[Lldb-commits] [lldb] [lldb] Improve trampoline function detection (PR #178695)

[Ebuka Ezike via lldb-commits]

https://github.com/da-viper created https://github.com/llvm/llvm-project/pull/178695

The mold linker adds a function symbol to the symbol table (.symtab) for every shared library exported function.
So if we try to step into a library, lldb will fail because the symbol at the address is not a trampline function.

Example:
```cpp
lib.c
int lib_add(int a, int b);
```
when we link the library to an executable, mold will create the normal trampoline functions in the .plt section but add an extra symbol in symbol table `lib_add$plt`.


I cannot think of a way to add another symbol in the trampoline's file address without linking with mold. 
Is there a way to write a test for this ? 

>From 8ef6eac983b5f31ddb75825180e1bd056f1a19d9 Mon Sep 17 00:00:00 2001
From: Ebuka Ezike <yerimyah1 at gmail.com>
Date: Thu, 29 Jan 2026 16:17:05 +0000
Subject: [PATCH] [lldb] Improve trampoline function detection

The mold linker adds a function symbol to the symbol table (.symtab) for
every shared library exported function.
So if we try to step into a library, lldb will fail because the symbol
at the address is not a trampline function.

Example:
```cpp
lib.c
int lib_add(int a, int b);
```
when we link the library to an executable, mold will create the normal
trampoline functions in the .plt section but add an extra symbol in
symbol table `lib_add$plt`.

This patch adds a new method Module::FindSymbolsContainingFileAddress()
to find symbols of a specific type that contain a given file address.
---
 lldb/include/lldb/Core/Module.h               |  4 +++
 lldb/source/Core/Module.cpp                   | 17 ++++++++++++
 .../POSIX-DYLD/DynamicLoaderPOSIXDYLD.cpp     | 27 +++++++++++++++++--
 3 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/lldb/include/lldb/Core/Module.h b/lldb/include/lldb/Core/Module.h
index 643b9a5c3bf54..87d828fb41eed 100644
--- a/lldb/include/lldb/Core/Module.h
+++ b/lldb/include/lldb/Core/Module.h
@@ -265,6 +265,10 @@ class Module : public std::enable_shared_from_this<Module>,
                                   lldb::SymbolType symbol_type,
                                   SymbolContextList &sc_list);
 
+  void FindSymbolsContainingFileAddress(const Address &addr,
+                                        lldb::SymbolType symbol_type,
+                                        SymbolContextList &sc_list);
+
   void FindSymbolsMatchingRegExAndType(
       const RegularExpression &regex, lldb::SymbolType symbol_type,
       SymbolContextList &sc_list,
diff --git a/lldb/source/Core/Module.cpp b/lldb/source/Core/Module.cpp
index 486af1a053344..436c4ce027d84 100644
--- a/lldb/source/Core/Module.cpp
+++ b/lldb/source/Core/Module.cpp
@@ -1316,6 +1316,23 @@ void Module::FindSymbolsWithNameAndType(ConstString name,
   }
 }
 
+void Module::FindSymbolsContainingFileAddress(const Address &addr,
+                                              lldb::SymbolType symbol_type,
+                                              SymbolContextList &sc_list) {
+  if (Symtab *symtab = GetSymtab()) {
+    std::vector<uint32_t> symbol_indexes;
+    symtab->ForEachSymbolContainingFileAddress(
+        addr.GetFileAddress(), [&, symbol_type](Symbol *match_sym) {
+          if (const lldb::SymbolType curr_type = match_sym->GetType();
+              curr_type == lldb::eSymbolTypeAny || curr_type == symbol_type) {
+            symbol_indexes.push_back(symtab->GetIndexForSymbol(match_sym));
+          }
+          return true;
+        });
+    SymbolIndicesToSymbolContextList(symtab, symbol_indexes, sc_list);
+  }
+}
+
 void Module::FindSymbolsMatchingRegExAndType(
     const RegularExpression &regex, SymbolType symbol_type,
     SymbolContextList &sc_list, Mangled::NamePreference mangling_preference) {
diff --git a/lldb/source/Plugins/DynamicLoader/POSIX-DYLD/DynamicLoaderPOSIXDYLD.cpp b/lldb/source/Plugins/DynamicLoader/POSIX-DYLD/DynamicLoaderPOSIXDYLD.cpp
index 6705ac139f0fb..8802b4b7cd27b 100644
--- a/lldb/source/Plugins/DynamicLoader/POSIX-DYLD/DynamicLoaderPOSIXDYLD.cpp
+++ b/lldb/source/Plugins/DynamicLoader/POSIX-DYLD/DynamicLoaderPOSIXDYLD.cpp
@@ -540,11 +540,34 @@ DynamicLoaderPOSIXDYLD::GetStepThroughTrampolinePlan(Thread &thread,
 
   StackFrame *frame = thread.GetStackFrameAtIndex(0).get();
   const SymbolContext &context = frame->GetSymbolContext(eSymbolContextSymbol);
-  const Symbol *sym = context.symbol;
 
-  if (sym == nullptr || !sym->IsTrampoline())
+  Symbol *sym = context.symbol;
+  if (sym == nullptr)
     return thread_plan_sp;
 
+  if (!sym->IsTrampoline()) {
+    if (sym->GetType() != lldb::eSymbolTypeCode)
+      return thread_plan_sp;
+
+    SymbolContextList addr_ctx_list;
+    context.module_sp->FindSymbolsContainingFileAddress(
+        context.GetFunctionOrSymbolAddress(), eSymbolTypeTrampoline,
+        addr_ctx_list);
+    if (addr_ctx_list.IsEmpty())
+      return thread_plan_sp;
+
+    for (const auto &sym_ctx : addr_ctx_list.SymbolContexts()) {
+      if (sym_ctx.symbol != nullptr) {
+        sym = sym_ctx.symbol;
+        break;
+      }
+    }
+
+    // may not find a match
+    if (sym == nullptr)
+      return thread_plan_sp;
+  }
+
   ConstString sym_name = sym->GetMangled().GetName(Mangled::ePreferMangled);
   if (!sym_name)
     return thread_plan_sp;