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

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

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.



-- 
H.J.