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Date:	Mon, 19 Aug 2013 13:39:19 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: Stack size on 64-bit Cygwin
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On Aug 19 07:04, Ryan Johnson wrote:
> On 19/08/2013 6:49 AM, Ryan Johnson wrote:
> >One thing I don't understand, though: shouldn't a stack overflow
> >normally manifest as a seg fault when trying to access the invalid
> >addresses, rather than silent memory corruption?

That would be helpful.

> >However, /proc/pid/maps for emacs shows:
> >>00010000-00020000 rw-s 00000000 0000:0000 0
> >>[win heap 1 default shared]
> >>00020000-00030000 rw-s 00000000 0000:0000 0 [win heap 2 default shared]
> >>00030000-001E4000 =3D=3D=3Dp 00000000 0000:0000 0 [stack (tid 4896)]
> >>001E4000-001E6000 rw-g 001B4000 0000:0000 0 [stack (tid 4896)]
> >>001E6000-00230000 rw-p 001B6000 0000:0000 0 [stack (tid 4896)]
> >GDB reports that thread 4896 is the main thread... so I guess
> >Windows doesn't reserve a red zone around its stack, but instead
> >chooses to place the main thread stack right next to the
> >fully-mapped global shared heap to maximize the potential for Fun?

Right.  I have no idea what the two shared mem regions preceeding the
stack are good for, though.


> Some googling turns up
> http://comments.gmane.org/gmane.comp.java.openjdk.hotspot.runtime.devel/7=
706
> >Windows only uses reserved but only partially committed memory for its s=
tacks. In order to detect when to
> >commit more stack, it installs  a one-shot guard page (btw the same type=
 of guard page that is used for the
> >hotspot yellow and red zone) right at the edge of the currently commited=
 stack zone. When a thread accesses
> >this guard page an exception is thrown which Windows catches internally,=
 commits more stack and
> >re-establishes the one-shot guard page at the new edge of the commited z=
one. When Windows detects such an
> >exception inside the _last 4 pages_ of a stack (I couldn't find any docu=
mentation for that on MSDN, I found
> >this value from manually testing on several Windows machines with 4k sta=
ck pages) it throws a STACK_OVERFLOW_EXCEPTION.
> So maybe emacs just had the incredibly bad luck to alloca() a large
> buffer right at end-of-stack and then somehow managed to skip over
> the 4 guard pages when accessing it?

That's unlikely since alloca is designed to probe the stack in 4K
steps.  And STATUS_STACK_OVERFLOW is translated to a SEGV by Cygwin's
exception handler.


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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