On Tue, Jun 10, 2025 at 05:56:01PM -0700, Song Liu wrote: > Hi Neil, > > Thanks for your suggestion! It does look like a good solution. > > On Tue, Jun 10, 2025 at 4:34 PM NeilBrown <neil@xxxxxxxxxx> wrote: > > > The above looks a lot like follow_dotdot(). This is good because it > > means that it is likely correct. But it is bad because it means there > > are two copies of essentially the same code - making maintenance harder. > > > > I think it would be good to split the part that you want out of > > follow_dotdot() and use that. Something like the following. > > > > You might need a small wrapper in landlock which would, for example, > > pass LOOKUP_BENEATH and replace path->dentry with the parent on success. > > > > NeilBrown > > > > diff --git a/fs/namei.c b/fs/namei.c > > index 4bb889fc980b..b81d07b4417b 100644 > > --- a/fs/namei.c > > +++ b/fs/namei.c > > @@ -2048,36 +2048,65 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd) > > return nd->path.dentry; > > } > > > > -static struct dentry *follow_dotdot(struct nameidata *nd) > > +/** > > + * path_walk_parent - Find the parent of the given struct path > > + * @path - The struct path to start from > > + * @root - A struct path which serves as a boundary not to be crosses > > + * @flags - Some LOOKUP_ flags > > + * > > + * Find and return the dentry for the parent of the given path (mount/dentry). > > + * If the given path is the root of a mounted tree, it is first updated to > > + * the mount point on which that tree is mounted. > > + * > > + * If %LOOKUP_NO_XDEV is given, then *after* the path is updated to a new mount, > > + * the error EXDEV is returned. > > + * If no parent can be found, either because the tree is not mounted or because > > + * the @path matches the @root, then @path->dentry is returned unless @flags > > + * contains %LOOKUP_BENEATH, in which case -EXDEV is returned. > > + * > > + * Returns: either an ERR_PTR() or the chosen parent which will have had the > > + * refcount incremented. > > + */ > > +struct dentry *path_walk_parent(struct path *path, struct path *root, int flags) > > We can probably call this __path_walk_parent() and make it static. > > Then we can add an exported path_walk_parent() that calls > __path_walk_parent() and adds extra logic. > > If this looks good to folks, I can draft v4 based on this idea. This looks good but it would be better if we could also do a full path walk within RCU when possible. > > Thanks, > Song > > [...] >
