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

[Reid Kleckner via llvm-dev]

I would suggest using a custom personality function for this. It will
optimize better and be much smaller than using a standard personality
function. It saves the LSDA tables.

LLVM supports custom personality functions, so only clang changes are
required. You could either do something like add a flag to override the EH
personality with a custom one, or come up with a new dedicated
fno-exceptions termination personality and add it to compiler-rt.

On Mon, Dec 7, 2020 at 3:31 PM Modi Mo via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> 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 and
> 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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