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Message-ID:	<29c48e0c-a4d4-4c79-d8e5-5686fe6d5a27@cornell.edu>
Date:	Sat, 2 Jul 2022 17:58:09 -0400
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Subject:	Re: [EXTERNAL] Re: Weird issue with file permissions
To:	"Lavrentiev, Anton (NIH/NLM/NCBI) [C]" <lavr AT ncbi DOT nlm DOT nih DOT gov>,
	"cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
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From:	Ken Brown <kbrown AT cornell DOT edu>
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On 7/2/2022 3:37 PM, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
>> what your test program was actually doing.  But you seem to be assuming that
>> calling fchmod on a socket descriptor should affect the permissions on the
>> socket file (assuming the socket is bound).  Is that documented anywhere?  POSIX
>> says that the behavior of fchmod on a socket descriptor is unspecified
> 
> The socket file descriptor for a bound UNIX sockets refers to an object in a filesystem
> (it's practically a file), which the bind() system call creates.  The access to the socket
> is controlled by the permission bits, when someone actually tries to connect to it,
> so permissions should be working for these objects (otherwise, there's no other way!)
> 
> And fchmod() for a bound Unix socket works on Linux and many other Unix flavors, actually.

That's not what I'm seeing when I run your test program on Linux:

$ ./sun
fstat mode = 140666
stat mode = 140777

$ ls -l .socket
srwxrwxrwx. 1 kbrown kbrown 0 Jul  2 17:47 .socket=

So calling fchmod on the socket descriptor did not change the permissions of the 
file to which the socket was bound.

And on freeBSD, calling fchmod on a socket descriptor is apparently an error:

   https://www.freebsd.org/cgi/man.cgi?query=fchmod&sektion=2&n=1

Ken

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