linux-kernel - Re: fanotify API: FMODE_NONOTIFY, FMODE_EXEC, FMODE

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Date:	Tue, 29 Apr 2014 22:10:06 +0200
From:	Jan Kara <jack@...e.cz>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Heinrich Schuchardt <xypron.glpk@....de>,
	linux-man <linux-man@...r.kernel.org>,
	Eric Paris <eparis@...hat.com>, Jan Kara <jack@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: fanotify API: FMODE_NONOTIFY, FMODE_EXEC, FMODE_NOCMTIME

  Hello,

On Tue 29-04-14 15:29:12, Michael Kerrisk (man-pages) wrote:
> Can you offer any insight on Heinrich's question, below?
> 
> On Sun, Apr 13, 2014 at 4:05 PM, Heinrich Schuchardt <xypron.glpk@....de> wrote:
> > On 06.04.2014 14:18, Michael Kerrisk (man-pages) wrote:
> >>>
> >>> ==
> >>> >
> >>> >  >> I notice that the FDs returned by read()s from the FAN FD have the
> >>> >  >> FMODE_NONOTIFY flag (fcntl(F_GETFL)) flag set. If you know what
> >>> > that's
> >>> >  >> about, it would be good to say something about. But, if not, do not
> >>> >  >> worry--just place a FIXME in the page source of fanotify(7)
> >>> >
> >>> >Fixed in fanotify.7
> >>> >If the listener accesses the file through the file descriptor provided
> >>> >no additional events are created.
> >>
> >> Ahh -- thanks for filling in that piece. I see that you refer to
> >> fcntl(2) when discussing that flag. But fcntl(2) does not
> >> mention that flag. I would rather see an explanation of this flag
> >> in the fanotify pages.
> >>
> >
> > I wrote a small test program and found:
> >
> > The flag FMODE_NONOTIFY can be read by function fcntl from userspace.
> > int flag = fcntl(fd, F_GETFL)
> >
> > In include/uapi/asm-generic/fcntl.h I found the following comment:
> >
> > /*
> >  * FMODE_EXEC is 0x20
> >  * FMODE_NONOTIFY is 0x1000000
> >  * These cannot be used by userspace O_* until internal and external open
> >  * flags are split.
> >  * -Eric Paris
> >  */
> >
> > The definition of FMODE_NONOTIFY is in include/linux/fs.h but this
> > include is only used to compile the Kernel and not supposed to be used by
> > userspace.
> >
> > I think it is quite annoying that fcntl can return a flag that is not
> > described in the manpage of fcntl and that is not defined in fcntl.h.
> >
> > But FMODE_NONOTIFY is not the only flag:
> >
> > I was able to pass
> > 0x20 (FMODE_EXEC), and
> > 0x800 (FMODE_NOCMTIME)
> > to fanotify_init and received them as flag in the file descriptors for the
> > fanotify events.
> > I wonder why fanotify_init does not check import parameter event_f_flags and
> > return an error if any inappropriate value is set.
  It seems to me fanotify_init() should really check event_f_flags have
only valid flags set. In particular exclude FMODE_EXEC, FMODE_NOCMTIME, or
FMODE_NONOTIFY.

> > Should I put this into the BUGS section?
> >
> > Should the name of the flag FMODE_NONOTIFY be mentioned at all in the man
> > pages?
> >
> > Or should we write:
> >
> > .I fd
> > This is an open file descriptor for the object being accessed or
> > .B FAN_NOFD
> > if a queue overflow occurred.
> > The file descriptor can be used to access the contents of the monitored file
> > or
> > directory.
> > It has an internal flag set, that suppresses fanotify event generation.
> > Hence when the receiver of the fanotify event accesses the notified file or
> > directory using this file descriptor no additional events will be created.
> > The reading application is responsible for closing the file descriptor.
  So this is what I would prefer. Just mention the file descriptor does not
generate new events. I would even go as far as masking kernel internal
flags like FMODE_EXEC or FMODE_NONOTIFY from the result of F_GETFL. What do
you think Al?

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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