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

[Christian Brauner]

On Tue, Jun 03, 2025 at 08:13:18AM -0700, Alexei Starovoitov wrote:
> On Mon, Jun 2, 2025 at 11:59 PM Song Liu <song at kernel.org> wrote:
> >
> > Introduce a path iterator, which reliably walk a struct path toward
> > the root. This path iterator is based on path_walk_parent. A fixed
> > zero'ed root is passed to path_walk_parent(). Therefore, unless the
> > user terminates it earlier, the iterator will terminate at the real
> > root.
> >
> > Signed-off-by: Song Liu <song at kernel.org>
> > ---
> >  kernel/bpf/Makefile    |  1 +
> >  kernel/bpf/helpers.c   |  3 +++
> >  kernel/bpf/path_iter.c | 58 ++++++++++++++++++++++++++++++++++++++++++
> >  kernel/bpf/verifier.c  |  5 ++++
> >  4 files changed, 67 insertions(+)
> >  create mode 100644 kernel/bpf/path_iter.c
> >
> > diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> > index 3a335c50e6e3..454a650d934e 100644
> > --- a/kernel/bpf/Makefile
> > +++ b/kernel/bpf/Makefile
> > @@ -56,6 +56,7 @@ obj-$(CONFIG_BPF_SYSCALL) += kmem_cache_iter.o
> >  ifeq ($(CONFIG_DMA_SHARED_BUFFER),y)
> >  obj-$(CONFIG_BPF_SYSCALL) += dmabuf_iter.o
> >  endif
> > +obj-$(CONFIG_BPF_SYSCALL) += path_iter.o
> >
> >  CFLAGS_REMOVE_percpu_freelist.o = $(CC_FLAGS_FTRACE)
> >  CFLAGS_REMOVE_bpf_lru_list.o = $(CC_FLAGS_FTRACE)
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index b71e428ad936..b190c78e40f6 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -3397,6 +3397,9 @@ BTF_ID_FLAGS(func, bpf_iter_dmabuf_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPAB
> >  BTF_ID_FLAGS(func, bpf_iter_dmabuf_destroy, KF_ITER_DESTROY | KF_SLEEPABLE)
> >  #endif
> >  BTF_ID_FLAGS(func, __bpf_trap)
> > +BTF_ID_FLAGS(func, bpf_iter_path_new, KF_ITER_NEW | KF_SLEEPABLE)
> > +BTF_ID_FLAGS(func, bpf_iter_path_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPABLE)
> > +BTF_ID_FLAGS(func, bpf_iter_path_destroy, KF_ITER_DESTROY | KF_SLEEPABLE)
> >  BTF_KFUNCS_END(common_btf_ids)
> >
> >  static const struct btf_kfunc_id_set common_kfunc_set = {
> > diff --git a/kernel/bpf/path_iter.c b/kernel/bpf/path_iter.c
> > new file mode 100644
> > index 000000000000..0d972ec84beb
> > --- /dev/null
> > +++ b/kernel/bpf/path_iter.c
> 
> I think Christian's preference was to keep
> everything in fs/bpf_fs_kfuncs.c

Yes. And since that also adds new fs helpers I want to take that through
the VFS tree, please. I'll provide a stable branch as we do with all
other subsystems.