[AArch64] Add -mmark-bti-property flag.

Writing the .note.gnu.property manually is error prone and hard to
maintain in the assembly files.
The -mmark-bti-property is for the assembler to emit the section with the
GNU_PROPERTY_AARCH64_FEATURE_1_BTI. To be used when C/C++ is compiled
with -mbranch-protection=bti.

This patch refactors the .note.gnu.property handling.

Reviewed By: chill, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D81930

Reland with test dependency on aarch64 target.

Author: DanielKristofKiss

clang/include/clang/Driver/Options.td

@@ -2385,6 +2385,9 @@ def mfix_cortex_a53_835769 : Flag<["-"], "mfix-cortex-a53-835769">,
 def mno_fix_cortex_a53_835769 : Flag<["-"], "mno-fix-cortex-a53-835769">,
   Group<m_aarch64_Features_Group>,
   HelpText<"Don't workaround Cortex-A53 erratum 835769 (AArch64 only)">;
+def mmark_bti_property : Flag<["-"], "mmark-bti-property">,
+  Group<m_aarch64_Features_Group>,
+  HelpText<"Add .note.gnu.property with BTI to assembly files (AArch64 only)">;
 foreach i = {1-31} in
   def ffixed_x#i : Flag<["-"], "ffixed-x"#i>, Group<m_Group>,
     HelpText<"Reserve the x"#i#" register (AArch64/RISC-V only)">;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7018,6 +7018,15 @@ void ClangAs::ConstructJob(Compilation &C, const JobAction &JA,
     }
     break;
 
+  case llvm::Triple::aarch64:
+  case llvm::Triple::aarch64_32:
+  case llvm::Triple::aarch64_be:
+    if (Args.hasArg(options::OPT_mmark_bti_property)) {
+      CmdArgs.push_back("-mllvm");
+      CmdArgs.push_back("-aarch64-mark-bti-property");
+    }
+    break;
+
   case llvm::Triple::riscv32:
   case llvm::Triple::riscv64:
     AddRISCVTargetArgs(Args, CmdArgs);

clang/test/Driver/arm64-markbti.S

@@ -0,0 +1,26 @@
+// REQUIRES: aarch64-registered-target
+
+// When -mmark-bti-property is passed the generated file object gets BTI marking.
+// RUN: %clang -target arm64-linux-none -mmark-bti-property -c -o - %s | llvm-readobj -n - | FileCheck -check-prefix=CHECK  -check-prefix=CHECK_GEN %s
+// RUN: %clang -target arm64-linux-none -DNOTE_PRESENT -c %s -o - | llvm-readobj -n - | FileCheck -check-prefix=CHECK  -check-prefix=CHECK_PRESET %s
+// RUN: %clang -target arm64-linux-none -mmark-bti-property -DNOTE_PRESENT -c %s -o - | llvm-readobj -n - | FileCheck -check-prefix=CHECK  -check-prefix=CHECK_PRESET %s
+// RUN: %clang -target arm64-linux-none -mmark-bti-property -DNOTE_PRESENT -c %s -o - 2>&1 |  FileCheck -check-prefix=CHECK_WARNING %s
+//
+// CHECK_WARNING: The .note.gnu.property is not emitted because it is already present.
+// CHECK: Name: .note.gnu.property
+// CHECK: Type: NT_GNU_PROPERTY_TYPE_0
+// CHECK_GEN: aarch64 feature: BTI
+// CHECK_PRESET: aarch64 feature: BTI, PAC
+
+#ifdef NOTE_PRESENT
+  .section .note.gnu.property, "a";
+  .balign 8;
+  .long 4;
+  .long 0x10;
+  .long 0x5
+  .asciz "GNU"
+  .long 0xc0000000
+  .long 4
+  .long 3
+  .long 0
+#endif

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

@@ -223,26 +223,9 @@ void AArch64AsmPrinter::emitStartOfAsmFile(Module &M) {
     return;
 
   // Emit a .note.gnu.property section with the flags.
-  MCSection *Cur = OutStreamer->getCurrentSectionOnly();
-  MCSection *Nt = MMI->getContext().getELFSection(
-      ".note.gnu.property", ELF::SHT_NOTE, ELF::SHF_ALLOC);
-  OutStreamer->SwitchSection(Nt);
-
-  // Emit the note header.
-  emitAlignment(Align(8));
-  OutStreamer->emitInt32(4);     // data size for "GNU\0"
-  OutStreamer->emitInt32(4 * 4); // Elf_Prop size
-  OutStreamer->emitInt32(ELF::NT_GNU_PROPERTY_TYPE_0);
-  OutStreamer->emitBytes(StringRef("GNU", 4)); // note name
-
-  // Emit the PAC/BTI properties.
-  OutStreamer->emitInt32(ELF::GNU_PROPERTY_AARCH64_FEATURE_1_AND);
-  OutStreamer->emitInt32(4);     // data size
-  OutStreamer->emitInt32(Flags); // data
-  OutStreamer->emitInt32(0);     // pad
-
-  OutStreamer->endSection(Nt);
-  OutStreamer->SwitchSection(Cur);
+  if (auto *TS = static_cast<AArch64TargetStreamer *>(
+          OutStreamer->getTargetStreamer()))
+    TS->emitNoteSection(Flags);
 }
 
 void AArch64AsmPrinter::emitFunctionHeaderComment() {

llvm/lib/Target/AArch64/MCTargetDesc/AArch64TargetStreamer.cpp

@@ -11,12 +11,23 @@
 //===----------------------------------------------------------------------===//
 
 #include "AArch64TargetStreamer.h"
+#include "AArch64MCAsmInfo.h"
+#include "AArch64Subtarget.h"
+#include "llvm/BinaryFormat/ELF.h"
 #include "llvm/MC/ConstantPools.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCSection.h"
+#include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCSubtargetInfo.h"
+#include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
 
+static cl::opt<bool> MarkBTIProperty(
+    "aarch64-mark-bti-property", cl::Hidden,
+    cl::desc("Add .note.gnu.property with BTI to assembly files"),
+    cl::init(false));
+
 //
 // AArch64TargetStreamer Implemenation
 //
@@ -37,8 +48,50 @@ void AArch64TargetStreamer::emitCurrentConstantPool() {
   ConstantPools->emitForCurrentSection(Streamer);
 }
 
-// finish() - write out any non-empty assembler constant pools.
-void AArch64TargetStreamer::finish() { ConstantPools->emitAll(Streamer); }
+// finish() - write out any non-empty assembler constant pools and
+//   write out note.gnu.properties if need.
+void AArch64TargetStreamer::finish() {
+  ConstantPools->emitAll(Streamer);
+
+  if (MarkBTIProperty)
+    emitNoteSection(ELF::GNU_PROPERTY_AARCH64_FEATURE_1_BTI);
+}
+
+void AArch64TargetStreamer::emitNoteSection(unsigned Flags) {
+  if (Flags == 0)
+    return;
+
+  MCStreamer &OutStreamer = getStreamer();
+  MCContext &Context = OutStreamer.getContext();
+  // Emit a .note.gnu.property section with the flags.
+  MCSectionELF *Nt = Context.getELFSection(".note.gnu.property", ELF::SHT_NOTE,
+                                           ELF::SHF_ALLOC);
+  if (Nt->isRegistered()) {
+    SMLoc Loc;
+    Context.reportWarning(
+        Loc,
+        "The .note.gnu.property is not emitted because it is already present.");
+    return;
+  }
+  MCSection *Cur = OutStreamer.getCurrentSectionOnly();
+  OutStreamer.SwitchSection(Nt);
+
+  // Emit the note header.
+  OutStreamer.emitValueToAlignment(Align(8).value());
+  OutStreamer.emitIntValue(4, 4);     // data size for "GNU\0"
+  OutStreamer.emitIntValue(4 * 4, 4); // Elf_Prop size
+  OutStreamer.emitIntValue(ELF::NT_GNU_PROPERTY_TYPE_0, 4);
+  OutStreamer.emitBytes(StringRef("GNU", 4)); // note name
+
+  // Emit the PAC/BTI properties.
+  OutStreamer.emitIntValue(ELF::GNU_PROPERTY_AARCH64_FEATURE_1_AND, 4);
+  OutStreamer.emitIntValue(4, 4);     // data size
+  OutStreamer.emitIntValue(Flags, 4); // data
+  OutStreamer.emitIntValue(0, 4);     // pad
+
+  OutStreamer.endSection(Nt);
+  OutStreamer.SwitchSection(Cur);
+}
 
 void AArch64TargetStreamer::emitInst(uint32_t Inst) {
   char Buffer[4];

llvm/lib/Target/AArch64/MCTargetDesc/AArch64TargetStreamer.h

@@ -33,6 +33,9 @@ class AArch64TargetStreamer : public MCTargetStreamer {
   /// Emit contents of constant pool for the current section.
   void emitCurrentConstantPool();
 
+  /// Callback used to implement the .note.gnu.property section.
+  void emitNoteSection(unsigned Flags);
+
   /// Callback used to implement the .inst directive.
   virtual void emitInst(uint32_t Inst);