[LLD][ARM] Support for R_ARM_TARGET2 relocation

[peter.smith]

Summary

The R_ARM_TARGET2 relocation is used in ARM exception tables to encode a data dependency that will only be dereferenced by code in the run-time support library. In a similar way to R_ARM_TARGET1 the handling of the relocation is target specific, it maps to one of R_ARM_ABS32, R_ARM_REL32 or R_ARM_GOT_PREL. The choice depends on the run-time library. R_ARM_GOT_PREL is used for linux and BSD, R_ARM_ABS32 and R_ARM_REL32 are used for bare-metal.

I've implemented R_ARM_TARGET2 in lld using the same command line option as GNU ld https://sourceware.org/binutils/docs/ld/ARM.html#ARM

The default is R_ARM_GOT_PREL as this is the default in Gold and the linux targeting BFD linkers.

[ruiu]

ELF/Driver.cpp

359

You are not using ReportError, so you may want to remove it.

[ruiu]

ELF/Target.cpp

1531–1532

Format.

[peter.smith]

Thanks for the comments (so far).

It is definitely the right thing to do to remove the ReportError so I've made it return the default if nothing arg is given. I've fixed the formatting as well, apologies about not catching that myself.

[ruiu]

LGTM with these changes.

ELF/Driver.cpp

349

auto* -> auto *

ELF/Target.cpp

1567

So the function seems to be used only once, so I'd inline it here and remove getTarget2Expr.

[peter.smith]

Thank you for the review. I've made the suggested changes and committed under 284404

ELF/Config.h

 enum class StripPolicy { None, All, Debug };
 
 // For --unresolved-symbols.
 enum class UnresolvedPolicy { NoUndef, ReportError, Warn, Ignore };
 
 // For --sort-section and linkerscript sorting rules.
 enum class SortSectionPolicy { Default, None, Alignment, Name, Priority };
 
+// For --target2
+enum class Target2Policy { Abs, Rel, GotRel, ReportError };
+
 struct SymbolVersion {
   llvm::StringRef Name;
   bool IsExternCpp;
   bool HasWildcards;
 };
 
 // This struct contains symbols version definition that
 // can be found in version script if it is used for link.
struct Configuration {
   bool ZNow;
   bool ZOrigin;
   bool ZRelro;
   bool ZWxneeded;
   DiscardPolicy Discard;
   SortSectionPolicy SortSection;
   StripPolicy Strip = StripPolicy::None;
   UnresolvedPolicy UnresolvedSymbols;
+  Target2Policy Target2 = Target2Policy::GotRel;
   BuildIdKind BuildId = BuildIdKind::None;
   ELFKind EKind = ELFNoneKind;
   uint16_t DefaultSymbolVersion = llvm::ELF::VER_NDX_GLOBAL;
   uint16_t EMachine = llvm::ELF::EM_NONE;
   uint64_t EntryAddr = 0;
   uint64_t ImageBase;
   uint64_t MaxPageSize;
   uint64_t ZStackSize;

ELF/Driver.cpp

if (S == "ignore-all" || S == "ignore-in-object-files")
       return UnresolvedPolicy::Ignore;
     if (S == "ignore-in-shared-libs" || S == "report-all")
       return UnresolvedPolicy::ReportError;
     error("unknown --unresolved-symbols value: " + S);
   }
   return UnresolvedPolicy::ReportError;
 }
 
+static Target2Policy getTarget2Option(opt::InputArgList &Args) {
+  if (auto* Arg = Args.getLastArg(OPT_target2)) {
+    StringRef S = Arg->getValue();
+    if (S == "rel")
+      return Target2Policy::Rel;
+    if (S == "abs")
+      return Target2Policy::Abs;
+    if (S == "got-rel")
+      return Target2Policy::GotRel;
+    error("unknown --target2 option: " + S);
+  }
+  return Target2Policy::ReportError;
+}
+
 static bool isOutputFormatBinary(opt::InputArgList &Args) {
   if (auto *Arg = Args.getLastArg(OPT_oformat)) {
     StringRef S = Arg->getValue();
     if (S == "binary")
       return true;
     error("unknown --oformat value: " + S);
   }
   return false;
void LinkerDriver::readConfigs(opt::InputArgList &Args) {
 
   for (auto *Arg : Args.filtered(OPT_undefined))
     Config->Undefined.push_back(Arg->getValue());
 
   Config->SortSection = getSortKind(Args);
 
   Config->UnresolvedSymbols = getUnresolvedSymbolOption(Args);
 
+  Config->Target2 = getTarget2Option(Args);
+
   if (auto *Arg = Args.getLastArg(OPT_dynamic_list))
     if (Optional<MemoryBufferRef> Buffer = readFile(Arg->getValue()))
       parseDynamicList(*Buffer);
 
   for (auto *Arg : Args.filtered(OPT_export_dynamic_symbol))
     Config->DynamicList.push_back(Arg->getValue());
 
   if (auto *Arg = Args.getLastArg(OPT_version_script))

ELF/Options.td

 def strip_debug: F<"strip-debug">, HelpText<"Strip debugging information">;
 
 def sysroot: J<"sysroot=">, HelpText<"Set the system root">;
 
 def target1_rel: F<"target1-rel">, HelpText<"Interpret R_ARM_TARGET1 as R_ARM_REL32">;
 
 def target1_abs: F<"target1-abs">, HelpText<"Interpret R_ARM_TARGET1 as R_ARM_ABS32">;
 
+def target2: J<"target2=">, MetaVarName<"<type>">, HelpText<"Interpret R_ARM_TARGET2 as <type>, where <type> is one of rel, abs, or got-rel.">;
+
 def threads: F<"threads">, HelpText<"Enable use of threads">;
 
 def trace: F<"trace">, HelpText<"Print the names of the input files">;
 
 def trace_symbol : J<"trace-symbol=">, HelpText<"Trace references to symbols">;
 
 def undefined: S<"undefined">,
   HelpText<"Force undefined symbol during linking">;

ELF/Target.cpp

ARMTargetInfo::ARMTargetInfo() {
   GotPltEntrySize = 4;
   PltEntrySize = 16;
   PltHeaderSize = 20;
   // ARM uses Variant 1 TLS
   TcbSize = 8;
   NeedsThunks = true;
 }
 
+RelExpr getTarget2Expr()
+{
+  if (Config->Target2 == Target2Policy::Rel)
+    return R_PC;
+  if (Config->Target2 == Target2Policy::Abs)
+    return R_ABS;
+  return R_GOT_PC;
+}
+
 RelExpr ARMTargetInfo::getRelExpr(uint32_t Type, const SymbolBody &S) const {
   switch (Type) {
   default:
     return R_ABS;
   case R_ARM_THM_JUMP11:
     return R_PC;
   case R_ARM_CALL:
   case R_ARM_JUMP24:
case R_ARM_GOT_BREL:
     // GOT(S) + A - GOT_ORG
     return R_GOT_OFF;
   case R_ARM_GOT_PREL:
   case R_ARM_TLS_IE32:
     // GOT(S) + A - P
     return R_GOT_PC;
   case R_ARM_TARGET1:
     return Config->Target1Rel ? R_PC : R_ABS;
+  case R_ARM_TARGET2:
+    return getTarget2Expr();
   case R_ARM_TLS_GD32:
     return R_TLSGD_PC;
   case R_ARM_TLS_LDM32:
     return R_TLSLD_PC;
   case R_ARM_BASE_PREL:
     // B(S) + A - P
     // FIXME: currently B(S) assumed to be .got, this may not hold for all
     // platforms.
case R_ARM_NONE:
     break;
   case R_ARM_ABS32:
   case R_ARM_BASE_PREL:
   case R_ARM_GOTOFF32:
   case R_ARM_GOT_BREL:
   case R_ARM_GOT_PREL:
   case R_ARM_REL32:
   case R_ARM_TARGET1:
+  case R_ARM_TARGET2:
   case R_ARM_TLS_GD32:
   case R_ARM_TLS_IE32:
   case R_ARM_TLS_LDM32:
   case R_ARM_TLS_LDO32:
   case R_ARM_TLS_LE32:
     write32le(Loc, Val);
     break;
   case R_ARM_PREL31:
default:
     return 0;
   case R_ARM_ABS32:
   case R_ARM_BASE_PREL:
   case R_ARM_GOTOFF32:
   case R_ARM_GOT_BREL:
   case R_ARM_GOT_PREL:
   case R_ARM_REL32:
   case R_ARM_TARGET1:
+  case R_ARM_TARGET2:
   case R_ARM_TLS_GD32:
   case R_ARM_TLS_LDM32:
   case R_ARM_TLS_LDO32:
   case R_ARM_TLS_IE32:
   case R_ARM_TLS_LE32:
     return SignExtend64<32>(read32le(Buf));
   case R_ARM_PREL31:
     return SignExtend64<31>(read32le(Buf));

test/ELF/arm-target2.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+// RUN: ld.lld %t.o -o %t 2>&1
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t | FileCheck %s
+// RUN: ld.lld %t.o --target2=got-rel -o %t2 2>&1
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t2 | FileCheck %s
+// RUN: ld.lld %t.o --target2=abs -o %t3 2>&1
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t3 | FileCheck -check-prefix=CHECK-ABS %s
+// RUN: ld.lld %t.o --target2=rel -o %t4 2>&1
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t4 | FileCheck -check-prefix=CHECK-REL %s
+// REQUIRES: arm
+
+// The R_ARM_TARGET2 is present in .ARM.extab sections. It can be handled as
+// either R_ARM_ABS32, R_ARM_REL32 or R_ARM_GOT_PREL. For ARM linux the default
+// is R_ARM_GOT_PREL. The other two options are primarily used for bare-metal,
+// they can be selected with the --target2=abs or --target2=rel option.
+ .syntax unified
+ .text
+ .globl _start
+ .align 2
+_start:
+ .type function, %function
+ .fnstart
+ bx lr
+ .personality   __gxx_personality_v0
+ .handlerdata
+ .word  _ZTIi(TARGET2)
+ .text
+ .fnend
+ .global __gxx_personality_v0
+ .type function, %function
+__gxx_personality_v0:
+ bx lr
+
+ .rodata
+_ZTIi:  .word 0
+
+// CHECK: Contents of section .ARM.extab:
+// 1011c + 1ee4 = 12000 = .got
+// CHECK-NEXT:  10114 f00e0000 b0b0b000 e41e0000
+
+// CHECK-ABS: Contents of section .ARM.extab:
+// 100e8 = .rodata
+// CHECK-ABS-NEXT: 100d4 300f0000 b0b0b000 e8000100
+
+// CHECK-REL: Contents of section .ARM.extab:
+// 100dc + c = 100e8 = .rodata
+// CHECK-REL-NEXT: 100d4 300f0000 b0b0b000 0c000000
+
+// CHECK: Contents of section .rodata:
+// CHECK-NEXT: 10128 00000000
+
+// CHECK-ABS: Contents of section .rodata:
+// CHECK-ABS-NEXT: 100e8 00000000
+
+// CHECK-REL: Contents of section .rodata:
+// CHECK-REL-NEXT: 100e8 00000000
+
+// CHECK: Contents of section .got:
+// 10128 = _ZTIi
+// CHECK-NEXT: 12000 28010100