On Mon, Mar 2, 2015 at 4:05 AM, Richard Biener
<richard.guent...@gmail.com> wrote:
> On Mon, Mar 2, 2015 at 11:09 AM, Alan Modra <amo...@gmail.com> wrote:
>> On Mon, Mar 02, 2015 at 09:40:01AM +0100, Richard Biener wrote:
>>> On Sat, Feb 28, 2015 at 5:42 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>> > Ue copy relocation in PIE improves performance. But copy relocation
>>> > can't be used to access protected symbols defined in shared libaries
>>> > and linker in binutils 2.26 enforces doesn't allow it. GCC doesn't
>>> > know if an external definition is protected or not. This option adds
>>> > -mcopyreloc-in-pie to give user an option to turn it off to avoid problem
>>> > at link-time. OK for trunk?
>>>
>>> I wonder if the linker can fix this up? That is, turn the relocation into
>>> a valid one?
>>
>> No it can't (*), nor can the dynamic linker. Copy relocs aren't
>> really the issue. They are just a means of initializing a linker
>> generated variable to be used in place of a variable in a shared
>> library. The issue is the linker generated .dynbss variable itself.
>>
>> Consider an ELF executable linked against a shared library, with the
>> executable referencing (but not defining) a variable defined in the
>> shared library. You'd expect that the executable and shared library
>> would both use the same location for the variable. Indeed, that is
>> true. Both executable and shared library use the shared library's
>> variable. Except there is a wrinkle. If the executable is non-PIC,
^^^^^^^^^^^^
This is the key here. This optimization makes PIE behave like normal
executable. Is it good or bad?
>> code in the executable will require dynamic text relocations as the
>> variable's address isn't known until run time. To avoid that, some
>> clever person thought: "Why not have the linker define the variable in
>> the executable? ELF run time linking semantics mean the shared
>> library will now use the linker defined copy, so we'll still just be
>> using one copy of the variable". Any everyone was happy. At least
>> until ELF visibility was invented.
>>
>> When ELF visibility comes into play, a variable defined in a shared
>> library with non-default visibility is *not* overridden by another
>> definition in the executable, be it an actual definition or a linker
>> generated one. There is no problem of course if there is an actual
>> definition in the executable. In that case the programmer would
>> expect to see two different variables used. However, if the shared
>> library contains a protected visibility variable, and the linker
>> introduces a copy, then it has changed the meaning of the program. At
>> the source level we only had one definition of the variable, but at
>> run time we'd end up using two different locations.
>>
>> *) Except by avoiding .dynbss copies and hence requiring dynamic text
>> relocations.
>
> Ah, I see (protected visibility has haunted us in the past...).
>
> So I think we need to turn the new option off by default.
>
--
H.J.
