linux-kernel - Re: [PATCH v2 3/8] vfs: Add O_DENYREAD/WRITE flags support for open syscall

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Date:	Thu, 7 Feb 2013 11:19:48 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Pavel Shilovsky <piastry@...rsoft.ru>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-cifs@...r.kernel.org, linux-nfs@...r.kernel.org,
	wine-devel@...ehq.org
Subject: Re: [PATCH v2 3/8] vfs: Add O_DENYREAD/WRITE flags support for open
 syscall

On Thu, Feb 07, 2013 at 08:00:13PM +0400, Pavel Shilovsky wrote:
> 2013/2/7 J. Bruce Fields <bfields@...ldses.org>:
> > On Thu, Feb 07, 2013 at 06:32:38PM +0400, Pavel Shilovsky wrote:
> >> 2013/2/7 J. Bruce Fields <bfields@...ldses.org>:
> >> > On Thu, Feb 07, 2013 at 01:53:46PM +0400, Pavel Shilovsky wrote:
> >> >> Nothing prevents it. If somebody grabbed a share mode lock on a file
> >> >> before we call deny_lock_file, we simply close this file and return
> >> >> -ETXTBSY.
> >> >
> >> > But leave the newly-created file there--ugh.
> >> >
> >> >> We can't grab it before atomic_open because we don't have an
> >> >> inode there.
> >> >
> >> > If you can get the lock while still holding the directory i_mutex can't
> >> > you prevent anyone else from looking up the new file until you've gotten
> >> > the lock?
> >> >
> >>
> >> Hm..., seems you are right, I missed this part:
> >> mutex_lock
> >> lookup_open -> atomic_open -> deny_lock_file
> >> mutex_unlock
> >>
> >> that means that nobody can open and of course set flock on the newly
> >> created file (because flock is done through file descriptor). So, it
> >> should be fine to call flock after f_ops->atomic_open in atomic_open
> >> function. Thanks.
> >
> > Whether that works may also depend on how the new dentry is set up?  If
> > it's hashed before you call flock then I suppose it's already visible to
> > others.
> 
> It seems it should be hashed in f_ops->atomic_open() (at least cifs
> and nfs do it this way). In do_last when we do an ordinary open, we
> don't hit parent i_mutex if lookup is succeeded through lookup_fast.
> lookup_fast can catch newly created dentry and set it's share mode
> before atomic_open codepath hits deny_lock_file.
> 
> Also, I noted that: atomic open does f_ops->atomic_open and then it
> processes may_open check; if may_open fails, the file is closed and
> open returns with a error code (but file is created anyway).

That would be a bug, I think.  E.g. "man 3posix open":

	No  files  shall  be created or modified if the function returns
	-1.

Looking at the code...  See the references to FILE_CREATED in
atomic_open--looks like that's trying to prevent may_open from failing
in this case.

> I think
> there is no difference between this case and the situation with
> deny_lock_file there.

Looks to me like it would be a bug in either case.

--b.
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