[ELF] Change GOT*_FROM_END (relative to end(.got)) to GOTPLT* (start(.got.plt))

Summary:
This should address remaining issues discussed in PR36555.

Currently R_GOT*_FROM_END are exclusively used by x86 and x86_64 to
express relocations types relative to the GOT base. We have
_GLOBAL_OFFSET_TABLE_ (GOT base) = start(.got.plt) but end(.got) !=
start(.got.plt)

This can have problems when _GLOBAL_OFFSET_TABLE_ is used as a symbol, e.g.
glibc dl_machine_dynamic assumes _GLOBAL_OFFSET_TABLE_ is start(.got.plt),
which is not true.

  extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
  return _GLOBAL_OFFSET_TABLE_[0]; // R_X86_64_GOTPC32

In this patch, we

* Change all GOT*_FROM_END to GOTPLT* to fix the problem.
* Add HasGotPltOffRel to denote whether .got.plt should be kept even if
  the section is empty.
* Simplify GotSection::empty and GotPltSection::empty by setting
  HasGotOffRel and HasGotPltOffRel according to GlobalOffsetTable early.

The change of R_386_GOTPC makes X86::writePltHeader simpler as we don't
have to compute the offset start(.got.plt) - Ebx (it is constant 0).

We still diverge from ld.bfd (at least in most cases) and gold in that
.got.plt and .got are not adjacent, but the advantage doing that is
unclear.

Reviewers: ruiu, sivachandra, espindola

Subscribers: emaste, mehdi_amini, arichardson, dexonsmith, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59594

llvm-svn: 356968

1 files changed, 5 insertions, 9 deletions

diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index 52359397371..5e67610cbd3 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -650,10 +650,8 @@ void GotSection::finalizeContents() {
 
 bool GotSection::empty() const {
   // We need to emit a GOT even if it's empty if there's a relocation that is
-  // relative to GOT(such as GOTOFFREL) or there's a symbol that points to a GOT
-  // (i.e. _GLOBAL_OFFSET_TABLE_) that the target defines relative to the .got.
-  return NumEntries == 0 && !HasGotOffRel &&
-         !(ElfSym::GlobalOffsetTable && !Target->GotBaseSymInGotPlt);
+  // relative to GOT(such as GOTOFFREL).
+  return NumEntries == 0 && !HasGotOffRel;
 }
 
 void GotSection::writeTo(uint8_t *Buf) {
@@ -1114,11 +1112,9 @@ void GotPltSection::writeTo(uint8_t *Buf) {
 }
 
 bool GotPltSection::empty() const {
-  // We need to emit a GOT.PLT even if it's empty if there's a symbol that
-  // references the _GLOBAL_OFFSET_TABLE_ and the Target defines the symbol
-  // relative to the .got.plt section.
-  return Entries.empty() &&
-         !(ElfSym::GlobalOffsetTable && Target->GotBaseSymInGotPlt);
+  // We need to emit GOTPLT even if it's empty if there's a relocation relative
+  // to it.
+  return Entries.empty() && !HasGotPltOffRel;
 }
 
 static StringRef getIgotPltName() {