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Subject: Re: Segfault if /proc/PID/maps is opened in parallel threads [CAUSE:
 RtlQueryProcessDebugInformation]
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Date: Tue, 27 May 2025 17:44:10 +0200
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From: Christian Franke via Cygwin <cygwin AT cygwin DOT com>
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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?

-- 
Regards,
Christian


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