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

Authored by LukeCheeseman on Dec 21 2018, 2:45 AM.

Description

[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