[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, 15 insertions, 11 deletions

diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
index ceb9a8b91cc..354533504a6 100644
--- a/lld/ELF/Relocations.cpp
+++ b/lld/ELF/Relocations.cpp
@@ -229,7 +229,7 @@ handleTlsRelocation(RelType Type, Symbol &Sym, InputSectionBase &C,
     return 1;
   }
 
-  if (oneof<R_TLSLD_GOT, R_TLSLD_GOT_FROM_END, R_TLSLD_PC, R_TLSLD_HINT>(
+  if (oneof<R_TLSLD_GOT, R_TLSLD_GOTPLT, R_TLSLD_PC, R_TLSLD_HINT>(
           Expr)) {
     // Local-Dynamic relocs can be relaxed to Local-Exec.
     if (!Config->Shared) {
@@ -274,7 +274,7 @@ handleTlsRelocation(RelType Type, Symbol &Sym, InputSectionBase &C,
   }
 
   if (oneof<R_TLSDESC, R_AARCH64_TLSDESC_PAGE, R_TLSDESC_CALL, R_TLSGD_GOT,
-            R_TLSGD_GOT_FROM_END, R_TLSGD_PC>(Expr)) {
+            R_TLSGD_GOTPLT, R_TLSGD_PC>(Expr)) {
     if (Config->Shared) {
       if (In.Got->addDynTlsEntry(Sym)) {
         uint64_t Off = In.Got->getGlobalDynOffset(Sym);
@@ -314,7 +314,7 @@ handleTlsRelocation(RelType Type, Symbol &Sym, InputSectionBase &C,
 
   // Initial-Exec relocs can be relaxed to Local-Exec if the symbol is locally
   // defined.
-  if (oneof<R_GOT, R_GOT_FROM_END, R_GOT_PC, R_AARCH64_GOT_PAGE_PC, R_GOT_OFF,
+  if (oneof<R_GOT, R_GOTPLT, R_GOT_PC, R_AARCH64_GOT_PAGE_PC, R_GOT_OFF,
             R_TLSIE_HINT>(Expr) &&
       !Config->Shared && !Sym.IsPreemptible) {
     C.Relocations.push_back({R_RELAX_TLS_IE_TO_LE, Type, Offset, Addend, &Sym});
@@ -376,13 +376,13 @@ static bool needsPlt(RelExpr Expr) {
 static bool needsGot(RelExpr Expr) {
   return oneof<R_GOT, R_GOT_OFF, R_HEXAGON_GOT, R_MIPS_GOT_LOCAL_PAGE,
                R_MIPS_GOT_OFF, R_MIPS_GOT_OFF32, R_AARCH64_GOT_PAGE_PC,
-               R_GOT_PC, R_GOT_FROM_END>(Expr);
+               R_GOT_PC, R_GOTPLT>(Expr);
 }
 
 // True if this expression is of the form Sym - X, where X is a position in the
 // file (PC, or GOT for example).
 static bool isRelExpr(RelExpr Expr) {
-  return oneof<R_PC, R_GOTREL, R_GOTREL_FROM_END, R_MIPS_GOTREL, R_PPC_CALL,
+  return oneof<R_PC, R_GOTREL, R_GOTPLTREL, R_MIPS_GOTREL, R_PPC_CALL,
                R_PPC_CALL_PLT, R_AARCH64_PAGE_PC, R_RELAX_GOT_PC>(Expr);
 }
 
@@ -398,11 +398,11 @@ static bool isRelExpr(RelExpr Expr) {
 static bool isStaticLinkTimeConstant(RelExpr E, RelType Type, const Symbol &Sym,
                                      InputSectionBase &S, uint64_t RelOff) {
   // These expressions always compute a constant
-  if (oneof<R_GOT_FROM_END, R_GOT_OFF, R_HEXAGON_GOT, R_TLSLD_GOT_OFF,
+  if (oneof<R_GOTPLT, R_GOT_OFF, R_HEXAGON_GOT, R_TLSLD_GOT_OFF,
             R_MIPS_GOT_LOCAL_PAGE, R_MIPS_GOTREL, R_MIPS_GOT_OFF,
             R_MIPS_GOT_OFF32, R_MIPS_GOT_GP_PC, R_MIPS_TLSGD,
             R_AARCH64_GOT_PAGE_PC, R_GOT_PC, R_GOTONLY_PC,
-            R_GOTONLY_PC_FROM_END, R_PLT_PC, R_TLSGD_GOT, R_TLSGD_GOT_FROM_END,
+            R_GOTPLTONLY_PC, R_PLT_PC, R_TLSGD_GOT, R_TLSGD_GOTPLT,
             R_TLSGD_PC, R_PPC_CALL_PLT, R_TLSDESC_CALL, R_AARCH64_TLSDESC_PAGE,
             R_HINT, R_TLSLD_HINT, R_TLSIE_HINT>(E))
     return true;
@@ -1065,11 +1065,15 @@ static void scanReloc(InputSectionBase &Sec, OffsetGetter &GetOffset, RelTy *&I,
       Expr = fromPlt(Expr);
   }
 
-  // This relocation does not require got entry, but it is relative to got and
-  // needs it to be created. Here we request for that.
-  if (oneof<R_GOTONLY_PC, R_GOTONLY_PC_FROM_END, R_GOTREL, R_GOTREL_FROM_END,
-            R_PPC_TOC>(Expr))
+  // If the relocation does not emit a GOT or GOTPLT entry but its computation
+  // uses their addresses, we need GOT or GOTPLT to be created.
+  //
+  // The 4 types that relative GOTPLT are all x86 and x86-64 specific.
+  if (oneof<R_GOTPLTONLY_PC, R_GOTPLTREL, R_GOTPLT, R_TLSGD_GOTPLT>(Expr)) {
+    In.GotPlt->HasGotPltOffRel = true;
+  } else if (oneof<R_GOTONLY_PC, R_GOTREL, R_PPC_TOC>(Expr)) {
     In.Got->HasGotOffRel = true;
+  }
 
   // Read an addend.
   int64_t Addend = computeAddend<ELFT>(Rel, End, Sec, Expr, Sym.isLocal());