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

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Message-ID: <20250611.Bee1Iohoh4We@digikod.net>
Date: Wed, 11 Jun 2025 17:42:13 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Song Liu <song@...nel.org>
Cc: NeilBrown <neil@...wn.name>, 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, 
	gnoack@...gle.com, m@...wtm.org
Subject: Re: [PATCH v3 bpf-next 1/5] namei: Introduce new helper function
 path_walk_parent()

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@...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.

This looks good but it would be better if we could also do a full path
walk within RCU when possible.

> 
> Thanks,
> Song
> 
> [...]
>