[ELF][PPC64] Don't report "relocation refers to a discarded section" for .toc

[MaskRay]

Summary

clang (as of 2019-06-12) / gcc (as of 8.2.1) PPC64 may emit a .rela.toc
which references an embedded switch table in a discarded .rodata/.text
section. The .toc and the .rela.toc are incorrectly not placed in the
comdat.

Technically a relocation from outside the group is not allowed by the ELF spec:

A symbol table entry with STB_LOCAL binding that is defined relative
to one of a group's sections, and that is contained in a symbol table
section that is not part of the group, must be discarded if the group
members are discarded. References to this symbol table entry from
outside the group are not allowed.

Don't report errors to work around the bug.

This should fix the ppc64le-lld-multistage-test bot while linking llvm-tblgen:

ld.lld: error: relocation refers to a discarded section: .rodata._ZNK4llvm3MVT13getSizeInBitsEv
>>> defined in utils/TableGen/CMakeFiles/llvm-tblgen.dir/CodeGenRegisters.cpp.o
>>> referenced by CodeGenRegisters.cpp
>>> utils/TableGen/CMakeFiles/llvm-tblgen.dir/CodeGenRegisters.cpp.o:(.toc+0x0)

Some other PPC specific sections may have similar problems. We can blacklist more
section names when problems occur.

// A simple program that reproduces the bug.
// Note .rela.toc (outside the group) references a section symbol (STB_LOCAL) in a group.
void puts(const char *);
struct A {
  void foo(int a) {
    switch (a) {
    case 0: puts("0"); break;
    case 1: puts("1"); puts("1"); break;
    case 2: puts("2"); break;
    case 3: puts("3"); puts("4"); break;
    case 4: puts("4"); break;
    case 5: puts("5"); puts("5"); break;
    case 6: puts("6"); break;
    }
  }
  int a;
};
void foo(A x) { x.foo(x.a); }

[MaskRay]

.

[ruiu]

ELF/Relocations.cpp

721

Hmm, has the compiler bug fixed? At least someone needs to fix this so that the compilers don't emit wrong code.

[MaskRay]

ELF/Relocations.cpp

721

Not yet.. Latest clang and powerpc64le gcc 6 (available on goldbolt.org) have the bug. You can find a reference from .toc to a local label in .rodata.blah (clang) or .text.blah (clang). The reference is a section relative relocation.

void puts(const char *);

struct A {
  void foo(int a) {
    switch (a) {
    case 0: puts("0"); break;
    case 1: puts("1"); puts("1"); break;
    case 2: puts("2"); break;
    case 3: puts("3"); puts("4"); break;
    case 4: puts("4"); break;
    case 5: puts("5"); puts("5"); break;
    case 6: puts("6"); break;
    }
  }
  int a;
};

void foo(A x) {
  x.foo(x.a);
}

[ruiu]

LGTM

ELF/Relocations.cpp

721

OK. We are not responsible for fixing the compiler issues, but we need to take a note which versions of the compilers have that bug. Please include that information to the comment.

[MaskRay]

Note down clang (as of 2019-06-12) and gcc (as of 8.2.1) have the bug.

[MaskRay]

Update description

[sfertile]

Thank You @MaskRay.

ELF/InputSection.cpp

if (Sym.Type == STT_SECTION) {
       // trivial for Elf_Rela, but for Elf_Rel we have to write to the
       // section data. We do that by adding to the Relocation vector.
 
       // .eh_frame is horribly special and can reference discarded sections. To
       // avoid having to parse and recreate .eh_frame, we just replace any
       // relocation in it pointing to discarded sections with R_*_NONE, which
       // hopefully creates a frame that is ignored at runtime. Also, don't warn
       // on .gcc_except_table and debug sections.
+      //
+      // See the comment in maybeReportUndefined for PPC64 .toc .
       auto *D = dyn_cast<Defined>(&Sym);
       if (!D) {
         if (!Sec->Name.startswith(".debug") &&
             !Sec->Name.startswith(".zdebug") && Sec->Name != ".eh_frame" &&
-            Sec->Name != ".gcc_except_table") {
+            Sec->Name != ".gcc_except_table" && Sec->Name != ".toc") {
           uint32_t SecIdx = cast<Undefined>(Sym).DiscardedSecIdx;
           Elf_Shdr_Impl<ELFT> Sec =
               CHECK(File->getObj().sections(), File)[SecIdx];
           warn("relocation refers to a discarded section: " +
                CHECK(File->getObj().getSectionName(&Sec), File) +
                "\n>>> referenced by " + getObjMsg(P->r_offset));
         }
         P->setSymbolAndType(0, 0, false);

ELF/Relocations.cpp

static bool maybeReportUndefined(Symbol &Sym, InputSectionBase &Sec,
   if (!Sym.isUndefined() || Sym.isWeak())
     return false;
 
   bool CanBeExternal = !Sym.isLocal() && Sym.computeBinding() != STB_LOCAL &&
                        Sym.Visibility == STV_DEFAULT;
   if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
     return false;
 
+  // clang (as of 2019-06-12) / gcc (as of 8.2.1) PPC64 may emit a .rela.toc
+  // which references a switch table in a discarded .rodata/.text section. The
+  // .toc and the .rela.toc are incorrectly not placed in the comdat. The ELF
+  // spec says references from outside the group to a STB_LOCAL symbol are not
+  // allowed. Work around the bug.
+  if (Config->EMachine == EM_PPC64 &&
+      cast<Undefined>(Sym).DiscardedSecIdx != 0 && Sec.Name == ".toc")
+    return false;
+
   auto Visibility = [&]() -> std::string {
     switch (Sym.Visibility) {
     case STV_INTERNAL:
       return "internal ";
     case STV_HIDDEN:
       return "hidden ";
     case STV_PROTECTED:
       return "protected ";

test/ELF/comdat-discarded-ppc64.s

+# REQUIRES: ppc
+# RUN: llvm-mc -filetype=obj -triple=powerpc64le %s -o %t.o
+# RUN: ld.lld %t.o %t.o -o /dev/null
+# RUN: ld.lld -r --fatal-warnings %t.o %t.o -o /dev/null
+
+## clang/gcc PPC64 may emit a .rela.toc which references a switch table in a
+## discarded .rodata/.text section. The .toc and the .rela.toc are incorrectly
+## not placed in the comdat.
+## Don't error "relocation refers to a discarded section".
+
+.section .text.foo,"axG",@progbits,foo,comdat
+.globl foo
+foo:
+.L0:
+
+.section .toc,"aw"
+.quad .L0