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Message-ID:	<521204EA.6020607@cs.utoronto.ca>
Date:	Mon, 19 Aug 2013 07:43:38 -0400
From:	Ryan Johnson <ryan DOT johnson AT cs DOT utoronto DOT ca>
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To:	cygwin AT cygwin DOT com
Subject:	Re: Stack size on 64-bit Cygwin
References:	<520E905A DOT 409 AT cornell DOT edu> <20130819093242 DOT GB18757 AT calimero DOT vinschen DOT de> <5211F83A DOT 30901 AT cs DOT utoronto DOT ca> <5211FBB6 DOT 6070108 AT cs DOT utoronto DOT ca> <20130819113919 DOT GA29385 AT calimero DOT vinschen DOT de>
In-Reply-To:	<20130819113919.GA29385@calimero.vinschen.de>

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 shared]
>>>> 00030000-001E4000 ===p 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 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 zone. When Windows detects such an
>>> exception inside the _last 4 pages_ of a stack (I couldn't find any documentation for that on MSDN, I found
>>> this value from manually testing on several Windows machines with 4k stack 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?

Ryan


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