[AArch64][PAC][MC][ELF] Support PAuth ABI compatibility tag

Emit `GNU_PROPERTY_AARCH64_FEATURE_PAUTH` property in
`.note.gnu.property` section depending on
`aarch64-elf-pauthabi-platform` and `aarch64-elf-pauthabi-version` llvm
module flags.

Author: kovdan01

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

@@ -265,6 +265,8 @@ void AArch64AsmPrinter::emitStartOfAsmFile(Module &M) {
         S, MCConstantExpr::create(Feat00Value, MMI->getContext()));
   }
 
+  // TODO: enhance naming to distinguish MachO ptrauth ABI version and ELF
+  // pauthabi platform and version
   if (TM.getTargetTriple().isOSBinFormatMachO())
     EmitPtrAuthVersion(M);
 
@@ -283,13 +285,24 @@ void AArch64AsmPrinter::emitStartOfAsmFile(Module &M) {
     if (Sign->getZExtValue())
       Flags |= ELF::GNU_PROPERTY_AARCH64_FEATURE_1_PAC;
 
-  if (Flags == 0)
-    return;
+  uint64_t PAuthABIPlatform = -1;
+  if (const auto *PAP = mdconst::extract_or_null<ConstantInt>(
+          M.getModuleFlag("aarch64-elf-pauthabi-platform")))
+    PAuthABIPlatform = PAP->getZExtValue();
+  uint64_t PAuthABIVersion = -1;
+  if (const auto *PAV = mdconst::extract_or_null<ConstantInt>(
+          M.getModuleFlag("aarch64-elf-pauthabi-version")))
+    PAuthABIVersion = PAV->getZExtValue();
+
+  if ((PAuthABIPlatform == uint64_t(-1)) != (PAuthABIVersion == uint64_t(-1)))
+    report_fatal_error(
+        "either both or no 'aarch64-elf-pauthabi-platform' and "
+        "'aarch64-elf-pauthabi-version' module flags must be present");
 
   // Emit a .note.gnu.property section with the flags.
   auto *TS =
       static_cast<AArch64TargetStreamer *>(OutStreamer->getTargetStreamer());
-  TS->emitNoteSection(Flags);
+  TS->emitNoteSection(Flags, PAuthABIPlatform, PAuthABIVersion);
 }
 
 void AArch64AsmPrinter::emitFunctionHeaderComment() {

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

@@ -56,10 +56,20 @@ void AArch64TargetStreamer::emitConstantPools() {
 void AArch64TargetStreamer::finish() {
   if (MarkBTIProperty)
     emitNoteSection(ELF::GNU_PROPERTY_AARCH64_FEATURE_1_BTI);
+  // TODO: do we need to handle GNU_PROPERTY_AARCH64_FEATURE_PAUTH here?
 }
 
-void AArch64TargetStreamer::emitNoteSection(unsigned Flags) {
-  if (Flags == 0)
+void AArch64TargetStreamer::emitNoteSection(unsigned Flags,
+                                            uint64_t PAuthABIPlatform,
+                                            uint64_t PAuthABIVersion) {
+  assert((PAuthABIPlatform == uint64_t(-1)) ==
+         (PAuthABIVersion == uint64_t(-1)));
+  uint64_t DescSz = 0;
+  if (Flags != 0)
+    DescSz += 4 * 4;
+  if (PAuthABIPlatform != uint64_t(-1))
+    DescSz += 4 + 4 + 8 * 2;
+  if (DescSz == 0)
     return;
 
   MCStreamer &OutStreamer = getStreamer();
@@ -80,15 +90,25 @@ void AArch64TargetStreamer::emitNoteSection(unsigned Flags) {
   // Emit the note header.
   OutStreamer.emitValueToAlignment(Align(8));
   OutStreamer.emitIntValue(4, 4);     // data size for "GNU\0"
-  OutStreamer.emitIntValue(4 * 4, 4); // Elf_Prop size
+  OutStreamer.emitIntValue(DescSz, 4); // Elf_Prop array 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
+  if (Flags != 0) {
+    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
+  }
+
+  // Emit the PAuth ABI compatibility info
+  if (PAuthABIPlatform != uint64_t(-1)) {
+    OutStreamer.emitIntValue(ELF::GNU_PROPERTY_AARCH64_FEATURE_PAUTH, 4);
+    OutStreamer.emitIntValue(8 * 2, 4); // data size
+    OutStreamer.emitIntValue(PAuthABIPlatform, 8);
+    OutStreamer.emitIntValue(PAuthABIVersion, 8);
+  }
 
   OutStreamer.endSection(Nt);
   OutStreamer.switchSection(Cur);

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

@@ -35,7 +35,8 @@ class AArch64TargetStreamer : public MCTargetStreamer {
   void emitCurrentConstantPool();
 
   /// Callback used to implement the .note.gnu.property section.
-  void emitNoteSection(unsigned Flags);
+  void emitNoteSection(unsigned Flags, uint64_t PAuthABIPlatform = -1,
+                       uint64_t PAuthABIVersion = -1);
 
   /// Callback used to implement the .inst directive.
   virtual void emitInst(uint32_t Inst);

llvm/test/CodeGen/AArch64/note-gnu-property-elf-pauthabi.ll

@@ -0,0 +1,50 @@
+; RUN: rm -rf %t && split-file %s %t && cd %t
+
+;--- ok.ll
+
+; RUN: llc -mtriple=aarch64-linux ok.ll               -o - | \
+; RUN:   FileCheck %s --check-prefix=ASM
+; RUN: llc -mtriple=aarch64-linux ok.ll -filetype=obj -o - |  \
+; RUN:   llvm-readelf --notes - | FileCheck %s --check-prefix=OBJ
+
+!llvm.module.flags = !{!0, !1}
+
+!0 = !{i32 1, !"aarch64-elf-pauthabi-platform", i32 2}
+!1 = !{i32 1, !"aarch64-elf-pauthabi-version", i32 31}
+
+; ASM: .section .note.gnu.property,"a",@note
+; ASM-NEXT: .p2align 3, 0x0
+; ASM-NEXT: .word 4
+; ASM-NEXT: .word 24
+; ASM-NEXT: .word 5
+; ASM-NEXT: .asciz "GNU"
+; 3221225473 = 0xc0000001 = GNU_PROPERTY_AARCH64_FEATURE_PAUTH
+; ASM-NEXT: .word 3221225473
+; ASM-NEXT: .word 16
+; ASM-NEXT: .xword 2
+; ASM-NEXT: .xword 31
+
+; OBJ: Displaying notes found in: .note.gnu.property
+; OBJ-NEXT:   Owner                 Data size	Description
+; OBJ-NEXT:   GNU                   0x00000018	NT_GNU_PROPERTY_TYPE_0 (property note)
+; OBJ-NEXT:   AArch64 PAuth ABI tag: platform 0x2 (linux), version 0x1f (PointerAuthCalls, PointerAuthReturns, PointerAuthVTPtrAddressDiscrimination, PointerAuthVTPtrTypeDiscrimination, PointerAuthInitFini)
+
+; ERR: either both or no 'aarch64-elf-pauthabi-platform' and 'aarch64-elf-pauthabi-version' module flags must be present
+
+;--- err1.ll
+
+; RUN: not --crash llc -mtriple=aarch64-linux err1.ll 2>&1 -o - | \
+; RUN:   FileCheck %s --check-prefix=ERR
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 1, !"aarch64-elf-pauthabi-platform", i32 2}
+
+;--- err2.ll
+
+; RUN: not --crash llc -mtriple=aarch64-linux err2.ll 2>&1 -o - | \
+; RUN:   FileCheck %s --check-prefix=ERR
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 1, !"aarch64-elf-pauthabi-version", i32 31}