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

[MaskRay]

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.

[MaskRay]

Add objdump -d to test zero displacement

[ruiu]

LGTM

ELF/InputSection.cpp

618–620

case R_DTPREL can be moved here.

[MaskRay]

Address review comment case R_DTPREL can be moved here.

[MaskRay]

Update description

[ruiu]

ELF/InputSection.cpp

618

I mean you could merge with this expression as this is the same expression as yours.

[MaskRay]

ELF/InputSection.cpp

618

It will be changed. In PPC64 and MIPS there is an offset.

[ruiu]

OK, then moving just below

ELF/InputSection.cpp

618

OK, but then moving this expression here doesn't make sense. I guess my point is to not make the code too future-proof. You can merge them and split it later, so that the code doesn't odd when you submit this. It's already committed, and it's not important, so I'm fine with this code for now though.

[MaskRay]

ELF/InputSection.cpp

618

I have realized the problem after I committed it. I will fix it soon when I create the patch for PPC.

It may look like return Sym.getVA(A) + getDTPOffset();

ELF/Arch/X86.cpp

RelExpr X86::getRelExpr(RelType Type, const Symbol &S,
   if (Type == R_386_TLS_LE || Type == R_386_TLS_LE_32 || Type == R_386_TLS_IE ||
       Type == R_386_TLS_GOTIE)
     Config->HasStaticTlsModel = true;
 
   switch (Type) {
   case R_386_8:
   case R_386_16:
   case R_386_32:
-  case R_386_TLS_LDO_32:
     return R_ABS;
+  case R_386_TLS_LDO_32:
+    return R_DTPREL;
   case R_386_TLS_GD:
     return R_TLSGD_GOTPLT;
   case R_386_TLS_LDM:
     return R_TLSLD_GOTPLT;
   case R_386_PLT32:
     return R_PLT_PC;
   case R_386_PC8:
   case R_386_PC16:

ELF/Arch/X86_64.cpp

if (Type == R_X86_64_GOTTPOFF)
     Config->HasStaticTlsModel = true;
 
   switch (Type) {
   case R_X86_64_8:
   case R_X86_64_16:
   case R_X86_64_32:
   case R_X86_64_32S:
   case R_X86_64_64:
+    return R_ABS;
   case R_X86_64_DTPOFF32:
   case R_X86_64_DTPOFF64:
-    return R_ABS;
+    return R_DTPREL;
   case R_X86_64_TPOFF32:
     return R_TLS;
   case R_X86_64_TLSLD:
     return R_TLSLD_PC;
   case R_X86_64_TLSGD:
     return R_TLSGD_PC;
   case R_X86_64_SIZE32:
   case R_X86_64_SIZE64:

ELF/InputSection.cpp

 }
 
 static uint64_t getRelocTargetVA(const InputFile *File, RelType Type, int64_t A,
                                  uint64_t P, const Symbol &Sym, RelExpr Expr) {
   switch (Expr) {
   case R_ABS:
   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:
     return Sym.getVA(A) - getARMStaticBase(Sym);
   case R_GOT:
   case R_RELAX_TLS_GD_TO_IE_ABS:
     return Sym.getGotVA() + A;
   case R_GOTONLY_PC:
for (const RelTy &Rel : Rels) {
     if (!RelTy::IsRela)
       Addend += Target->getImplicitAddend(BufLoc, Type);
 
     Symbol &Sym = getFile<ELFT>()->getRelocTargetSym(Rel);
     RelExpr Expr = Target->getRelExpr(Type, Sym, BufLoc);
     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) + "'";
       if (Expr != R_PC) {
         error(Msg);
         return;
       }
 

ELF/Relocations.h

 using RelType = uint32_t;
 
 // List of target-independent relocation types. Relocations read
 // from files are converted to these types so that the main code
 // doesn't have to know about architecture-specific details.
 enum RelExpr {
   R_ABS,
   R_ADDEND,
+  R_DTPREL,
   R_GOT,
   R_GOT_OFF,
   R_GOT_PC,
   R_GOTONLY_PC,
   R_GOTPLTONLY_PC,
   R_GOTPLT,
   R_GOTPLTREL,
   R_GOTREL,

ELF/Relocations.cpp

if (oneof<R_TLSLD_GOT, R_TLSLD_GOTPLT, R_TLSLD_PC, R_TLSLD_HINT>(
     if (In.Got->addTlsIndex())
       In.RelaDyn->addReloc(Target->TlsModuleIndexRel, In.Got,
                            In.Got->getTlsIndexOff(), nullptr);
     C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
     return 1;
   }
 
   // Local-Dynamic relocs can be relaxed to Local-Exec.
-  if (Expr == R_ABS && !Config->Shared) {
+  // TODO Delete R_ABS after all R_*_DTPREL_* relocations migrate to R_DTPREL.
+  if ((Expr == R_ABS || Expr == R_DTPREL) && !Config->Shared) {
     C.Relocations.push_back(
         {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_LD_TO_LE), Type,
          Offset, Addend, &Sym});
     return 1;
   }
 
   // Local-Dynamic sequence where offset of tls variable relative to dynamic
   // thread pointer is stored in the got.

 // non-interposable function in the same executable. This function
 // will return true for such relocation.
 //
 // If this function returns false, that means we need to emit a
 // dynamic relocation so that the relocation will be fixed at load-time.
 static bool isStaticLinkTimeConstant(RelExpr E, RelType Type, const Symbol &Sym,
                                      InputSectionBase &S, uint64_t RelOff) {
   // These expressions always compute a constant
-  if (oneof<R_GOTPLT, R_GOT_OFF, R_HEXAGON_GOT, R_TLSLD_GOT_OFF,
+  if (oneof<R_DTPREL, 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_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))
+            R_AARCH64_GOT_PAGE_PC, R_GOT_PC, R_GOTONLY_PC, 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;
 
   // These never do, except if the entire file is position dependent or if
   // only the low bits are used.
   if (E == R_GOT || E == R_PLT || E == R_TLSDESC)
     return Target->usesOnlyLowPageBits(Type) || !Config->Pic;
 
   if (Sym.IsPreemptible)

test/ELF/i386-tls-ld-preemptable.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=i386 %s -o %t.o
+# RUN: ld.lld %t.o -shared -o %t.so
+# RUN: llvm-objdump -d --no-show-raw-insn %t.so | FileCheck %s
+
+# CHECK: 100b:       movl    (%eax), %eax
+
+# We used to error on R_386_TLS_LDO_32 to preemptable symbols.
+# i is STB_GLOBAL and preemptable.
+  leal i@TLSLDM(%ebx), %eax
+  calll __tls_get_addr@PLT
+  movl i@DTPOFF(%eax), %eax # R_386_TLS_LDO_32
+
+.section .tbss,"awT",@nobits
+.globl i
+i:
+  .long 0
+  .size i, 4

test/ELF/x86-64-tls-ld-preemptable.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld %t.o -shared -o %t.so
+# RUN: llvm-objdump -d --no-show-raw-insn %t.so | FileCheck %s
+
+# CHECK:      100c:       leaq    (%rax), %rax
+# CHECK-NEXT: 1013:       movabsq 0, %rax
+
+# We used to error on R_X86_64_DTPOFF{32,64} to preemptable symbols.
+# i is STB_GLOBAL and preemptable.
+  leaq i@TLSLD(%rip), %rdi
+  callq __tls_get_addr@PLT
+  leaq i@DTPOFF(%rax), %rax # R_X86_64_DTPOFF32
+  movabsq i@DTPOFF, %rax # R_X86_64_DTPOFF64
+
+.section .tbss,"awT",@nobits
+.globl i
+i:
+  .long 0
+  .size i, 4