stuff to deal with a 
fundamentally broken concept -- protected visibility.

Jeff