[PATCH bpf-next 3/4] bpf: Introduce path iterator

[Song Liu]

On Mon, Jun 2, 2025 at 8:40 AM Alexei Starovoitov
<alexei.starovoitov at gmail.com> wrote:
>
> On Mon, Jun 2, 2025 at 6:27 AM Song Liu <song at kernel.org> wrote:
> >
> > On Mon, Jun 2, 2025 at 2:27 AM Christian Brauner <brauner at kernel.org> wrote:
> > >
> > > On Thu, May 29, 2025 at 11:00:51AM -0700, Song Liu wrote:
> > > > On Thu, May 29, 2025 at 10:38 AM Al Viro <viro at zeniv.linux.org.uk> wrote:
> > > > >
> > > > > On Thu, May 29, 2025 at 09:53:21AM -0700, Song Liu wrote:
> > > > >
> > > > > > Current version of path iterator only supports walking towards the root,
> > > > > > with helper path_parent. But the path iterator API can be extended
> > > > > > to cover other use cases.
> > > > >
> > > > > Clarify the last part, please - call me paranoid, but that sounds like
> > > > > a beginning of something that really should be discussed upfront.
> > > >
> > > > We don't have any plan with future use cases yet. The only example
> > > > I mentioned in the original version of the commit log is "walk the
> > > > mount tree". IOW, it is similar to the current iterator, but skips non
> > > > mount point iterations.
> > > >
> > > > Since we call it "path iterator", it might make sense to add ways to
> > > > iterate the VFS tree in different patterns. For example, we may
> > >
> > > No, we're not adding a swiss-army knife for consumption by out-of-tree
> > > code. I'm not opposed to adding a sane iterator for targeted use-cases
> > > with a clear scope and internal API behavior as I've said multiple times
> > > already on-list and in-person.
> > >
> > > I will not merge anything that will endup exploding into some fancy
> > > "walk subtrees in any order you want".
> >
> > We are not proposing (and AFAICT never proposed) to have a
> > swiss-army knife that "walk subtrees in any order you want". Instead,
> > we are proposing a sane iterator that serves exactly one use case
> > now. I guess the concern is that it looks extensible. However, I made
> > the API like this so that it can be extended, with thorough reviews, to
> > cover another sane use case. If there is still concern with this. We
> > sure can make current code not extensible. In case there is a
> > different sane use case, we will introduce another iterator after
> > thorough reviews.
>
> It's good that the iterator is extensible, but to achieve that
> there is no need to introduce "enum bpf_path_iter_mode"
> which implies some unknown walk patterns.
> Just add "u64 flags" to bpf_iter_path_new() and
> if (!flags) return -EINVAL;
> Then we'll have a way to extend that kfunc if really necessary.
> Deleting and introducing new kfuncs/iterators is not a big deal,
> but reserving 'flags' as an option for extension is almost
> always a good backup.

Sounds good! I will prepare v2 with a flags field.

Thanks,
Song