Re: Preventing preemption of 'protected' symbols in GNU ld 2.26 [aka should we revert the fix for 65248]

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Apr 15, 2016 at 11:56 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Apr 15, 2016 at 2:49 PM, Jeff Law <law@redhat.com> wrote:
>>
>> So in the immediate term, if we drop the problem 65248 patch, we're back in
>> a state where the DSO and the executable can have two different views of
>> certain objects.  In which case we really need a solution in place to flag
>> that as an error, which is #1 in your list of mitigations.
>
> From program behavior perspective, there is nothing wrong with
> my changes to ld, ld.so and GCC.  The main drawback is my
> changes make protected symbol pretty much useless in term
> of faster access within the shared object.  All these are due to
> incompatibility between protected symbol and copy relocation.

I agree.  There's nothing to do for GCC 6.

> Given a shared library that defines a variable, and a non-PIC
> executable that references that variable, the linker makes a duplicate
> of the variable in the executable .dynbss section and arranges to have
> the copy initialized by the dynamic loader with a copy relocation.
> .dynbss is a linker created section that becomes part of the
> executable bss segment.  The idea is that at run-time both the
> executable and the shared library will use the executable copy of the
> variable.  It's a rather ancient linker hack to avoid dynamic text
> relocations, invented well before symbol visibility.

So what other choice does the linker have here?  AFAICS it's wrong
to create the .dynbss copy for protected symbols.  So why not
simply create 'dynamic text relocations' then?  Is that possible with
a pure linker change?

That said, correctness trumps optimization.  A correctness fix that
works with old objects trumps one that requires a compiler change.
Requiring a compiler change to get back optimization while preserving
correctness is fine.

To summarize: there is currently no testcase for a wrong-code issue
because there is no wrong-code issue.  There's a missed-optimization
issue (the library is less optimized) but if we fix that by revering HJs
patch we _do_ have a testcase for a wrong-code issue that re-appears.

=> nothing to block GCC 6 which behaves strictly better than GCC 5 here.

Richard.

>
> --
> H.J.