[llvm-dev] Catching exceptions while unwinding through -fno-exceptions code

[Modi Mo via llvm-dev]

If you don’t need to capture more information and can just terminate, you can directly register std::terminate as the personality routine as opposed to __gxx_personality_v0 or __CxxFrameHandler3/4 (Windows) which lets you omit other metadata and work cross-platform.

Modi

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Everett Maus via llvm-dev <llvm-dev at lists.llvm.org>
Reply-To: Everett Maus <evmaus at google.com>
Date: Monday, December 7, 2020 at 12:47 PM
To: "llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Catching exceptions while unwinding through -fno-exceptions code

Hey all:

I wanted to bring up something that was discussed a few years ago around the behavior of exceptions when interacting with code compiled with -fno-exceptions. (In https://lists.llvm.org/pipermail/llvm-dev/2017-February/109992.html<https://lists.llvm.org/pipermail/llvm-dev/2017-February/109992.html> and https://lists.llvm.org/pipermail/llvm-dev/2017-February/109995.html<https://lists.llvm.org/pipermail/llvm-dev/2017-February/109995.html>)

It's possible to compile (and link/etc.) code with -fexceptions for some compilation units and -fno-exceptions for others.  Unlike the behavior of noexcept (which requires termination), this doesn't have a specified behavior in the C++ standard as far as I can tell.  However, it can lead to memory leaks & other issues (e.x. with TSAN, it messes up the tracking of the current stack frame).

I'd be interested in looking into potentially doing the work to add an option to clang/etc. to terminate when an exception traverses code compiled with -fno-exceptions, instead of simply allowing the unwinder to walk through the stack frame & leak memory/etc. (possibly behind a flag?).  This particular issue bit a team I work closely with, and I'd imagine it could be causing subtle issues for other clang users.

I'm mostly concerned with solving this on Linux/x86_64, although if there's a way to solve it more generally I'm open to looking into doing that instead.

I /think/ the right place to change this (from the discussions I linked) would be in the LLVM -> assembly layer, adding an appropriate .gcc_except_table for functions that are determined to be unable to throw exceptions (either due to noexcept or due to -fno-exceptions). Then the unwinder would find .eh_frame but no entry in the .gcc_except_table and should terminate (via  __gxx_personality_v0).

Am I understanding that correctly?  What's the best way to propose this sort of change to clang? (document/just try to look at putting together a PR/other?)

Alternatively--one other thing that occurred to me is that it could be reasonably cheap to simply add try/catch blocks that report an UBSAN error in all methods that shouldn't be able to throw an exception.  This obviously doesn't fix the code-generation problem and would lead to larger binary sizes, but that seems less bad for an UBSAN build in particular.  That would likely meet my needs around wanting a way to automatically detect this behavior/problem, but might not address the more generic issue.

Thanks,
--
--EJM
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