Strip .note.gnu.build-id sections if --build-id is given.

[ruiu]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=36203

[ruiu]

• Add -r to produce a re-linkable object file.

ELF/InputFiles.cpp

InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &Sec) {
   // The linkonce feature is a sort of proto-comdat. Some glibc i386 object
   // files contain definitions of symbol "__x86.get_pc_thunk.bx" in linkonce
   // sections. Drop those sections to avoid duplicate symbol errors.
   // FIXME: This is glibc PR20543, we should remove this hack once that has been
   // fixed for a while.
   if (Name.startswith(".gnu.linkonce."))
     return &InputSection::Discarded;
 
+  // If we are creating a new .build-id section, strip existing .build-id
+  // sections so that the output won't have more than one .build-id.
+  // This is not usually a problem because input object files normally don't
+  // have .build-id sections, but you can create such files by
+  // "ld.{bfd,gold,lld} -r --build-id", and we want to guard against it.
+  if (Name == ".note.gnu.build-id" && Config->BuildId != BuildIdKind::None)
+    return &InputSection::Discarded;
+
   // The linker merges EH (exception handling) frames and creates a
   // .eh_frame_hdr section for runtime. So we handle them with a special
   // class. For relocatable outputs, they are just passed through.
   if (Name == ".eh_frame" && !Config->Relocatable)
     return make<EhInputSection>(*this, Sec, Name);
 
   if (shouldMerge(Sec))
     return make<MergeInputSection>(*this, Sec, Name);

test/ELF/relocatable-build-id.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1.o
+# RUN: ld.lld --build-id=0xcafebabe -o %t2.o %t1.o -r
+# RUN: ld.lld --build-id=0xdeadbeef -o %t.exe %t2.o
+# RUN: llvm-objdump -s %t.exe | FileCheck %s
+
+# CHECK-NOT: cafebabe
+# CHECK: deadbeef
+
+.global _start
+_start:
+  ret