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From:	Chris Faylor <cgf AT cygnus DOT com>
Date:	Thu, 3 Feb 2000 18:26:21 -0500
To:	"Fifer, Eric" <EFifer AT sanwaint DOT com>
Cc:	"'Ray Easton'" <ray DOT easton AT wcom DOT com>, cygwin AT sourceware DOT cygnus DOT com
Subject:	Re: call_handler, interrupt_now and interruptible
Message-ID:	<20000203182621.B2383@cygnus.com>
Mail-Followup-To:	"Fifer, Eric" <EFifer AT sanwaint DOT com>,
	'Ray Easton' <ray DOT easton AT wcom DOT com>, cygwin AT sourceware DOT cygnus DOT com
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In-Reply-To:	<779F20BCCE5AD31186A50008C75D99791716D3@SILLDN_MAIL1>; from EFifer@sanwaint.com on Thu, Feb 03, 2000 at 06:38:43PM -0000

On Thu, Feb 03, 2000 at 06:38:43PM -0000, Fifer, Eric wrote:
>
>"Ray Easton" writes:
>>> Anyways, is this dangerous to do?
>>
>>If 'this' means interrupting a DLL while it is "paused on a system
>>call", yes, it is extermely dangerous and can leave the NT kernel in a
>>corrupt state.  (This has nothing to do with Cygwin -- this is purely
>>an NT issue.)
>
>Actually, by 'this' I meant comment out 'pchigh == 0x60000000'.  And,
>when I wrote 'system call' I meant a Cygwin system call like read, but
>since most things are emulated on top of Win32 calls, eventually there
>might be a Win32 call like ReadFile.  AFAICT, Cygwin signals are emulated
>with Win32 threads and semaphores.  A Cygwin process has a "sig" thread
>and a "main" thread, when a signal is sent, the "sig" thread receives it,
>does a SuspendThread on the "main" thread and does a GetThreadContext,
>if it is "interruptible" the context is changed and the signal handler
>is pushed on the stack and the new context is set with SetThreadContext
>then followed by a ResumeThread.  I can see how this could be dangerous
>although I'm fairly ignorant of Win32, so I'm not sure exactly what you
>can do that will leave the NT kernel in a corrupt state.

The answer to your question is that this *is* dangerous to do.  It allows
cygwin "system calls" to be interrupted.

The code that you are investigating is essentially a stopgap until such
time as I have time to make it more foolproof.

As you've discovered, if you are using a snapshot, you're living on the
edge.

cgf

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