linux-kernel - Re: [PATCH v2 bpf-next 3/4] bpf: Introduce path iterator

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Message-ID: <CAEf4BzZpG05EHK20RDKJq2BXHKtZ4Z7CLvhQNJN5AUpTDYcMOw@mail.gmail.com>
Date: Wed, 4 Jun 2025 13:37:50 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Song Liu <song@...nel.org>
Cc: 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, jack@...e.cz, 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 v2 bpf-next 3/4] bpf: Introduce path iterator

On Tue, Jun 3, 2025 at 4:20 PM Song Liu <song@...nel.org> wrote:
>
> On Tue, Jun 3, 2025 at 2:45 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Tue, Jun 3, 2025 at 2:09 PM Song Liu <song@...nel.org> wrote:
> > >
> > > On Tue, Jun 3, 2025 at 11:40 AM Andrii Nakryiko
> > > <andrii.nakryiko@...il.com> wrote:
> > > [...]
> > > > > +__bpf_kfunc struct path *bpf_iter_path_next(struct bpf_iter_path *it)
> > > > > +{
> > > > > +       struct bpf_iter_path_kern *kit = (void *)it;
> > > > > +       struct path root = {};
> > > > > +
> > > > > +       if (!path_walk_parent(&kit->path, &root))
> > > > > +               return NULL;
> > > > > +       return &kit->path;
> > > > > +}
> > > > > +
> > > > > +__bpf_kfunc void bpf_iter_path_destroy(struct bpf_iter_path *it)
> > > > > +{
> > > > > +       struct bpf_iter_path_kern *kit = (void *)it;
> > > > > +
> > > > > +       path_put(&kit->path);
> > > >
> > > > note, destroy() will be called even if construction of iterator fails
> > > > or we exhausted iterator. So you need to make sure that you have
> > > > bpf_iter_path state where you can detect that there is no path present
> > > > and skip path_put().
> > >
> > > In bpf_iter_path_next(), when path_walk_parent() returns false, we
> > > still hold reference to kit->path, then _destroy() will release it. So we
> > > should be fine, no?
> >
> > you still need to handle iterators that failed to be initialized,
> > though? And one can argue that if path_walk_parent() returns false, we
> > need to put that last path before returning NULL, no?
>
> kit->path is zero'ed on initialization failures, so we can path_put() it
> safely. For _next() returns NULL case, we can either put kit->path
> in _destroy(), which is the logic now, or put kit->path in the last
> _next() call and make _destroy() a no-op in that case. I don't have
> a strong preference either way.

I didn't realize path_put() is a no-op for zeroed-out struct path. I'd
probably leave a comment for future selves, I don't have strong
preference otherwise.

>
> Thanks,
> Song