On Tue, Mar 03, 2015 at 06:20:05AM -0800, H.J. Lu wrote: > For x86-64, protected data symbols are impossible.
Impossible? This is not even true currently since -fPIC emits code that looks like it would fully support protected visibiliy variables in shared libraries. If you meant to say it is impossible with non-PIC, then even that statement is going too far. All you'd need to do is have GNU ld emit dynamic text relocations and possibly add some reloc support to ld.so, and you'd have support so long as shared libraries loaded within 2G of the executable. A little horrible, but quite possible. With some more work, GNU ld could edit the current code sequences emitted by gcc for non-PIC, to a branch to a patch area where you use a PIC code sequence. See also Cary's suggestion at https://groups.google.com/forum/#!msg/generic-abi/9JX9vdstoVA/g4UGTmRdXJcJ Or you could just acknowledge that non-PIC has limitations. Another similar one (same root cause of no GOT indirection) is with weak symbols, where extern int foo () __attribute__ ((weak)); if (foo) foo (); just doesn't work for foo in a shared library. The non-PIC "if (foo)" effectively becomes either "if (0)" or "if (1)" at link time, unless you emit dynamic text relocations or edit the code. -- Alan Modra Australia Development Lab, IBM
