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

Wed Dec 9 12:14:19 PST 2020

Using existing personality functions requires emitting an LLVM IR cleanup
around every function when building in this -fterminate-exceptions mode.
Then all functions would have IR like this:

invoke void @foo(...) unwind to %terminate
invoke void @bar(...) unwind to %terminate
...
landingpad cleanup ...
  call void @myterminate()

On Tue, Dec 8, 2020 at 12:56 PM James Y Knight <jyknight at google.com> wrote:

> Why is adding a new personality function useful? Can't you share a single
> LSDA table for every noexcept function in a TU (both those implicitly
> noexcept due to -fno-exceptions and for those marked "noexcept")?
>
> On Tue, Dec 8, 2020 at 1:05 PM Reid Kleckner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> 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
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201209/31cb0ed0/attachment.html>