linux-kernel - Re: [PATCH] open: introduce O

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Date:	Fri, 28 Aug 2009 07:04:40 -0600
From:	Eric Blake <ebb9@....net>
To:	Florian Weimer <fweimer@....de>
CC:	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	bug-coreutils@....org, bug-gnulib@....org,
	Ulrich Drepper <drepper@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] open: introduce O_NOSTD

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According to Florian Weimer on 8/28/2009 6:52 AM:
> * Eric Blake:
> 
>> Your version fails to clear the cloexec bit of the final fd if the
>> original caller didn't request O_CLOEXEC.
> 
> Okay, but you can fix that in a race-free manner (but I thought that
> this was implied by open_safer).

The current semantics of gnulib's open_safer is that the result is
guaranteed to be 3 or larger.  It would require an audit of all gnulib
clients of the open_safer method to see whether it also makes sense to
change the semantics of open_safer to also guarantee that fds start life
with the cloexec bit set.  But maybe that is a change worth making in
gnulib, with applications intending to give an fd to a child process being
required to explicitly clear the cloexec bit.

>> Also, your suggestion has a definite race in that you are calling
>> open() multiple times rather than cloning an existing fd after the
>> first open(), such that another process could alter which file is
>> visited between your first and last open().
> 
> Sure, but this is an unobservable differen.ce

It is absolutely observable - if the user passed O_CREAT|O_EXCL as part of
their flags, then the second open() will inappropriately fail.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@....net
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