[ELF][X86] Allow R_386_TLS_LDO_32 and R_X86_64_DTPOFF{32,64} to preemptable local-dynamic symbols

Summary:
Fixes PR35242. A simplified reproduce:

    thread_local int i; int f() { return i; }

% {g++,clang++} -fPIC -shared -ftls-model=local-dynamic -fuse-ld=lld a.cc
ld.lld: error: can't create dynamic relocation R_X86_64_DTPOFF32 against symbol: i in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output

In isStaticLinkTimeConstant(), Syn.IsPreemptible is true, so it is not
seen as a constant. The error is then issued in processRelocAux().

A symbol of the local-dynamic TLS model cannot be preempted but it can
preempt symbols of the global-dynamic TLS model in other DSOs.
So it makes some sense that the variable is not static.

This patch fixes the linking error by changing getRelExpr() on
R_386_TLS_LDO_32 and R_X86_64_DTPOFF{32,64} from R_ABS to R_DTPREL.
R_PPC64_DTPREL_* and R_MIPS_TLS_DTPREL_* need similar fixes, but they are not handled in this patch.

As a bonus, we use `if (Expr == R_ABS && !Config->Shared)` to find
ld-to-le opportunities. R_ABS is overloaded here for such STT_TLS symbols.
A dedicated R_DTPREL is clearer.

Differential Revision: https://reviews.llvm.org/D60945

llvm-svn: 358870

1 files changed, 3 insertions, 1 deletions

diff --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index a32015f4b14..060374ad6d4 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -616,6 +616,8 @@ static uint64_t getRelocTargetVA(const InputFile *File, RelType Type, int64_t A,
   case R_RELAX_TLS_LD_TO_LE_ABS:
   case R_RELAX_GOT_PC_NOPIC:
     return Sym.getVA(A);
+  case R_DTPREL:
+    return Sym.getVA(A);
   case R_ADDEND:
     return A;
   case R_ARM_SBREL:
@@ -806,7 +808,7 @@ void InputSection::relocateNonAlloc(uint8_t *Buf, ArrayRef<RelTy> Rels) {
     if (Expr == R_NONE)
       continue;
 
-    if (Expr != R_ABS) {
+    if (Expr != R_ABS && Expr != R_DTPREL) {
       std::string Msg = getLocation<ELFT>(Offset) +
                         ": has non-ABS relocation " + toString(Type) +
                         " against symbol '" + toString(Sym) + "'";