[PATCH 2/6] leases: fix write-open/read-lease race

[Mimi Zohar]

On Wed, 2011-09-21 at 11:01 -0400, J. Bruce Fields wrote: 
> On Wed, Sep 21, 2011 at 10:58:13AM -0400, J. Bruce Fields wrote:
> > In setlease, we use i_writecount to decide whether we can give out a
> > read lease.
> > 
> > In open, we break leases before incrementing i_writecount.
> > 
> > There is therefore a window between the break lease and the i_writecount
> > increment when setlease could add a new read lease.
> > 
> > This would leave us with a simultaneous write open and read lease, which
> > shouldn't happen.
> 
> And maybe someone that knows the open code better than me could confirm
> whether it's reasonable to move the break_lease() call to __dentry_open
> like this....
> 
> --b.

Moving break_lease() from may_open() to __dentry_open(), places the call
immediately after the call to __get_file_write_access(), which
increments i_writecount.

Currently, break_lease() is being called before the transition from
put_filp() to fput(). The move doesn't change this, so I would assume it
should be ok.

Mimi

> > 
> > Signed-off-by: J. Bruce Fields <bfields at redhat.com>
> > ---
> >  fs/namei.c |    5 +----
> >  fs/open.c  |    4 ++++
> >  2 files changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 2826db3..6ff59e5 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -2044,10 +2044,7 @@ static int may_open(struct path *path, int acc_mode, int flag)
> >  	if (flag & O_NOATIME && !inode_owner_or_capable(inode))
> >  		return -EPERM;
> >  
> > -	/*
> > -	 * Ensure there are no outstanding leases on the file.
> > -	 */
> > -	return break_lease(inode, flag);
> > +	return 0;
> >  }
> >  
> >  static int handle_truncate(struct file *filp)
> > diff --git a/fs/open.c b/fs/open.c
> > index f711921..22c41b5 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -685,6 +685,10 @@ static struct file *__dentry_open(struct dentry *dentry, struct vfsmount *mnt,
> >  	if (error)
> >  		goto cleanup_all;
> >  
> > +	error = break_lease(inode, f->f_flags);
> > +	if (error)
> > +		goto cleanup_all;
> > +
> >  	if (!open && f->f_op)
> >  		open = f->f_op->open;
> >  	if (open) {
> > -- 
> > 1.7.4.1
> >