On Thu, Jun 5, 2025 at 12:27 PM Matt Bobrowski <mattbobrowski@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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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