[compiler-rt] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (#138571) (PR #143230)

[Anatoly Trosinenko via llvm-commits]

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@@ -681,7 +681,18 @@ DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_jumpto)
   // context struct, because it is allocated on the stack, and an exception
   // could clobber the de-allocated portion of the stack after sp has been
   // restored.
-  ldr    x16,     [x0, #0x0F8]
+
+  ldr    x16,     [x0, #0x0F8]  // load sp into scratch
+  ldr    lr,      [x0, #0x100]  // restore pc into lr
+
+#if __has_feature(ptrauth_calls)
+  // The LR is signed with its address inside the register state.  Time
+  // to resign to be a regular ROP signed pointer
+  add    x1, x0, #0x100
+  autib  lr, x1
+  pacib  lr, x16  // signed the scratch register for sp
+#endif
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atrosinenko wrote:

Likewise, we should probably use `autib1716` and `pacib1716` here to retain compatibility with Armv8.2-a.

https://github.com/llvm/llvm-project/pull/143230