Re: [PATCH] PR target/65248: [5 Regression] Copy relocation in PIE against protected symbol

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, Mar 2, 2015 at 11:09 AM, Alan Modra <amodra@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.tools@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,
> 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.

Richard.

> --
> Alan Modra
> Australia Development Lab, IBM