[PATCH] gold: Add -z noreloc-overflow option

[Fangrui Song]

On 2020-11-03, Hang Deng (handeng) via Binutils wrote:
>HJ, Cary,
>
>Thanks, we tried your patch for X86_64, it works well.
>
>And the patch below also works with arm64,  but I am not familiar with  the test case and review/commit process, can you please help to commit this patch as well?
>
>Thanks,
>Hunter
>
>diff --git a/gold/aarch64.cc b/gold/aarch64.cc
>index b207dcdc..29da1e35 100644
>--- a/gold/aarch64.cc
>+++ b/gold/aarch64.cc
>@@ -5972,7 +5972,6 @@ Target_aarch64<size, big_endian>::Scan::local(
>     case elfcpp::R_AARCH64_NONE:
>       break;
>
>-    case elfcpp::R_AARCH64_ABS32:
>     case elfcpp::R_AARCH64_ABS16:
>       if (parameters->options().output_is_position_independent())
>        {
>@@ -5981,6 +5980,13 @@ Target_aarch64<size, big_endian>::Scan::local(
>        }
>       break;
>
>+    case elfcpp::R_AARCH64_ABS32:
>+      if ( parameters->options().output_is_position_independent() &&
>+          !parameters->options().noreloc_overflow()) {
>+              gold_error(_("%s: unsupported reloc %u in pos independent link."),
>+                       object->name().c_str(), r_type);
>+           break;
>+       }

Cary raised the point that -z noreloc-overflow should be related to the
link-time value, rather than the possibility of runtime overflow.

In -shared/-pie links, R_AARCH64_ABS32 referencing a non-preemptible SHN_ABS symbol should be allowed.

>     case elfcpp::R_AARCH64_ABS64:
>       // If building a shared library or pie, we need to mark this as a dynmic
>       // reloction, so that the dynamic loader can relocate it.
>
>
>On 2020/11/3, 2:53 AM, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>
>    On Mon, Nov 2, 2020 at 10:05 AM Cary Coutant <ccoutant@gmail.com> wrote:
>    >
>    > > +    case elfcpp::R_X86_64_32S:
>    > > +      // -z noreloc-overflow
>    > > +      if (parameters->options().noreloc_overflow())
>    > > +       return;
>    > > +      // Fall through.
>    > > +
>    > >      default:
>    > >        // This prevents us from issuing more than one error per reloc
>    > >        // section.  But we can still wind up issuing more than one
>    >
>    > This will issue the wrong error message in the normal (check overflow)
>    > case, won't it? You'll get "requires unsupported dynamic reloc"
>    > instead of "requires R_X86_64_32S reloc which may overflow at
>    > runtime".
>
>    Since R_X86_64_32S is handled by the default label, the error
>    message without -z noreloc-overflow is unchanged.

Probably unrelated but many other relocation types e.g.  R_X86_64_8
R_X86_64_16 lack checks as they do in GNU ld.

>    > Also, shouldn't -z noreloc-overflow turn off link-time overflow
>    > checking? All this patch does is disable the warning about a possible
>    > overflow at runtime. If that's all you want, I think the option is
>    > misnamed.
>    >
>
>    This patch disables overflow checks for R_X86_64_32S and
>    R_X86_64_32 as the tests show.
>
>    --
>    H.J.
>