Re: Preventing preemption of 'protected' symbols in GNU ld 2.26

[Cary Coutant <ccoutant@gmail.com>]

> And FWIW, there are some folks on the GCC side of things that think that
> HJ's change for 65248 is broken and needs to be reverted before gcc-6
> releases.

Thanks; I was thinking the next step was to move this discussion over
to the GCC side to see if anyone there was in agreement.

> It would help me immensely on the GCC side if things if you and Alan could
> easily summarize correct behavior and the impact if we were to just revert
> HJ's change.  A testcase would be amazingly helpful too.

On the compiler side, the main thing to check is that any reference to
a protected data symbol, whether extern, common, weak, or initialized,
is accessed with a pc-relative or GOT-relative addressing mode (i.e.,
not indirect through a GOT entry). This is the fundamental reason to
declare a data symbol protected.

The testcases in gcc.target/i386/pr65248-[1234].c look suitable to me,
if we simply switch "scan-assembler-not" with "scan-assembler"
throughout. These basically take a source like this:

__attribute__((visibility("protected")))
int xxx;

int
foo ()
{
  return xxx;
}

and compile with -fpic. Prior to GCC 5, we got

   movl    xxx(%rip), %eax

but with GCC 5, we now get

   movq    xxx@GOTPCREL(%rip), %rax
   movl    (%rax), %eax

There was another change to GCC at r218397, in Dec. 2014, to make data
access in PIE mode work more like non-PIC mode. This required linker
changes to enable the use of COPY relocations when generating PIE
output. This patch was originally developed by Sriraman Tallam on the
Google branch, but was checked on trunk in by HJ after considerable
discussion. As far as I'm concerned (and I advised Sri during the
development of this patch), this change was fine -- it simply
recognizes that a position-independent executable does not need to use
PIC-style references to data when a pc-relative or got-relative access
is possible. As in non-PIC mode, these accesses work fine for data
defined in the main program, but result in occasional COPY relocations
if the data happens to be defined in a shared library. Unfortunately,
I think this was the patch that set the rest in motion -- when
developing test cases, HJ noted that COPY relocations to protected
symbols were not allowed, and set out to fix that.

Ultimately, I'd like to see GCC support the use of declspec(dllimport)
or __attribute__((dllimport)) to declare a global variable known to be
defined externally in a shared library. It should then generate a
PIC-style reference even in non-PIC mode. This could be used to
eliminate all COPY relocations.

On the linker side, the linkers need to check that a reference from a
main program to a protected symbol will result in neither a text
relocation nor a COPY relocation, but instead issue an error message
explaining fairly clearly that a protected symbol in a shared library
cannot be referenced from non-PIC code. See PR ld/15228 and PR
gold/19823 (the latter being the bug report that started this current
thread).

-cary