From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: "E. Madison Bray" <erik.m.bray@gmail.com>
Cc: cygwin@cygwin.com
Subject: Re: Consider exposing mmap_is_attached_or_noreserve
Date: Wed, 27 Feb 2019 16:32:00 -0000 [thread overview]
Message-ID: <20190227162403.GD4133@calimero.vinschen.de> (raw)
In-Reply-To: <20190227161712.GB4133@calimero.vinschen.de>
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On Feb 27 17:17, 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 :}
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?
Can you test this?
Thanks,
Corinna
--
Corinna Vinschen
Cygwin Maintainer
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next prev parent reply other threads:[~2019-02-27 16:24 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-27 15:51 E. Madison Bray
2019-02-27 16:19 ` Corinna Vinschen
2019-02-27 16:32 ` Corinna Vinschen [this message]
2019-02-28 13:12 ` E. Madison Bray
2019-02-28 18:01 ` Corinna Vinschen
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