DMARC-Filter: OpenDMARC Filter v1.4.2 delorie.com 563B3SR3087242
Authentication-Results: delorie.com; dmarc=pass (p=none dis=none) header.from=cygwin.com
Authentication-Results: delorie.com; spf=pass smtp.mailfrom=cygwin.com
DKIM-Filter: OpenDKIM Filter v2.11.0 delorie.com 563B3SR3087242
Authentication-Results: delorie.com;
	dkim=pass (1024-bit key, unprotected) header.d=cygwin.com header.i=@cygwin.com header.a=rsa-sha256 header.s=default header.b=fhdjhbnp
X-Recipient: archive-cygwin AT delorie DOT com
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 89D0D3852FF9
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cygwin.com;
	s=default; t=1751540605;
	bh=Sai5MnFjq2mK4GTKOEpmKRM06/zBJxr+HWKG5OdzoAM=;
	h=Date:To:Subject:References:In-Reply-To:List-Id:List-Unsubscribe:
	 List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc:
	 From;
	b=fhdjhbnpNW+U6brgevHUVmyAd1yphWOVG4Vg/KojmnuFkciMqc6agKWWK9nDDUfG1
	 Cfd6IMqNmdDOjT4OmXr2UQ0pTqcQXhwJOxnOW3/wxMx0TgF9juD4DeC+Z6la6vXW0+
	 MLUrlqdJra5adFN2kTCRPebkx9/Cdrd3ogVIV3wg=
X-Original-To: cygwin AT cygwin DOT com
Delivered-To: cygwin AT cygwin DOT com
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 368F83852743
Date: Thu, 3 Jul 2025 13:03:00 +0200
To: cygwin AT cygwin DOT com
Subject: Re: Segfault if /proc/PID/maps is opened in parallel threads [CAUSE:
 RtlQueryProcessDebugInformation]
Message-ID: <aGZjZKoT86ptfahW@calimero.vinschen.de>
Mail-Followup-To: cygwin AT cygwin DOT com
References: <2f7ae08b-80c9-4497-f07c-5d527bbb7ec2 AT t-online DOT de>
 <17bba06c-4bdd-01df-8477-235aff98b2cb AT t-online DOT de>
 <9deb7d60-b05d-ad7f-e316-64781687953b AT t-online DOT de>
 <68cbc37b-4825-6e3f-5918-f4fee20d7c05 AT t-online DOT de>
 <aGVeI9glhLe2UzTu AT calimero DOT vinschen DOT de>
MIME-Version: 1.0
Content-Disposition: inline
In-Reply-To: <aGVeI9glhLe2UzTu@calimero.vinschen.de>
X-BeenThere: cygwin AT cygwin DOT com
X-Mailman-Version: 2.1.30
Precedence: list
List-Id: General Cygwin discussions and problem reports <cygwin.cygwin.com>
List-Unsubscribe: <https://cygwin.com/mailman/options/cygwin>,
 <mailto:cygwin-request AT cygwin DOT com?subject=unsubscribe>
List-Archive: <https://cygwin.com/pipermail/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-request AT cygwin DOT com?subject=help>
List-Subscribe: <https://cygwin.com/mailman/listinfo/cygwin>,
 <mailto:cygwin-request AT cygwin DOT com?subject=subscribe>
From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
Reply-To: cygwin AT cygwin DOT com
Cc: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Errors-To: cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com
Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

On Jul  2 18:28, Corinna Vinschen via Cygwin wrote:
> On Jul  2 16:18, Christian Franke via Cygwin wrote:
> > On Tue, 27 May 2025 17:44:10 +0200 Christian Franke wrote:
> > > On Sun, 25 May 2025 16:46:49 +0200, Christian Franke wrote:
> > > > On Sat, 24 May 2025 15:19:10 +0200, Christian Franke wrote:
> > > > > If /proc/PID/maps is opened in parallel threads, the process PID
> > > > > may segfault.
> > > > > 
> > > > > Testcase:
> > > > > 
> > > > > ...
> > > > > 
> > > > > Problem is not reproducible with any of the other /proc/PID/* files.
> > > > > 
> > > > 
> > > > A closer look shows that the problem is unrelated to thread safety.
> > > > The segfault also occurs when the following command is run
> > > > simultaneously in two terminals:
> > > > 
> > > > $ while cat /proc/1234/maps > /dev/null; do printf .; done
> > > > .............cat: /proc/1234/maps: No such file or directory
> > > > 
> > > 
> > > The root of the problem is the call of RtlQueryProcessDebugInformation()
> > > here:
> > > https://cygwin.com/git/?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/fhandler/process.cc;h=8fae9be5#l630
> > > 
> > > 
> > > If this part is removed, the segfault does no longer occur. Then
> > > /proc/PID/maps still work but the "[win heap ...]" information is
> > > missing. No patch provided because I don't know whether this could
> > > possibly be fixed without removing this functionality.
> > > 
> > > A test with a separate test program shows that the problem is unrelated
> > > to Cygwin: Parallel calls to RtlQueryProcessDebugInformation() with the
> > > same PID could be used to crash this process. It also "worked" with
> > > Notepad.exe.
> > > 
> > > Possible Windows (Version 10.0.22631.5039) bug?
> 
> Looks very much like a Windows bug to me.  But the function is supposed
> to be run under a debugger, so there's a chance multiple parallel calls
> were never part of the picture.  And given the function is undocumented,
> I guess we have to keep the broken pieces...
> 
> > Testcase (attached):
> > [...]
> > Conclusion: Possible Windows bug in RtlQueryProcessDebugInformation().
> > Cygwin should not use this function to provide "nice to have" types of
> > information like "[win heap...]" if this may crash the target process.
> 
> RtlQueryProcessDebugInformation isn't only called to emulate maps,
> it is also called from within dlsym().
> 
> A valid patch might be to move the RtlQueryProcessDebugInformation
> into its own function which adds a global lock.  But obviouly the 
> function can be called in a non-Cygwin process in parallel, so there's
> no 100% safe way to do it.
> 
> Any other idea?

I just added two patches, one of them dropping RtlQueryProcessDebugInformation
in favor of EnumProcessModules in dlsym(), the other by adding a global
mutex to format_process_maps while accessing the debug buffer.  I could
reproduce the crash even if RtlQueryProcessDebugInformation was serialized.
I could only avoid the crash by serializing the entire code from
RtlCreateQueryDebugBuffer up to and including RtlDestroyQueryDebugBuffer.

This ignores native processes which have a hard time to access
/proc anyway.


HTH,
Corinna

-- 
Problem reports:      https://cygwin.com/problems.html
FAQ:                  https://cygwin.com/faq/
Documentation:        https://cygwin.com/docs.html
Unsubscribe info:     https://cygwin.com/ml/#unsubscribe-simple