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

[Alexei Starovoitov]

On Thu, May 29, 2025 at 9:53 AM Song Liu <song at kernel.org> wrote:
>
> Hi Al and Jan,
>
> Thanks for your review!
>
> On Thu, May 29, 2025 at 4:58 AM Jan Kara <jack at suse.cz> wrote:
> >
> > On Wed 28-05-25 23:37:24, Al Viro wrote:
> > > On Wed, May 28, 2025 at 03:26:22PM -0700, Song Liu wrote:
> > > > Introduce a path iterator, which reliably walk a struct path.
> > >
> > > No, it does not.  If you have no external warranty that mount
> > > *and* dentry trees are stable, it's not reliable at all.
> >
> > I agree that advertising this as "reliable walk" is misleading. It is
> > realiable in the sense that it will not dereference freed memory, leak
> > references etc. As you say it is also reliable in the sense that without
> > external modifications to dentry & mount tree, it will crawl the path to
> > root. But in presence of external modifications the only reliability it
> > offers is "it will not crash". E.g. malicious parallel modifications can
> > arbitrarily prolong the duration of the walk.
>
> How about we describe this as:
>
> Introduce a path iterator, which safely (no crash) walks a struct path.
> Without malicious parallel modifications, the walk is guaranteed to
> terminate. The sequence of dentries maybe surprising in presence
> of parallel directory or mount tree modifications and the iteration may
> not ever finish in face of parallel malicious directory tree manipulations.

Hold on. If it's really the case then is the landlock susceptible
to this type of attack already ?
landlock may infinitely loop in the kernel ?