linux-kernel - Re: [PATCHv8.1] fanotify: enable close-on-exec on events' fd when requested in fanotify

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Date:	Wed, 1 Oct 2014 15:36:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Yann Droneaud <ydroneaud@...eya.com>
Cc:	Heinrich Schuchardt <xypron.glpk@....de>,
	Eric Paris <eparis@...hat.com>,
	Richard Guy Briggs <rgb@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	stable@...r.kernel.org, linux-api@...r.kernel.org,
	Jan Kara <jack@...e.cz>,
	Lino Sanfilippo <LinoSanfilippo@....de>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>
Subject: Re: [PATCHv8.1] fanotify: enable close-on-exec on events' fd when
 requested in fanotify_init()

On Mon, 29 Sep 2014 10:49:15 +0200 Yann Droneaud <ydroneaud@...eya.com> wrote:

> According to commit 80af258867648 ('fanotify: groups can specify
> their f_flags for new fd'), file descriptors created as part of
> file access notification events inherit flags from the
> event_f_flags argument passed to syscall fanotify_init(2).
> 
> So while it is legal for userspace to call fanotify_init() with
> O_CLOEXEC as part of its second argument, O_CLOEXEC is currently
> silently ignored.
> 
> Indeed event_f_flags are only given to dentry_open(), which only
> seems to care about O_ACCMODE and O_PATH in do_dentry_open(),
> O_DIRECT in open_check_o_direct() and O_LARGEFILE in
> generic_file_open().
> 
> But it seems logical to set close-on-exec flag on the file
> descriptor if userspace is allowed to request it with O_CLOEXEC.
> 
> In fact, according to some lookup on http://codesearch.debian.net/
> and various search engine, there's already some userspace code
> requesting it:
> 
> - in systemd's readahead[2]:
> 
>     fanotify_fd = fanotify_init(FAN_CLOEXEC|FAN_NONBLOCK, O_RDONLY|O_LARGEFILE|O_CLOEXEC|O_NOATIME);
> 
> - in clsync[3]:
> 
>     #define FANOTIFY_EVFLAGS (O_LARGEFILE|O_RDONLY|O_CLOEXEC)
> 
>     int fanotify_d = fanotify_init(FANOTIFY_FLAGS, FANOTIFY_EVFLAGS);
> 
> - in examples [4] from "Filesystem monitoring in the Linux
>   kernel" article[5] by Aleksander Morgado:
> 
>     if ((fanotify_fd = fanotify_init (FAN_CLOEXEC,
>                                       O_RDONLY | O_CLOEXEC | O_LARGEFILE)) < 0)

So we have a number of apps which are setting O_CLOEXEC, but it doesn't
actually work.  With this change it *will* work, so the behaviour of
those apps might change, possibly breaking them?

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