Re: [PATCH 2/3] Revert "[AArch64][BZ #17711] Fix extern protected data handling"

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, May 26, 2022 at 1:03 PM Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>
> Hi Florian,
>
> Sure, something basic like this shows the issues:
>
> int x;
> int f(void) { return ++x; }
> int main(void) { return f(); }
>
> compile with -O2 -fPIC -flto -shared:

-fPIC is the issue.   x can be marked as "export" with the
protected visibility.  GCC 12 with

-fvisibility=protected -flto  -mno-direct-extern-access

works on x86-64:

0000000000001040 <main>:
    1040: 8b 05 d6 2f 00 00    mov    0x2fd6(%rip),%eax        # 401c <x>
    1046: 83 c0 01              add    $0x1,%eax
    1049: 89 05 cd 2f 00 00    mov    %eax,0x2fcd(%rip)        # 401c <x>
    104f: c3                    ret

0000000000001110 <f>:
    1110: 8b 05 06 2f 00 00    mov    0x2f06(%rip),%eax        # 401c <x>
    1116: 83 c0 01              add    $0x1,%eax
    1119: 89 05 fd 2e 00 00    mov    %eax,0x2efd(%rip)        # 401c <x>
    111f: c3                    ret

>
> 00000000000004f0 <f@plt>:
>  4f0:   b0000090        adrp    x16, 11000 <__cxa_finalize@GLIBC_2.17>
>  4f4:   f9400611        ldr     x17, [x16, #8]
>  4f8:   91002210        add     x16, x16, #0x8
>  4fc:   d61f0220        br      x17
>
> 0000000000000510 <main>:
>  510:   17fffff8        b       4f0 <f@plt>
>
> 0000000000000600 <f>:
>  600:   90000081        adrp    x1, 10000 <__FRAME_END__+0xf8f8>
>  604:   f947e821        ldr     x1, [x1, #4048]
>  608:   b9400020        ldr     w0, [x1]
>  60c:   11000400        add     w0, w0, #0x1
>  610:   b9000020        str     w0, [x1]
>  614:   d65f03c0        ret
>
> So f() does not get inlined into main, it is redirected via PLT, and it uses a GOT
> indirection to access the global. The underlying problem is that ELF assumes by
> default that you want interposition/export for all symbols. In reality that is
> almost never needed. LLVM makes -fno-semantic-interposition the default,
> which solves the PLT and inlining issue, but not the unnecessary GOT indirections.
>
> Cheers,
> Wilco



-- 
H.J.