Re: [PATCH 04/11] VFS: introduce dentry_lookup_continue()

["NeilBrown" <neil@brown.name>]

On Mon, 18 Aug 2025, Amir Goldstein wrote:
> On Wed, Aug 13, 2025 at 1:53 AM NeilBrown <neil@brown.name> wrote:
> >
> > A few callers operate on a dentry which they already have - unlike the
> > normal case where a lookup proceeds an operation.
> >
> > For these callers dentry_lookup_continue() is provided where other
> > callers would use dentry_lookup().  The call will fail if, after the
> > lock was gained, the child is no longer a child of the given parent.
> >
> > There are a couple of callers that want to lock a dentry in whatever
> > its current parent is.  For these a NULL parent can be passed, in which
> > case ->d_parent is used.  In this case the call cannot fail.
> >
> > The idea behind the name is that the actual lookup occurred some time
> > ago, and now we are continuing with an operation on the dentry.
> >
> > When the operation completes done_dentry_lookup() must be called.  An
> > extra reference is taken when the dentry_lookup_continue() call succeeds
> > and will be dropped by done_dentry_lookup().
> >
> > This will be used in smb/server, ecryptfs, and overlayfs, each of which
> > have their own lock_parent() or parent_lock() or similar; and a few
> > other places which lock the parent but don't check if the parent is
> > still correct (often because rename isn't supported so parent cannot be
> > incorrect).
> >
> > Signed-off-by: NeilBrown <neil@brown.name>
> > ---
> >  fs/namei.c            | 39 +++++++++++++++++++++++++++++++++++++++
> >  include/linux/namei.h |  2 ++
> >  2 files changed, 41 insertions(+)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 7af9b464886a..df21b6fa5a0e 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -1874,6 +1874,45 @@ struct dentry *dentry_lookup_killable(struct mnt_idmap *idmap,
> >  }
> >  EXPORT_SYMBOL(dentry_lookup_killable);
> >
> > +/**
> > + * dentry_lookup_continue: lock a dentry if it is still in the given parent, prior to dir ops
> > + * @child: the dentry to lock
> > + * @parent: the dentry of the assumed parent
> > + *
> > + * The child is locked - currently by taking i_rwsem on the parent - to
> > + * prepare for create/remove operations.  If the given parent is not
> > + * %NULL and is no longer the parent of the dentry after the lock is
> > + * gained, the lock is released and the call fails (returns
> > + * ERR_PTR(-EINVAL).
> > + *
> > + * On success a reference to the child is taken and returned.  The lock
> > + * and reference must both be dropped by done_dentry_lookup() after the
> > + * operation completes.
> > + */
> > +struct dentry *dentry_lookup_continue(struct dentry *child,
> > +                                     struct dentry *parent)
> > +{
> > +       struct dentry *p = parent;
> > +
> > +again:
> > +       if (!parent)
> > +               p = dget_parent(child);
> > +       inode_lock_nested(d_inode(p), I_MUTEX_PARENT);
> > +       if (child->d_parent != p) {
> 
> || d_unhashed(child))
> 
> ;)

As you say!

> 
> and what about silly renames? are those also d_unhashed()?

With NFS it is not unhashed (i.e.  it is still hashed, but with a
different name).  I haven't checked AFS.

But does it matter?  As long as it has the right parent and is not
unhashed, it is a suitable dentry to pass to vfs_unlink() etc.

If this race happened with NFS then ovl could try to remove the .nfsXXX
file and would get ETXBUSY due to DCACH_NFSFS_RENAMED.  I don't think
this is a problem.

If we really wanted to be sure the name hadn't changed we could do a
lookup and check that the same dentry is returned.

OVL is by nature exposed to possible races if something else tried to
modify the upper directory tree.  I don't think it needs to provide
perfect semantics in that case, it only needs to fail-safe.  I think
this recent change is enough to be safe in the face of concurrent
unlinks.

Thanks,
NeilBrown

 
> Thanks,
> Amir.
>