linux-kernel - Re: [PATCH bpf-next] bpf: ringbuf: Support consuming BPF_MAP_TYPE

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Message-ID: <vru2zgphyfywjcqikolwotsfun2bgtrnfmwvfls5ra4tznsydr@46w5rq7gqepz>
Date: Tue, 10 Sep 2024 15:07:24 -0600
From: Daniel Xu <dxu@...uu.xyz>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, 
	Eddy Z <eddyz87@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, 
	Shuah Khan <shuah@...nel.org>, John Fastabend <john.fastabend@...il.com>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Mykola Lysenko <mykolal@...com>, LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH bpf-next] bpf: ringbuf: Support consuming
 BPF_MAP_TYPE_RINGBUF from prog

On Tue, Sep 10, 2024 at 01:41:41PM GMT, Andrii Nakryiko wrote:
> On Tue, Sep 10, 2024 at 11:36 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Mon, Sep 9, 2024 at 5:55 PM Daniel Xu <dxu@...uu.xyz> wrote:
> > >
> > > Right now there exists prog produce / userspace consume and userspace
> > > produce / prog consume support. But it is also useful to have prog
> > > produce / prog consume.
> > >
> > > For example, we want to track the latency overhead of cpumap in
> > > production. Since we need to store enqueue timestamps somewhere and
> > > cpumap is MPSC, we need an MPSC data structure to shadow cpumap. BPF
> > > ringbuf is such a data structure. Rather than reimplement (possibly
> > > poorly) a custom ringbuffer in BPF, extend the existing interface to
> > > allow the final quadrant of ringbuf usecases to be filled. Note we
> > > ignore userspace to userspace use case - there is no need to involve
> > > kernel for that.
> > >
> > > Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> > > ---
> > >  kernel/bpf/verifier.c                         |  6 +-
> > >  tools/testing/selftests/bpf/Makefile          |  3 +-
> > >  .../selftests/bpf/prog_tests/ringbuf.c        | 50 +++++++++++++++
> > >  .../bpf/progs/test_ringbuf_bpf_to_bpf.c       | 64 +++++++++++++++++++
> > >  4 files changed, 120 insertions(+), 3 deletions(-)
> > >  create mode 100644 tools/testing/selftests/bpf/progs/test_ringbuf_bpf_to_bpf.c
> > >
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index 53d0556fbbf3..56bfe559f228 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -9142,7 +9142,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
> > >                     func_id != BPF_FUNC_ringbuf_query &&
> > >                     func_id != BPF_FUNC_ringbuf_reserve_dynptr &&
> > >                     func_id != BPF_FUNC_ringbuf_submit_dynptr &&
> > > -                   func_id != BPF_FUNC_ringbuf_discard_dynptr)
> > > +                   func_id != BPF_FUNC_ringbuf_discard_dynptr &&
> > > +                   func_id != BPF_FUNC_user_ringbuf_drain)
> > >                         goto error;
> > >                 break;
> > >         case BPF_MAP_TYPE_USER_RINGBUF:
> > > @@ -9276,7 +9277,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
> > >                         goto error;
> > >                 break;
> > >         case BPF_FUNC_user_ringbuf_drain:
> > > -               if (map->map_type != BPF_MAP_TYPE_USER_RINGBUF)
> > > +               if (map->map_type != BPF_MAP_TYPE_USER_RINGBUF &&
> > > +                   map->map_type != BPF_MAP_TYPE_RINGBUF)
> > >                         goto error;
> >
> > I think it should work.
> >
> > Andrii,
> >
> > do you see any issues with such use?
> >
> 
> Not from a quick glance. Both ringbufs have the same memory layout, so
> user_ringbuf_drain() should probably work fine for normal BPF ringbuf
> (and either way bpf_user_ringbuf_drain() has to protect from malicious
> user space, so its code is pretty unassuming).
> 
> We should make it very explicit, though, that the user is responsible
> for making sure that bpf_user_ringbuf_drain() will not be called
> simultaneously in two threads, kernel or user space.

I see an atomic_try_cmpxchg() protecting the drain. So it should be
safe, right? What are they supposed to expect?

> 
> Also, Daniel, can you please make sure that dynptr we return for each
> sample is read-only? We shouldn't let consumer BPF program ability to
> corrupt ringbuf record headers (accidentally or otherwise).

Sure.

> 
> And as a thought exercise. I wonder what would it take to have an
> open-coded iterator returning these read-only dynptrs for each
> consumed record? Maybe we already have all the pieces together. So
> consider looking into that as well.
> 
> P.S. And yeah "user_" part in helper name is kind of unfortunate given
> it will work for both ringbufs. Can/should we add some sort of alias
> for this helper so it can be used with both bpf_user_ringbuf_drain()
> and bpf_ringbuf_drain() names?

You mean register a new helper that shares the impl? Easy enough, but I
thought we didn't want to add more uapi helpers.

Thanks,
Daniel