linux-kernel - Re: [PATCH v3 bpf-next 1/5] namei: Introduce new helper function path_walk

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Message-ID: <CAPhsuW6oet8_LbL+6mVi7Lc4U_8i7O-PN5F1zOm5esV52sBu0A@mail.gmail.com>
Date: Tue, 10 Jun 2025 17:56:01 -0700
From: Song Liu <song@...nel.org>
To: NeilBrown <neil@...wn.name>
Cc: Jan Kara <jack@...e.cz>, bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, 
	kernel-team@...a.com, andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, 
	daniel@...earbox.net, martin.lau@...ux.dev, viro@...iv.linux.org.uk, 
	brauner@...nel.org, kpsingh@...nel.org, mattbobrowski@...gle.com, 
	amir73il@...il.com, repnop@...gle.com, jlayton@...nel.org, 
	josef@...icpanda.com, mic@...ikod.net, gnoack@...gle.com, m@...wtm.org
Subject: Re: [PATCH v3 bpf-next 1/5] namei: Introduce new helper function path_walk_parent()

Hi Neil,

Thanks for your suggestion! It does look like a good solution.

On Tue, Jun 10, 2025 at 4:34 PM NeilBrown <neil@...wn.name> 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.

Thanks,
Song

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