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

Authored by LukeCheeseman on Sep 7 2018, 8:46 AM.


  • 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.

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Event Timeline

LukeCheeseman created this revision.Sep 7 2018, 8:46 AM
  • std::move ExtraAugementation string in constructor
  • tidy up MIR test

Before I review the code itself, have you agreed the .append_augmentation directive with the GCC/GAS developers to make sure we implement it the same way?

If the design isn't yet set in stone, I have a few comments on it:

  • Why doesn't it begin with ".cfi_", while every other related directive does? There's a complete list of them at
  • Why are we adding a generic directive to append to the augmentation string, rather than a directive to select the B key? This looks very similar to .cfi_signal_frame, which also adds one character to the augmentation string.
LukeCheeseman edited the summary of this revision. (Show Details)
  • Replace .append_augmentation <string> with .cfi_b_key_frame
olista01 added inline comments.Sep 20 2018, 2:45 AM
1585 ↗(On Diff #165754)

This emits a pair of double quotes, which that parser doesn't expect. Also, there is no test for this.

1729 ↗(On Diff #165754)

80 columns (I'd suggest using git-clang-format on the patch).

4185 ↗(On Diff #165754)

Outdated comment.

4186 ↗(On Diff #165754)

This should probably be restricted to AArch64, ideally by putting it in the AArch64 asm parser.

olista01 added inline comments.Oct 9 2018, 5:53 AM
1582 ↗(On Diff #167740)

This will cause an assertion if we are not emitting unwind tables, so we need to skip this this as is currently done in AsmPrinter::emitCFIInstruction.

  • Addressed @olista01 comment on triggering an assertion when we don't emit unwind info
  • The emit b key assembly directive isn't emitted when we don't emit unwind info
olista01 accepted this revision.Dec 20 2018, 8:08 AM

LGTM, thanks!

This revision is now accepted and ready to land.Dec 20 2018, 8:08 AM
This revision was automatically updated to reflect the committed changes.

FYI – The following test fails if the ARM 32-bit target is NOT built alongside the 64-bit ARM target: MC/ELF/cfi-b-key-frame.s

FYI – The following test fails if the ARM 32-bit target is NOT built alongside the 64-bit ARM target: MC/ELF/cfi-b-key-frame.s

Ah, thanks! I was just investigating to see what was going on here.

My mistake. Something else is going on. I assumed based on past precedent that this was what was going on. (I'm not sure how many people at ARM are verifying that their AArch64 commits work when the 32-bit target is disabled).

Turns out my setup was busted and I thought I was building/testing AArch64 when I wasn't. I think you need to add // REQUIRES: aarch64-registered-target to the file.