[lldb] Restore register state if PrepareTrivialCall fails (#129038)

Fixes #124269

PrepareTrivalCall always had the possibility of failing, but given that
it only wrote to general purpose registers, if it did, you had bigger
problems.

When it failed, we did not mark the thread plan valid and when it was
torn down we didn't try to restore the register state. This meant that
if you tried to continue, the program was unlikely to work.

When I added AArch64 GCS support, I needed to handle the situation where
the GCS pointer points to unmapped memory and we fail to write the extra
entry we need. So I added code to restore the gcspr_el0 register
specifically if this happened, and ordered the operations so that we
tried this first.

In this change I've made the teardown of an invalid thread plan restore
the register state if one was saved. It may be there isn't one if
ConstructorSetup fails, but this is ok because that function does not
modify anything.

Now that we're doing that, I don't need the GCS specific code anymore,
and all thread plans are protected from this in the rare event something
does fail.

Testing is done by the existing GCS test case that points the gcspr into
unmapped memory which causes PrepareTrivialCall to fail. I tried adding
a simulated test using a mock gdb server. This was not possible because
they all use DynamicLoaderStatic which disables all JIT features.

Author: DavidSpickett

lldb/source/Plugins/ABI/AArch64/ABISysV_arm64.cpp

@@ -102,12 +102,7 @@ static Status PushToLinuxGuardedControlStack(addr_t return_addr,
   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.
-    reg_ctx->WriteRegisterFromUnsigned(gcspr_el0_info, gcspr_el0 + 8);
+    // gcspr_el0 will be restored by the ThreadPlan's DoTakedown.
     return Status("Failed to write new Guarded Control Stack entry.");
   }
 
@@ -150,8 +145,6 @@ bool ABISysV_arm64::PrepareTrivialCall(Thread &thread, addr_t sp,
   if (args.size() > 8)
     return false;
 
-  // Do this first, as it's got the most chance of failing (though still very
-  // low).
   if (GetProcessSP()->GetTarget().GetArchitecture().GetTriple().isOSLinux()) {
     Status err = PushToLinuxGuardedControlStack(return_addr, reg_ctx, thread);
     // If we could not manage the GCS, the expression will certainly fail,

lldb/source/Target/ThreadPlanCallFunction.cpp

@@ -174,8 +174,20 @@ void ThreadPlanCallFunction::ReportRegisterState(const char *message) {
 
 void ThreadPlanCallFunction::DoTakedown(bool success) {
   Log *log = GetLog(LLDBLog::Step);
+  Thread &thread = GetThread();
 
   if (!m_valid) {
+    // If ConstructorSetup was succesfull but PrepareTrivialCall was not,
+    // we will have a saved register state and potentially modified registers.
+    // Restore those.
+    if (m_stored_thread_state.register_backup_sp)
+      if (!thread.RestoreRegisterStateFromCheckpoint(m_stored_thread_state))
+        LLDB_LOGF(
+            log,
+            "ThreadPlanCallFunction(%p): Failed to restore register state from "
+            "invalid plan that contained a saved register state.",
+            static_cast<void *>(this));
+
     // Don't call DoTakedown if we were never valid to begin with.
     LLDB_LOGF(log,
               "ThreadPlanCallFunction(%p): Log called on "
@@ -185,7 +197,6 @@ void ThreadPlanCallFunction::DoTakedown(bool success) {
   }
 
   if (!m_takedown_done) {
-    Thread &thread = GetThread();
     if (success) {
       SetReturnValue();
     }