On Mon, 8 Jul 2013 10:02:23 -0400
Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
> On Mon, Jul 08, 2013 at 09:30:55AM -0400, Jeff Layton wrote:
> > As Al Viro points out, there is an unlikely, but possible race between
> > opening a file and setting a lease on it. generic_add_lease is done with
> > the i_lock held, but the inode->i_flock check in break_lease is
> > lockless. It's possible for another task doing an open to do the entire
> > pathwalk and call break_lease between the point where generic_add_lease
> > checks for a conflicting open and adds the lease to the list. If this
> > occurs, we can end up with a lease set on the file with a conflicting
> > open.
> >
> > To guard against that, check again for a conflicting open after adding
> > the lease to the i_flock list. If the above race occurs, then we can
> > simply unwind the lease setting and return -EAGAIN.
>
> Maybe it's an entirely theoretical question at this point, but in the
> absence of any lock or memory barrier on the lease-setter's side I still
> don't understand what guarantees that the opener calling break_lease
> will see the new value of i_flock.
>
> --b.
Ok, I think I see what you mean. The concern you have is that
break_lease still may not see a populated i_flock list even after
locks_insert_lock is called since it's not being checked with any
locking? So this patch would tighten up the race window w/o eliminating
it...
locks_insert_lock will acquire a percpu spinlock to put it on the
percpu hlist, but I'm not 100% sure that's sufficient as a memory
barrier here.
Would an explicit smp_wmb() after locks_insert_lock paired with a
smp_rmb() early in break_lease be sufficient?
Also, there's a bug in this patch as well, which I've got fixed in
my tree. I'll fix that in the next version. Details below...
>
>
> >
> > Cc: Bruce Fields <bfields@xxxxxxxxxxxx>
> > Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> > fs/locks.c | 31 ++++++++++++++++++++++++-------
> > 1 file changed, 24 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/locks.c b/fs/locks.c
> > index b27a300..9f7f647 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -1455,6 +1455,19 @@ int fcntl_getlease(struct file *filp)
> > return type;
> > }
> >
> > +static int
> > +check_conflicting_open(struct dentry *dentry, long arg)
> > +{
> > + struct inode *inode = dentry->d_inode;
> > +
> > + if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0))
> > + return -EAGAIN;
> > + if ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||
> > + (atomic_read(&inode->i_count) > 1)))
> > + return -EAGAIN;
> > + return 0;
> > +}
> > +
> > static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp)
> > {
> > struct file_lock *fl, **before, **my_before = NULL, *lease;
> > @@ -1464,12 +1477,8 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
> >
> > lease = *flp;
> >
> > - error = -EAGAIN;
> > - if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0))
> > - goto out;
> > - if ((arg == F_WRLCK)
> > - && ((d_count(dentry) > 1)
> > - || (atomic_read(&inode->i_count) > 1)))
> > + error = check_conflicting_open(dentry, arg);
> > + if (error)
> > goto out;
> >
> > /*
> > @@ -1514,8 +1523,16 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
> > goto out;
> >
> > locks_insert_lock(before, lease);
> > - return 0;
> >
> > + /*
> > + * The check in break_lease() is lockless. It's possible for another
> > + * open to race in after we did the earlier check for a conflicting
> > + * open but before the lease was inserted. Check again for a
> > + * conflicting open and cancel the lease if there is one.
> > + */
> > + error = check_conflicting_open(dentry, arg);
> > + if (error)
> > + locks_delete_lock(flp);
^^^^^
This isn't safe since the caller will try to free *flp
on error, so we need to be a bit more careful here and
only dequeue the lock w/o freeing it.
> > out:
> > return error;
> > }
> > --
> > 1.8.1.4
> >
--
Jeff Layton <jlayton@xxxxxxxxxx>
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