Allow use define symbols to override linker defined ones

[• rafael]

Summary

I hit an internal linker script that was defining _DYNAMIC instead of letting the linker do it. It turns out that both bfd and gold allow that.

This is pretty easy to implement, just make the linker defined symbol weak. This should have no impact in the case where there is no user defined symbol: The visibility is hidden, which causes the output to still be local.

[emaste]

Was the MIPS duplicate _gp issue entirely a false error now fixed by this change?

[atanasyan]

In D26801#598636, @emaste wrote:

Was the MIPS duplicate _gp issue entirely a false error now fixed by this change?

This fix removes false error message but the linker will not use _gp value provided in the linker script. That needs to be fixed separately. I will do that soon.

[ruiu]

LGTM with a comment saying that new symbols are added as weak symbols so that they can be overriden by user-defined linker script symbols.

[davide]

lgtm

ELF/Writer.cpp

if (!S->isUndefined() && !S->isShared())
     return S->symbol();
   return Symtab<ELFT>::X->addSynthetic(Name, Sec, Val, StOther);
 }
 
 template <class ELFT>
 static Symbol *addRegular(StringRef Name, InputSectionBase<ELFT> *IS,
                           typename ELFT::uint Value) {
   typename ELFT::Sym LocalHidden = {};
-  LocalHidden.setBindingAndType(STB_LOCAL, STT_NOTYPE);
+  LocalHidden.setBindingAndType(STB_WEAK, STT_NOTYPE);
   LocalHidden.setVisibility(STV_HIDDEN);
   LocalHidden.st_value = Value;
   return Symtab<ELFT>::X->addRegular(Name, LocalHidden, IS, nullptr);
 }
 
 template <class ELFT>
 static Symbol *addOptionalRegular(StringRef Name, InputSectionBase<ELFT> *IS,
                                   typename ELFT::uint Value) {

test/ELF/duplicate-internal.s

-# Should print an expected message in case of conflict with an internally generated symbol.
-
-# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t.o
-# RUN: not ld.lld -shared %t.o -o %t.so 2>&1 | FileCheck %s
-
-# CHECK: duplicate symbol '_gp' in {{.*}} and (internal)
-
-# REQUIRES: mips
-
-  .globl  _gp
-_gp = 0

test/ELF/linkerscript/dynamic-sym.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: echo "_DYNAMIC = 0x123;" > %t.script
+# RUN: ld.lld -T %t.script %t.o -shared -o %t.so
+# RUN: llvm-readobj -t %t.so | FileCheck %s
+
+# CHECK:      Symbol {
+# CHECK:        Name: _DYNAMIC
+# CHECK-NEXT:   Value: 0x123
+# CHECK-NEXT:   Size: 0
+# CHECK-NEXT:   Binding: Local
+# CHECK-NEXT:   Type: None
+# CHECK-NEXT:   Other [
+# CHECK-NEXT:     STV_HIDDEN
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Section: Absolute
+# CHECK-NEXT: }