[PATCH v2 bpf-next 1/4] namei: Introduce new helper function path_walk_parent()

[Mickaël Salaün]

On Mon, Jun 02, 2025 at 11:59:17PM -0700, 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.
> 
> Signed-off-by: Song Liu <song at kernel.org>
> ---
>  fs/namei.c            | 52 +++++++++++++++++++++++++++++++++++++++++++
>  include/linux/namei.h |  2 ++
>  2 files changed, 54 insertions(+)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 4bb889fc980b..7d5bf2bb604f 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1424,6 +1424,58 @@ static bool choose_mountpoint(struct mount *m, const struct path *root,
>  	return found;
>  }
>  
> +/**
> + * 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
> + * not changed.
> + *
> + * The logic of path_walk_parent() is similar to follow_dotdot(), except
> + * that path_walk_parent() will continue walking for !path_connected case.
> + * This effectively means we are walking from disconnectedbind mount to the
> + * original mount point. If this behavior is not desired, the caller can
> + * add a check like:
> + *
> + *   if (path_walk_parent(&path) && !path_connected(path.mnt, path.dentry)
> + *           // continue walking
> + *   else
> + *           // stop walking
> + *
> + * 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;
> +
> +	if (path_equal(path, root))
> +		return false;
> +
> +	if (unlikely(path->dentry == path->mnt->mnt_root)) {
> +		struct path p;
> +
> +		if (!choose_mountpoint(real_mount(path->mnt), root, &p))
> +			return false;
> +		path_put(path);
> +		*path = p;
> +		return true;

It should not return here but continue with the following checks until
the potential dget_parent() call.

I sent a test to check this issue:
https://lore.kernel.org/r/20250606110811.211297-1-mic@digikod.net


> +	}
> +
> +	if (unlikely(IS_ROOT(path->dentry)))
> +		return false;
> +
> +	parent = dget_parent(path->dentry);
> +	dput(path->dentry);
> +	path->dentry = parent;
> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(path_walk_parent);
> +
>  /*
>   * Perform an automount
>   * - return -EISDIR to tell follow_managed() to stop and return the path we
> 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 *);
> -- 
> 2.47.1
> 
>