linux-kernel - Re: [PATCH v4 6/7] events: use get_unused_fd_flags(0) instead of get_unused

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Date:	Wed, 30 Oct 2013 23:00:13 +0100
From:	Yann Droneaud <ydroneaud@...eya.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk
Subject: Re: [PATCH v4 6/7] events: use get_unused_fd_flags(0) instead of
 get_unused_fd()

Le mercredi 30 octobre 2013 à 22:18 +0100, Yann Droneaud a écrit :
> Hi,
> 
> Le 30.10.2013 21:20, Peter Zijlstra a écrit :
> > On Wed, Oct 30, 2013 at 08:47:46PM +0100, Yann Droneaud wrote:
> >> This patch replaces calls to get_unused_fd() with equivalent call to
> >> get_unused_fd_flags(0) to preserve current behavor for existing code.
> >> 
> >> The hard coded flag value (0) should be reviewed on a per-subsystem 
> >> basis,
> >> and, if possible, set to O_CLOEXEC.
> > 
> > And how am I supposed to know if that is 'possible'? You provide a 
> > total
> > number of 0 useful clues on how to determine this.
> 

I'm sorry for sending this email so unreadable ... "unformatted" by
RoundCube webmail.
Please find something more readable below:

> Fair.
>
> Short: Will it break kernel ABI ?
>        If no, use O_CLOEXEC, if yes, use 0.
>
> Long: If userspace expect to retrieve a file descriptor with plain
>       old Unix(tm) semantics, O_CLOEXEC must not be the default, as it
>       could break some applications expecting that the file descriptor
>       will be inherited during exec().
>
>       But for some subsystems, such as InfiniBand, KVM, VFIO, it make no
>       sense to have file descriptors inherited since those are tied to
>       resources that will vanish when a another program will replace the
>       current one by mean of exec(), so it's safe to use O_CLOEXEC in
>       such cases.
>
>       For others, like XFS, the file descriptor is retrieved by one
>       program and will be used by a different program, executed as a
>       child. In this case, setting O_CLOEXEC would break existing
>       application, which do not expect to have to call fcntl(fd,
>       F_SETFD, FD_CLOEXEC) to make it available across exec().
>
>       If file descriptor created by events subsystem are not tied to the
>       current process resources, it's likely legal to use it in a
>       different process context, thus O_CLOEXEC must not be the default.
>
> Aside: If O_CLOEXEC cannot be made the default, it would be interesting
>        to think to extend the API to have a (set of) function(s) taking
>        a flags parameter so that userspace can set O_CLOEXEC if wanted.
>        And I have a patch for this :)
>
> PS: I like the title of this article: "Excuse me son, but your code is
>     leaking !!!" [1] by Dan Walsh but one should probably read PEP-446
>     "Make newly created file descriptors non-inheritable" [2] instead
>     since it has lot more background information on file descriptor
>     leaking.
>
> [1] http://danwalsh.livejournal.com/53603.html
> [2] http://www.python.org/dev/peps/pep-0446/
>
>
> Regards.
>

-- 
Yann Droneaud
OPTEYA


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