[Lldb-commits] [lldb] [lldb] Restore register state if PrepareTrivialCall fails (PR #129038)

[David Spickett via lldb-commits]

https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/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 unlikely to work.

When I added GCS, 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 such 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.

>From 969124c6e28338e2a386852232da18cf432ae0d2 Mon Sep 17 00:00:00 2001
From: David Spickett <david.spickett at linaro.org>
Date: Wed, 26 Feb 2025 10:01:39 +0000
Subject: [PATCH] [lldb] Restore register state if PrepareTrivialCall fails

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
unlikely to work.

When I added GCS, 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
such 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.
---
 lldb/source/Plugins/ABI/AArch64/ABISysV_arm64.cpp |  7 +------
 lldb/source/Target/ThreadPlanCallFunction.cpp     | 13 ++++++++++++-
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/lldb/source/Plugins/ABI/AArch64/ABISysV_arm64.cpp b/lldb/source/Plugins/ABI/AArch64/ABISysV_arm64.cpp
index 280ec5ba37100..eb7c3a429f03d 100644
--- a/lldb/source/Plugins/ABI/AArch64/ABISysV_arm64.cpp
+++ b/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 when the ThreadPlan is destroyed.
     return Status("Failed to write new Guarded Control Stack entry.");
   }
 
diff --git a/lldb/source/Target/ThreadPlanCallFunction.cpp b/lldb/source/Target/ThreadPlanCallFunction.cpp
index 50dcb66b9719f..218111d4faf60 100644
--- a/lldb/source/Target/ThreadPlanCallFunction.cpp
+++ b/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();
     }