[ELF] Don't crash if relocation target section is discarded

[evgeny777]

Summary

lld crashes in this case, while ld/gold emit warning message.

[ruiu]

ELF/Symbols.cpp

64

Please include a filename and symbol name in the error message.

65

Isn't it better to return 0 instead of D.Value?

[evgeny777]

ELF/Symbols.cpp

64

symbol name is typically nothing more than '.section + addend'.

[evgeny777]

Addressed review comments

[ruiu]

LGTM

ELF/Symbols.cpp

64

Ah, right. Since this is an error path, maybe this should suffice.

warn(SC->getFile()->getName() + ": relocation refers to discarded section");

ELF/Symbols.cpp

 #include "Symbols.h"
 #include "Error.h"
 #include "InputFiles.h"
 #include "InputSection.h"
 #include "OutputSections.h"
 #include "Target.h"
 
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/Path.h"
 
 using namespace llvm;
 using namespace llvm::object;
 using namespace llvm::ELF;
 
 using namespace lld;
 using namespace lld::elf;
 
 template <class ELFT>
+static std::string getSectionName(InputSectionBase<ELFT> *S) {
+  StringRef Filename = S->getFile()->getName();
+  return (sys::path::filename(Filename) + "(" + S->Name + ")").str();
+}
+
+template <class ELFT>
 static typename ELFT::uint getSymVA(const SymbolBody &Body,
                                     typename ELFT::uint &Addend) {
   typedef typename ELFT::uint uintX_t;
 
   switch (Body.kind()) {
   case SymbolBody::DefinedSyntheticKind: {
     auto &D = cast<DefinedSynthetic<ELFT>>(Body);
     const OutputSectionBase<ELFT> *Sec = D.Section;
case SymbolBody::DefinedRegularKind: {
     // 0.
     if (SC == &InputSection<ELFT>::Discarded)
       return 0;
 
     // This is an absolute symbol.
     if (!SC)
       return D.Value;
 
+    if (!SC->Live) {
+      warn("relocation refers to discarded section '" + getSectionName(SC) + "'");
+      return 0;
+    }
+
     uintX_t Offset = D.Value;
     if (D.isSection()) {
       Offset += Addend;
       Addend = 0;
     }
     uintX_t VA = (SC->OutSec ? SC->OutSec->getVA() : 0) + SC->getOffset(Offset);
     if (D.isTls() && !Config->Relocatable) {
       if (!Out<ELFT>::TlsPhdr)

test/ELF/linkerscript/discard-section.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: echo "SECTIONS { /DISCARD/ : { *(.aaa*) } }" > %t.script
-# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: ld.lld -o %t1 --script %t.script %t 2>&1 | FileCheck --check-prefix=WARN %s
 # RUN: llvm-objdump -section-headers %t1 | FileCheck %s
+
+# WARN: relocation refers to discarded section {{.+}}(.aaa)
 # CHECK-NOT: .aaa
 
 .section .aaa,"a"
-aaa:
+aab:
   .quad 0
+
+.section .zzz,"a"
+  .quad aab