[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, 13 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp b/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
index a65ddd478f4..384b474abe8 100644
--- a/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ b/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -176,6 +176,7 @@ private:
   bool parseDirectiveReq(StringRef Name, SMLoc L);
   bool parseDirectiveUnreq(SMLoc L);
   bool parseDirectiveCFINegateRAState();
+  bool parseDirectiveCFIBKeyFrame();
 
   bool validateInstruction(MCInst &Inst, SMLoc &IDLoc,
                            SmallVectorImpl<SMLoc> &Loc);
@@ -5030,6 +5031,8 @@ bool AArch64AsmParser::ParseDirective(AsmToken DirectiveID) {
     parseDirectiveInst(Loc);
   else if (IDVal == ".cfi_negate_ra_state")
     parseDirectiveCFINegateRAState();
+  else if (IDVal == ".cfi_b_key_frame")
+    parseDirectiveCFIBKeyFrame();
   else if (IsMachO) {
     if (IDVal == MCLOHDirectiveName())
       parseDirectiveLOH(IDVal, Loc);
@@ -5410,6 +5413,16 @@ bool AArch64AsmParser::parseDirectiveCFINegateRAState() {
   return false;
 }
 
+/// parseDirectiveCFIBKeyFrame
+/// ::= .cfi_b_key
+bool AArch64AsmParser::parseDirectiveCFIBKeyFrame() {
+  if (parseToken(AsmToken::EndOfStatement,
+                 "unexpected token in '.cfi_b_key_frame'"))
+    return true;
+  getStreamer().EmitCFIBKeyFrame();
+  return false;
+}
+
 bool
 AArch64AsmParser::classifySymbolRef(const MCExpr *Expr,
                                     AArch64MCExpr::VariantKind &ELFRefKind,