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

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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); }

Reviewers: ruiu, sfertile, espindola

Reviewed By: ruiu

Subscribers: emaste, nemanjai, arichardson, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63182

llvm-svn: 363126
This commit is contained in:
Fangrui Song 2019-06-12 07:35:42 +00:00
parent ed4cd44870
commit 11549e5c46
3 changed files with 29 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
lld/ELF/InputSection.cpp
View File

@ -438,11 +438,13 @@ void InputSection::copyRelocations(uint8_t *Buf, ArrayRef<RelTy> Rels) {
// 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];

9
lld/ELF/Relocations.cpp
View File

@ -718,6 +718,15 @@ static bool maybeReportUndefined(Symbol &Sym, InputSectionBase &Sec,
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:

17
lld/test/ELF/comdat-discarded-ppc64.s Normal file
View File

@ -0,0 +1,17 @@
# 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