On Tue, Sep 10, 2024 at 2:07 PM Daniel Xu <dxu@xxxxxxxxx> wrote: > > 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@xxxxxxxxx> wrote: > > > > > > On Mon, Sep 9, 2024 at 5:55 PM Daniel Xu <dxu@xxxxxxxxx> 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@xxxxxxxxx> > > > > --- > > > > 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? User space can ignore rb->busy and start consuming in parallel. Ok, given we had this atomic_try_cmpxchg() it was rather OK for user ringbuf, but it's not so great for BPF ringbuf, way too easy to screw up... > > > > > 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. No, not a new helper. Just an alternative name for it, "bpf_ringbuf_drain()". Might not be worth doing, I haven't checked what would be the best way to do this, can't tell right now. > > Thanks, > Daniel
