[llvm] [BOLT][AArch64] Handle OpNegateRAState to enable optimizing binaries with pac-ret hardening (PR #120064)

[Gergely Bálint via llvm-commits]

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+# Optimizing binaries with pac-ret hardening
+
+This is a design document about processing the `DW_CFA_AARCH64_negate_ra_state` DWARF instruction in BOLT. As is describes internal design decisions, the intended audiance is BOLT developers. The document is an updated version of the [RFC posted on the LLVM Discourse](https://discourse.llvm.org/t/rfc-bolt-aarch64-handle-opnegaterastate-to-enable-optimizing-binaries-with-pac-ret-hardening/86594).
+
+
+`DW_CFA_AARCH64_negate_ra_state` is also referred to as  `.cfi_negate_ra_state` in assembly, or `OpNegateRAState` is BOLT sources. In this document, I will use **negate-ra-state** as a shorthand.
+
+## Introduction
+
+### Pointer Authentication
+
+Refer to the [pac-ret section of the BOLT-binary-analysis document](BinaryAnalysis.md#pac-ret-analysis).
+
+### DW_CFA_AARCH64_negate_ra_state
+
+The negate-ra-state CFI is a vendor-specific Call Frame Instruction defined in the [Arm ABI](https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst#id1).
+
+```
+The DW_CFA_AARCH64_negate_ra_state operation negates bit[0] of the RA_SIGN_STATE pseudo-register.
+```
+
+This bit indicates to the unwinder whether the current return address is signed or not (hence the name). The unwinder uses this information to authenticate the pointer, and remove the Pointer Authentication Code (PAC) bits. Incorrect negate-ra-state placement can lead to the unwinder trying to authenticate an unsigned pointer (which segfaults), or skipping authenticating a signed pointer, and trying to access an incorrect location (also leading to a segfault).
+
+(Note: not *all* unwinders do this. Some use the `xpac` instruction to strip the PAC bits without authenticating the pointer. This is incorrect, as it allows control-flow modification in the case of unwinding.)
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bgergely0 wrote:

It is an _incorrect_ implementation of control flow integrity, as it leaves a gap (unwinding) in the integrity enforcing. So not incorrect as "possibly segfaulting". Could be called bad practise as well.

https://github.com/llvm/llvm-project/pull/120064