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

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

On 15/04/16 17:16, 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

protected visibility indicates

  "that references within the defining module bind to
  the definition in that module. That is, the declared
  entity cannot be overridden by another module."

here "definition in that module" does not mean addresses
at runtime, but the c language level definition.
(c semantics can only indicate abstract machine behaviour,
not relocation types and in memory layout at runtime).

so there is no conflict between copy relocation and
protected visibility: the definition is not overridden,
it's an implementation detail that the address happens
to be in the data section of the main executable instead
of the defining module.

i think your fixes in gcc, etc are ok.
(i understand that ppl want to optimize this but that
should be a separate discussion once there is consensus
about the correct semantics).

> 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.
> 
> 
>