[lldb] Don't clear a Module's UnwindTable when adding a SymbolFile

[jasonmolenda]

Fixing a crash in lldb when symbols.auto-download setting is enabled. When doing a backtrace, this feature has lldb search for a SymbolFile for stack frames when we are backtracing, and add them either synchoronously or asynchronously, depending on the specific setting used.

Module::SetSymbolFileFileSpec clears the Module's UnwindTable, once we find a new SymbolFile. We may be adding a source of unwind information that we did not have when lldb was working only with the executable binary.

What happens in practice is that we're using a reference to the Module's UnwindTable, and then the other thread getting the SymbolFile clears it and now the first thread is referring to freed memory and we can crash. When built with address sanitizer, it crashes much more reliably.

Given that unwind information used for exception handling -- eh_frame, compact unwind -- is present in executable binaries, the only thing we're likely to add would be DWARF's debug_frame if that was also available. The actual value of re-creating the UnwindTable when we have added a SymbolFile is not large.

I also tried fixing this by changing the Module to have a shared_ptr to the UnwindTable, so we could have two different UnwindTable's in use simultaneously for a brief period. This would be fine TODAY, but it introduces a very subtle bug that someone will have a heck of a time figuring out in the future.

In the end, I believe the safest approach is to sacrifice the possible marginal gain of reconstructing the UnwindTable once a SymbolFile has been added, to sidestep this whole problem area.

Also, in Module::GetUnwindTable(), call DownloadSymbolFileAsync before we create the UnwindTable for the first time, in case the symbol file is fetched synchronously, we will have it for that possible marginal gain.

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Jason Molenda (jasonmolenda)

Changes

Fixing a crash in lldb when symbols.auto-download setting is enabled. When doing a backtrace, this feature has lldb search for a SymbolFile for stack frames when we are backtracing, and add them either synchoronously or asynchronously, depending on the specific setting used.

Module::SetSymbolFileFileSpec clears the Module's UnwindTable, once we find a new SymbolFile. We may be adding a source of unwind information that we did not have when lldb was working only with the executable binary.

What happens in practice is that we're using a reference to the Module's UnwindTable, and then the other thread getting the SymbolFile clears it and now the first thread is referring to freed memory and we can crash. When built with address sanitizer, it crashes much more reliably.

Given that unwind information used for exception handling -- eh_frame, compact unwind -- is present in executable binaries, the only thing we're likely to add would be DWARF's debug_frame if that was also available. The actual value of re-creating the UnwindTable when we have added a SymbolFile is not large.

I also tried fixing this by changing the Module to have a shared_ptr to the UnwindTable, so we could have two different UnwindTable's in use simultaneously for a brief period. This would be fine TODAY, but it introduces a very subtle bug that someone will have a heck of a time figuring out in the future.

In the end, I believe the safest approach is to sacrifice the possible marginal gain of reconstructing the UnwindTable once a SymbolFile has been added, to sidestep this whole problem area.

Also, in Module::GetUnwindTable(), call DownloadSymbolFileAsync before we create the UnwindTable for the first time, in case the symbol file is fetched synchronously, we will have it for that possible marginal gain.

---

Full diff: https://github.com/llvm/llvm-project/pull/86603.diff

1 Files Affected:

• (modified) lldb/source/Core/Module.cpp (+1-6)

diff --git a/lldb/source/Core/Module.cpp b/lldb/source/Core/Module.cpp
index 8ffa35518b3c36..a520523a96521a 100644
--- a/lldb/source/Core/Module.cpp
+++ b/lldb/source/Core/Module.cpp
@@ -1239,9 +1239,9 @@ void Module::SectionFileAddressesChanged() {
 
 UnwindTable &Module::GetUnwindTable() {
   if (!m_unwind_table) {
-    m_unwind_table.emplace(*this);
     if (!m_symfile_spec)
       SymbolLocator::DownloadSymbolFileAsync(GetUUID());
+    m_unwind_table.emplace(*this);
   }
   return *m_unwind_table;
 }
@@ -1359,15 +1359,10 @@ void Module::SetSymbolFileFileSpec(const FileSpec &file) {
         // one
         obj_file->ClearSymtab();
 
-        // Clear the unwind table too, as that may also be affected by the
-        // symbol file information.
-        m_unwind_table.reset();
-
         // The symbol file might be a directory bundle ("/tmp/a.out.dSYM")
         // instead of a full path to the symbol file within the bundle
         // ("/tmp/a.out.dSYM/Contents/Resources/DWARF/a.out"). So we need to
         // check this
-
         if (FileSystem::Instance().IsDirectory(file)) {
           std::string new_path(file.GetPath());
           std::string old_path(obj_file->GetFileSpec().GetPath());

[JDevlieghere]

Thanks for fixing this!