Problem with exception handling on alt stacks

Corinna Vinschen
Fri Jun 1 10:08:00 GMT 2018

On May 31 15:21, Erik Bray wrote:
> Hi all,
> I recently ran into a tricky problem when working with cysignals [1]
> on Cygwin:  Cysignals provides signal handlers for code written in C
> or Cython running in a Python interpreter, which converts otherwise
> unhandled signals into Python exceptions.
> It recently started using sigaltstack [2] to provide an alternate
> stack to run signal handlers on, specifically for SIGSEGV (in case the
> cause thereof was a stack overflow).
> Cygwin supports this in principle, but things get pretty flaky at that
> point, because if any exception occurs while running on an altstack,
> Cygwin will handle it if the exception came from Cygwin itself, but
> not otherwise, per this note [3]
> > TODO: What we do here is to handle only __try/__except blocks in Cygwin.
> > "Normal" exceptions will simply exit the process. Still, better
> > than nothing... */
> Is that really better than nothing though?  The result is that the
> process dies with a (Cygwin) exit code of 0.  This is quite
> surprising!  Even if there were really nothing else to be done I
> wonder if Cygwin could try as hard as possible to at least set a
> non-zero exit code in this case.
> Now, it's true what comment above that says:
> > normal SEH handlers are skipped, because the OS exception
> > handling considers the current (alternate) stack "broken"
> However, I was able to work around this in cysignals [4] by adding a
> VectoredContinueHandler which, in this case, is at least able to
> convert the exception code to the appropriate POSIX return code and
> exit the process.  And superficially it seems to work quite reliably.
> But I wonder if there isn't something "wrong" about this from the
> Cygwin perspective, and if it shouldn't at least try to do the same...

Good point.  I'm not sure I have time to look into this for the time
being, but I'm not at all opposed to patches in this area.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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