Re: [PATCH v4 bpf-next 1/5] namei: Introduce new helper function path_walk_parent()

[Tingmao Wang <m@maowtm.org>]

On 6/11/25 23:02, Song Liu wrote:
> This helper walks an input path to its parent. Logic are added to handle
> walking across mount tree.
> 
> This will be used by landlock, and BPF LSM.
> 
> Suggested-by: Neil Brown <neil@brown.name>
> Signed-off-by: Song Liu <song@kernel.org>
> ---
>  fs/namei.c            | 99 +++++++++++++++++++++++++++++++++++++------
>  include/linux/namei.h |  2 +
>  2 files changed, 87 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 4bb889fc980b..bc65361c5d13 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2048,36 +2048,107 @@ 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.
> + *        - If @root is zero'ed, walk all the way to global root.
> + * @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.
> + */
> +static struct dentry *__path_walk_parent(struct path *path, const struct path *root, int flags)
>  {
>  	struct dentry *parent;
>  
> -	if (path_equal(&nd->path, &nd->root))
> +	if (path_equal(path, root))
>  		goto in_root;
> -	if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
> -		struct path path;
> +	if (unlikely(path->dentry == path->mnt->mnt_root)) {
> +		struct path new_path;
>  
> -		if (!choose_mountpoint(real_mount(nd->path.mnt),
> -				       &nd->root, &path))
> +		if (!choose_mountpoint(real_mount(path->mnt),
> +				       root, &new_path))
>  			goto in_root;
> -		path_put(&nd->path);
> -		nd->path = path;
> -		nd->inode = path.dentry->d_inode;
> -		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
> +		path_put(path);
> +		*path = new_path;
> +		if (unlikely(flags & LOOKUP_NO_XDEV))
>  			return ERR_PTR(-EXDEV);
>  	}
>  	/* rare case of legitimate dget_parent()... */
> -	parent = dget_parent(nd->path.dentry);
> -	if (unlikely(!path_connected(nd->path.mnt, parent))) {
> +	parent = dget_parent(path->dentry);
> +	if (unlikely(!path_connected(path->mnt, parent))) {

This is checking path_connected here but also in follow_dotdot,
path_connected is checked again. Is this check meant to be here?  It will
also change the landlock behaviour right?

(For some reason patch 2 rejects when I tried to apply it on v6.16-rc1, so
I haven't actually tested this patch to see if this is really an issue)

>  		dput(parent);
>  		return ERR_PTR(-ENOENT);
>  	}
>  	return parent;
>  
>  in_root:
> -	if (unlikely(nd->flags & LOOKUP_BENEATH))
> +	if (unlikely(flags & LOOKUP_BENEATH))
>  		return ERR_PTR(-EXDEV);
> -	return dget(nd->path.dentry);
> +	return dget(path->dentry);
> +}
> +
> +/**
> + * path_walk_parent - Walk to the parent of path
> + * @path: input and output path.
> + * @root: root of the path walk, do not go beyond this root. If @root is
> + *        zero'ed, walk all the way to real root.
> + *
> + * Given a path, find the parent path. Replace @path with the parent path.
> + * If we were already at the real root or a disconnected root, @path is
> + * released and zero'ed.
> + *
> + * Returns:
> + *  true  - if @path is updated to its parent.
> + *  false - if @path is already the root (real root or @root).
> + */
> +bool path_walk_parent(struct path *path, const struct path *root)
> +{
> +	struct dentry *parent;
> +
> +	parent = __path_walk_parent(path, root, LOOKUP_BENEATH);
> +
> +	if (IS_ERR(parent))
> +		goto false_out;
> +
> +	if (parent == path->dentry) {
> +		dput(parent);
> +		goto false_out;
> +	}
> +	dput(path->dentry);
> +	path->dentry = parent;
> +	return true;
> +
> +false_out:
> +	path_put(path);
> +	memset(path, 0, sizeof(*path));
> +	return false;
> +}
> +
> +static struct dentry *follow_dotdot(struct nameidata *nd)
> +{
> +	struct dentry *parent = __path_walk_parent(&nd->path, &nd->root, nd->flags);
> +
> +	if (IS_ERR(parent))
> +		return parent;
> +	if (unlikely(!path_connected(nd->path.mnt, parent))) {
> +		dput(parent);
> +		return ERR_PTR(-ENOENT);
> +	}
> +	nd->inode = nd->path.dentry->d_inode;
> +	return parent;
>  }
>  
>  static const char *handle_dots(struct nameidata *nd, int type)
> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index 5d085428e471..cba5373ecf86 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -85,6 +85,8 @@ extern int follow_down_one(struct path *);
>  extern int follow_down(struct path *path, unsigned int flags);
>  extern int follow_up(struct path *);
>  
> +bool path_walk_parent(struct path *path, const struct path *root);
> +
>  extern struct dentry *lock_rename(struct dentry *, struct dentry *);
>  extern struct dentry *lock_rename_child(struct dentry *, struct dentry *);
>  extern void unlock_rename(struct dentry *, struct dentry *);