Re: Consider exposing mmap_is_attached_or_noreserve

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

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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