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

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Message-ID: <CAPhsuW46grJgJrXovKuksGXM0HfYg-hmmfroUUkBJTsPL4bSxQ@mail.gmail.com>
Date: Thu, 5 Jun 2025 14:14:17 -0700
From: Song Liu <song@...nel.org>
To: Matt Bobrowski <mattbobrowski@...gle.com>
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, 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 Thu, Jun 5, 2025 at 12:27 PM Matt Bobrowski <mattbobrowski@...gle.com> wrote:
>
> On Mon, Jun 02, 2025 at 11:59:19PM -0700, Song Liu 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@...nel.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)
>
> Hm, I'd expect KF_TRUSTED_ARGS to be enforced onto
> bpf_iter_path_new(), no? Shouldn't this only be operating on a stable
> struct path reference?

Good catch! Added KF_TRUSTED_ARGS. also added a test with
untrusted pointer.

>
> > +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)
>
> At this point, the claim is that such are only to be used from the
> context of the BPF LSM. If true, I'd expect these BPF kfuncs to be
> part of bpf_fs_kfunc_set_ids once moved into fs/bpf_fs_kfuncs.c.

I moved this to fs/bpf_fs_kfuncs.c in the next version.

>
> >  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
> > @@ -0,0 +1,58 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> > +#include <linux/bpf.h>
> > +#include <linux/bpf_mem_alloc.h>
> > +#include <linux/namei.h>
> > +#include <linux/path.h>
> > +
> > +/* open-coded iterator */
> > +struct bpf_iter_path {
> > +     __u64 __opaque[3];
> > +} __aligned(8);
> > +
> > +struct bpf_iter_path_kern {
> > +     struct path path;
> > +     __u64 flags;
> > +} __aligned(8);
> > +
> > +__bpf_kfunc_start_defs();
> > +
> > +__bpf_kfunc int bpf_iter_path_new(struct bpf_iter_path *it,
> > +                               struct path *start,
> > +                               __u64 flags)
> > +{
> > +     struct bpf_iter_path_kern *kit = (void *)it;
> > +
> > +     BUILD_BUG_ON(sizeof(*kit) > sizeof(*it));
> > +     BUILD_BUG_ON(__alignof__(*kit) != __alignof__(*it));
> > +
> > +     if (flags) {
> > +             memset(&kit->path, 0, sizeof(struct path));
>
> This warrants a comment for sure. Also why not just zero it out
> entirely?

Added some comments in v3. Also "flags" is removed in v3.

Thanks,
Song

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