[Lldb-commits] [lldb] [LLDB] Fix potential data race in Symtab initialization (PR #192753)

[Adrian Prantl via lldb-commits]

https://github.com/adrian-prantl created https://github.com/llvm/llvm-project/pull/192753

Claude pointed out to me that Symtab::FindFunctionSymbols doesn't lock the mutex before checking m_name_indexes_computed and recomputing it. On top of that all the initialization flags are bitfields, which makes any unguarded concurrent accesses UB. Changing them to bools should no longer be necessary after introducing a lock, but several of the public methods trust that their caller holds the lock so I'm opting to remove this footgun just in case.

rdar://174988238

>From cecec0bd259bee13d4d115a68d0d40dc8815a8f5 Mon Sep 17 00:00:00 2001
From: Adrian Prantl <aprantl at apple.com>
Date: Fri, 17 Apr 2026 16:00:49 -0700
Subject: [PATCH] [LLDB] Fix potential data race in Symtab initialization

Claude pointed out to me that Symtab::FindFunctionSymbols doesn't lock
the mutex before checking m_name_indexes_computed and recomputing
it. On top of that all the initialization flags are bitfields, which
makes any unguarded concurrent accesses UB. Changing them to bools
should no longer be necessary after introducing a lock, but several of
the public methods trust that their caller holds the lock so I'm
opting to remove this footgun just in case.

rdar://174988238
---
 lldb/include/lldb/Symbol/Symtab.h | 10 ++++++----
 lldb/source/Symbol/Symtab.cpp     |  1 +
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/lldb/include/lldb/Symbol/Symtab.h b/lldb/include/lldb/Symbol/Symtab.h
index 57627d2dde7d2..9c7173c1153a3 100644
--- a/lldb/include/lldb/Symbol/Symtab.h
+++ b/lldb/include/lldb/Symbol/Symtab.h
@@ -277,10 +277,12 @@ class Symtab {
   /// Maps function names to symbol indices (grouped by FunctionNameTypes)
   std::map<lldb::FunctionNameType, UniqueCStringMap<uint32_t>>
       m_name_to_symbol_indices;
-  mutable std::recursive_mutex
-      m_mutex; // Provide thread safety for this symbol table
-  bool m_file_addr_to_index_computed : 1, m_name_indexes_computed : 1,
-    m_loaded_from_cache : 1, m_saved_to_cache : 1;
+  /// Provide thread safety for this symbol table.
+  mutable std::recursive_mutex m_mutex;
+  bool m_file_addr_to_index_computed = false;
+  bool m_name_indexes_computed = false;
+  bool m_loaded_from_cache = false;
+  bool m_saved_to_cache = false;
 
 private:
   UniqueCStringMap<uint32_t> &
diff --git a/lldb/source/Symbol/Symtab.cpp b/lldb/source/Symbol/Symtab.cpp
index c7cd900f41cbb..57a07e3d70c2f 100644
--- a/lldb/source/Symbol/Symtab.cpp
+++ b/lldb/source/Symbol/Symtab.cpp
@@ -1126,6 +1126,7 @@ void Symtab::FindFunctionSymbols(ConstString name, uint32_t name_type_mask,
     }
   }
 
+  std::lock_guard<std::recursive_mutex> guard(m_mutex);
   if (!m_name_indexes_computed)
     InitNameIndexes();