[AArch64] Emit PAC/BTI .note.gnu.property flags

This patch make LLVM emit the processor specific program property types
defined in AArch64 ELF spec
https://developer.arm.com/docs/ihi0056/f/elf-for-the-arm-64-bit-architecture-aarch64-abi-2019q2-documentation

A file containing no functions gets both property flags.  Otherwise, a property
is set iff all the functions in the file have the corresponding attribute.

Patch by Daniel Kiss and Momchil Velikov.

Differential Revision: https://reviews.llvm.org/D71019

1 files changed, 60 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
index 7ea7915c2ca..11f3273760d 100644
--- a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
+++ b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
@@ -84,6 +84,7 @@ public:
     return MCInstLowering.lowerOperand(MO, MCOp);
   }
 
+  void EmitStartOfAsmFile(Module &M) override;
   void EmitJumpTableInfo() override;
   void emitJumpTableEntry(const MachineJumpTableInfo *MJTI,
                           const MachineBasicBlock *MBB, unsigned JTI);
@@ -181,6 +182,65 @@ private:
 
 } // end anonymous namespace
 
+void AArch64AsmPrinter::EmitStartOfAsmFile(Module &M) {
+  if (!TM.getTargetTriple().isOSBinFormatELF())
+    return;
+
+  // Assemble feature flags that may require creation of a note section.
+  unsigned Flags = ELF::GNU_PROPERTY_AARCH64_FEATURE_1_BTI |
+                   ELF::GNU_PROPERTY_AARCH64_FEATURE_1_PAC;
+
+  if (any_of(M, [](const Function &F) {
+        return !F.isDeclaration() &&
+               !F.hasFnAttribute("branch-target-enforcement");
+      })) {
+    Flags &= ~ELF::GNU_PROPERTY_AARCH64_FEATURE_1_BTI;
+  }
+
+  if ((Flags & ELF::GNU_PROPERTY_AARCH64_FEATURE_1_BTI) == 0 &&
+      any_of(M, [](const Function &F) {
+        return F.hasFnAttribute("branch-target-enforcement");
+      })) {
+    errs() << "warning: some functions compiled with BTI and some compiled "
+              "without BTI\n"
+           << "warning: not setting BTI in feature flags\n";
+  }
+
+  if (any_of(M, [](const Function &F) {
+        if (F.isDeclaration())
+          return false;
+        Attribute A = F.getFnAttribute("sign-return-address");
+        return !A.isStringAttribute() || A.getValueAsString() == "none";
+      })) {
+    Flags &= ~ELF::GNU_PROPERTY_AARCH64_FEATURE_1_PAC;
+  }
+
+  if (Flags == 0)
+    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->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 AArch64AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI)
 {
   EmitSled(MI, SledKind::FUNCTION_ENTER);