[Bug ld/15228] New: copy relocations against protected symbols should be disallowed

[luto at mit dot edu]

http://sourceware.org/bugzilla/show_bug.cgi?id=15228

             Bug #: 15228
           Summary: copy relocations against protected symbols should be
                    disallowed
           Product: binutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
        AssignedTo: address@hidden
        ReportedBy: address@hidden
    Classification: Unclassified


Take this code:

-- begin lib.cc --
struct A
{
  A() : x(1) {}
  int x;
};

__attribute__((visibility("protected"))) A a;
-- end lib.cc --

--begin main.cc--
struct A
{
  A() : x(1) {}
  int x;
};

extern A a;

extern "C" void abort(void);

int main()
{
  if (a.x != 1)
    abort();
}
--end main.cc--

and build it with:

$ g++ -shared -fPIC -o lib.so lib.cc
$ g++ lib.so main.cc

ld sees a protected defined object called a of size 4 in lib.so and an
undefined object called a in main.o.  It helpfully generates a copy relocation.
 The resulting code cannot possibly work, and, in fact, it aborts.  Everything
works fine with -fPIE.

IMO it would be much better to fail to link than to produce a binary that ld.so
is happy to load (maybe that's an ld bug) but that is more or less guaranteed
to execute incorrectly.


(FWIW, Windows has solved this problem using dllimport for many years, but
that's another story.)

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