[Lldb-commits] [lldb] [lldb][AArch64] Fix expression evaluation with Guarded Control Stacks (PR #123918)

[David Spickett via lldb-commits]

================
@@ -60,6 +60,69 @@ ABISysV_arm64::CreateInstance(lldb::ProcessSP process_sp, const ArchSpec &arch)
   return ABISP();
 }
 
+static Status PushToLinuxGuardedControlStack(addr_t return_addr,
+                                             RegisterContext *reg_ctx,
+                                             Thread &thread) {
+  Status err;
+
+  // If the Guarded Control Stack extension is present we may need to put the
+  // return address onto that stack.
+  const RegisterInfo *gcs_features_enabled_info =
+      reg_ctx->GetRegisterInfoByName("gcs_features_enabled");
+  if (!gcs_features_enabled_info)
+    return err;
+
+  uint64_t gcs_features_enabled = reg_ctx->ReadRegisterAsUnsigned(
+      gcs_features_enabled_info, LLDB_INVALID_ADDRESS);
+  if (gcs_features_enabled == LLDB_INVALID_ADDRESS)
+    return Status("Could not read GCS features enabled register.");
+
+  // Only attempt this if GCS is enabled. If it's not enabled then gcspr_el0
+  // may point to unmapped memory.
+  if ((gcs_features_enabled & 1) == 0)
+    return err;
+
+  const RegisterInfo *gcspr_el0_info =
+      reg_ctx->GetRegisterInfoByName("gcspr_el0");
+  if (!gcspr_el0_info)
+    return Status("Could not get register info for gcspr_el0.");
+
+  uint64_t gcspr_el0 =
+      reg_ctx->ReadRegisterAsUnsigned(gcspr_el0_info, LLDB_INVALID_ADDRESS);
+  if (gcspr_el0 == LLDB_INVALID_ADDRESS)
+    return Status("Could not read gcspr_el0.");
+
+  // A link register entry on the GCS is 8 bytes.
+  gcspr_el0 -= 8;
+  if (!reg_ctx->WriteRegisterFromUnsigned(gcspr_el0_info, gcspr_el0))
+    return Status(
+        "Attempted to decrement gcspr_el0, but could not write to it.");
+
+  Status error;
+  size_t wrote = thread.GetProcess()->WriteMemory(gcspr_el0, &return_addr,
+                                                  sizeof(return_addr), error);
+  if ((wrote != sizeof(return_addr) || error.Fail())) {
+    // When PrepareTrivialCall fails, the register context is not restored,
+    // unlike when an expression fails to execute. This is arguably a bug,
+    // see https://github.com/llvm/llvm-project/issues/124269.
+    // For now we are handling this here specifically. We can assume this
+    // write will work as the one to decrement the register did.
----------------
DavidSpickett wrote:

See this comment.

https://github.com/llvm/llvm-project/pull/123918