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Date: Mon, 19 Aug 2013 14:36:12 +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 07:43, Ryan Johnson wrote:
> On 19/08/2013 7:39 AM, Corinna Vinschen wrote:
> >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 share=
d]
> >>>>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=
/7706
> >>>Windows only uses reserved but only partially committed memory for its=
 stacks. In order to detect when to
> >>>commit more stack, it installs  a one-shot guard page (btw the same ty=
pe of guard page that is used for the
> >>>hotspot yellow and red zone) right at the edge of the currently commit=
ed stack zone. When a thread accesses
> >>>this guard page an exception is thrown which Windows catches internall=
y, commits more stack and
> >>>re-establishes the one-shot guard page at the new edge of the commited=
 zone. When Windows detects such an
> >>>exception inside the _last 4 pages_ of a stack (I couldn't find any do=
cumentation for that on MSDN, I found
> >>>this value from manually testing on several Windows machines with 4k s=
tack 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.
> ... and yet somehow emacs managed to get around that protection
> (unintentially), leading to all that fun over the last week. What
> went wrong?

Good question.  I don't know.


Corinna

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

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