[ELF] implemented -z defs option

[grimar]

Summary

Just do not allow to link shared library if there are
undefined symbols.

This fixes PR27447

[• rafael]

LGTM. Thanks!

ELF/Config.h

struct Configuration {
   bool StripDebug;
   bool SysvHash = true;
   bool Threads;
   bool Trace;
   bool Verbose;
   bool VersionScript = false;
   bool WarnCommon;
   bool ZCombreloc;
+  bool ZDefs;
   bool ZExecStack;
   bool ZNodelete;
   bool ZNow;
   bool ZOrigin;
   bool ZRelro;
   BuildIdKind BuildId = BuildIdKind::None;
   ELFKind EKind = ELFNoneKind;
   uint16_t EMachine = llvm::ELF::EM_NONE;

ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->LtoO = getInteger(Args, OPT_lto_O, 2);
   if (Config->LtoO > 3)
     error("invalid optimization level for LTO: " + getString(Args, OPT_lto_O));
   Config->LtoJobs = getInteger(Args, OPT_lto_jobs, 1);
   if (Config->LtoJobs == 0)
     error("number of threads must be > 0");
 
   Config->ZCombreloc = !hasZOption(Args, "nocombreloc");
+  Config->ZDefs = hasZOption(Args, "defs");
   Config->ZExecStack = hasZOption(Args, "execstack");
   Config->ZNodelete = hasZOption(Args, "nodelete");
   Config->ZNow = hasZOption(Args, "now");
   Config->ZOrigin = hasZOption(Args, "origin");
   Config->ZRelro = !hasZOption(Args, "norelro");
 
   if (Config->Relocatable)
     Config->StripAll = false;

ELF/Writer.cpp

else
     scanRelocs(S, EObj.rels(&RelSec));
 }
 
 template <class ELFT>
 static void reportUndefined(SymbolTable<ELFT> &Symtab, SymbolBody *Sym) {
   if (!Config->NoUndefined) {
     if (Config->Relocatable)
       return;
-    if (Config->Shared)
+    if (Config->Shared && !Config->ZDefs)
       if (Sym->symbol()->Visibility == STV_DEFAULT)
         return;
   }
 
   std::string Msg = "undefined symbol: " + Sym->getName().str();
   if (InputFile *File = Sym->getSourceFile<ELFT>())
     Msg += " in " + getFilename(File);
   if (Config->NoinhibitExec)

test/ELF/zdefs.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld -shared %t.o -o %t1.so
+
+# RUN: not ld.lld -z defs -shared %t.o -o %t1.so 2>&1 | FileCheck -check-prefix=ERR %s
+# ERR: undefined symbol: foo
+
+callq foo@PLT