Stackoverflow crash when evaluating an invalid expression

Subprojects LLDB
1

Hello all,

I am currently investigating an issue where LLDB is crashing due to a stack overflow when attempting to evaluate an expression. I have seen the same issue in 6.0.1 and have reproduced it in 7.0.1. Any help to diagnose and fix would be greatly appreciated as I am trying to meet a release deadline early next week!

The facts:

  • The expression being evaluated is not a valid expression in the C++ domain. For example the expression is a datatype like ‘Foobar’. (For reasons unexplained I am constrained to supporting this use case.)
  • The crash occurs when using the C++ LLDB API but not when trying to evaluate the expression via the LLDB shell or the LLDB Python script shell. However, when doing ‘expr Foobar’ there is no output and the operation is completely silent. It is similar when trying to do ‘lldb.frame.EvaluateExpression(“Foobar”)’ in the Python shell as well. I would expect to get some error output or an SbValue in an error state but no such luck.
  • I was able to capture a stack trace (attached) and it seems to be a recursive loop bottoming out. For brevity two ‘loops’ of stacktrace are included here:
  1. liblldb.dll!clang::ASTContext::getASTRecordLayout(const clang::RecordDecl * D) Line 2965 C++
  2. liblldb.dll!`anonymous namespace’::EmptySubobjectMap::ComputeEmptySubobjectSizes() Line 216 C++
  3. liblldb.dll!`anonymous namespace’::EmptySubobjectMap::EmptySubobjectMap(const clang::ASTContext & Context, const clang::CXXRecordDecl * Class) Line 172 C++
  4. liblldb.dll!clang::ASTContext::getASTRecordLayout(const clang::RecordDecl * D) Line 2965 C++
  5. liblldb.dll!`anonymous namespace’::EmptySubobjectMap::ComputeEmptySubobjectSizes() Line 216 C++
  6. liblldb.dll!`anonymous namespace’::EmptySubobjectMap::EmptySubobjectMap(const clang::ASTContext & Context, const clang::CXXRecordDecl * Class) Line 172 C++
  7. liblldb.dll!clang::ASTContext::getASTRecordLayout(const clang::RecordDecl * D) Line 2965 C++

Help please :S

Thanks,
Ben

lldb EvaluateExpression StackOverflow Crash - Stacktrace.txt (435 KB)

2

Hi Ben,

I think I already answered this last week:
http://lists.llvm.org/pipermail/lldb-dev/2019-February/014789.html

I don't think you'll get an answer here without posting the
problematic source or giving any more information as I described in my
mail.

Cheers,
- Raphael

3

Hey Raphael,

Yes, you did advise me to drop a D->dumpColor() call in to getASTRecordLayout(). For frustrating reasons I still haven’t been able to capture those logs but when I do I will report back.

Thanks so much for your help and quick response!

Ben

4

Ah I see what happened. My original e-mail was delayed because it was too large but it was recently accepted by a moderator.

5

You can also dump() the AST to a file on disk if that is easier to
capture than stderr. See the overload of `dump()` that takes a stream:
https://clang.llvm.org/doxygen/ASTDumper_8cpp_source.html#l00225

- Raphael

6

Looks like I am hitting the same issue. Was there a fix checked in for this?

7

I don’t think we ever received a reproducer for this that would allow us to investigate this (or write a fix).

  • Raphael