[Dwarf/AArch64] Return address signing B key dwarf support

- When signing return addresses with -msign-return-address=<scope>{+<key>},
  either the A key instructions or the B key instructions can be used. To
  correctly authenticate the return address, the unwinder/debugger must know
  which key was used to sign the return address.
- When and exception is thrown or a break point reached, it may be necessary to
  unwind the stack. To accomplish this, the unwinder/debugger must be able to
  first authenticate an the return address if it has been signed.
- To enable this, the augmentation string of CIEs has been extended to allow
  inclusion of a 'B' character. Functions that are signed using the B key
  variant of the instructions should have and FDE whose associated CIE has a 'B'
  in the augmentation string.
- One must also be able to preserve these semantics when first stepping from a
  high level language into assembly and then, as a second step, into an object
  file. To achieve this, I have introduced a new assembly directive
  '.cfi_b_key_frame ', that tells the assembler the current frame uses return
  address signing with the B key.
- This ensures that the FDE is associated with a CIE that has 'B' in the
  augmentation string.

Differential Revision: https://reviews.llvm.org/D51798

llvm-svn: 349895

1 files changed, 9 insertions, 4 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
index 800dd449b49..538a8d7e8fb 100644
--- a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
@@ -816,10 +816,15 @@ void AArch64FrameLowering::emitPrologue(MachineFunction &MF,
   DebugLoc DL;
 
   if (ShouldSignReturnAddress(MF)) {
-    BuildMI(
-        MBB, MBBI, DL,
-        TII->get(ShouldSignWithAKey(MF) ? AArch64::PACIASP : AArch64::PACIBSP))
-        .setMIFlag(MachineInstr::FrameSetup);
+    if (ShouldSignWithAKey(MF))
+      BuildMI(MBB, MBBI, DL, TII->get(AArch64::PACIASP))
+          .setMIFlag(MachineInstr::FrameSetup);
+    else {
+      BuildMI(MBB, MBBI, DL, TII->get(AArch64::EMITBKEY))
+          .setMIFlag(MachineInstr::FrameSetup);
+      BuildMI(MBB, MBBI, DL, TII->get(AArch64::PACIBSP))
+          .setMIFlag(MachineInstr::FrameSetup);
+    }
 
     unsigned CFIIndex =
         MF.addFrameInst(MCCFIInstruction::createNegateRAState(nullptr));