Re: Consider exposing mmap_is_attached_or_noreserve

["E. Madison Bray" <erik.m.bray@gmail.com>]

On Wed, Feb 27, 2019 at 5:17 PM Corinna Vinschen wrote:
>
> On Feb 27 16:38, E. Madison Bray wrote:
> > Hello,
> >
> > A very technical request regarding Cygwin internals: In mmap.c there
> > is a function mmap_is_attached_or_noreserve(void *addr, size_t len)
> > which is called from Cygwin's exception handler in the case of a
> > STATUS_ACCESS_VIOLATION.
> >
> > This is called in case an access violation occurs in memory that was
> > allocated with Cygwin's mmap() with the MAP_NORESERVE flag, and allows
> > us to commit the relevant pages when they are accessed.
> >
> > After a successful call of mmap_is_attached_or_noreserve(), the Cygwin
> > exception handler returns with ExceptionContinueExecution.
> > Unfortunately, if the application happens to have a Vectored Continue
> > Handler registered which happens to do something in the case of
> > STATUS_ACCESS_VIOLATION (see [1]) there is no obvious way to tell if
> > we're handling this sort of case.
> >
> > Normally this isn't too much of a problem: E.g. we could just check
> > the address that caused the access violation and see if its status is
> > now MEM_COMMIT (i.e. Cygwin ran its exception handler and all is
> > good).  However, due to the bug described in [1], if an exception
> > occurs in code running on a sigaltstack, the Cygwin exception handler
> > isn't run.
> >
> > This makes for a tricky to handle use case:  What if some code in a
> > signal handler function tries to access uncommitted memory in a
> > MAP_NORESERVE mmap?  It's probably an unusual, undesirable case, and I
> > haven't personally encountered it *yet*, but I could imagine some
> > cases where it might happen.
> >
> > In order to handle such a case it might be nice if
> > mmap_is_attached_or_noreserve were able to be called by user code,
> > perhaps as a new cygwin_internal(...) call.  I'd happily provide a
> > patch, but I fear this might be an X/Y problem that I'm not seeing.
>
> Honestly, I'm not overly keen to expose this stuff.  Wouldn't it
> make more sense to fix Cygwin's sigaltstack implementation to handle
> these cases gracefully?  You're apparently not shy working with
> Windows exception handling.  Patches more than welcome!  I'm not
> happy not having found a solution to this problem :}

I can theoretically imagine a case where might be a problem totally
outside the context of the altstack issue: e.g. maybe a cygwin
application that has to link with a native Windows DLL that happens to
register some vectored continue handler that does something with
STATUS_ACCESS_VIOLATION exceptions.  Of course, absent a real example
I wouldn't push for it.

I completely agree it would be better to have a solution to the actual  problem.

> Oh, wait!  Maybe there is a simple solution.  Patch 9a5abcc896bd
> added a single line
>
>   exception protect;
>
> to the pthread::thread_init_wrapper method.
>
> What if adding the same line to the altstack_wrapper function
> would help for altstack as well?

You know, I actually noticed this just recently, because I noticed
that pthreads also run on a stack allocated by Cygwin, and I wondered
how exception handling would work in that case.  I think I was looking
at it in the context of the thread last month that resulted in that
fix, but I forgot to ask you if this could work for the altstack issue
as well.

> Can you test this?

I'll give it a try and report back with a patch if it works.

The biggest risk is if a stack overflow happens while running on the
altstack--in this case even the Cygwin exception handling could fail
and the application will just crash.  But for other, less extreme
cases having this would be better than nothing.

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