Re: [PATCH 2/5] bpf: Define new BPF_MAP_TYPE_USER_RINGBUF map type

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Aug 8, 2022 at 8:54 AM David Vernet <void@manifault.com> wrote:
>
> We want to support a ringbuf map type where samples are published from
> user-space to BPF programs. BPF currently supports a kernel -> user-space
> circular ringbuffer via the BPF_MAP_TYPE_RINGBUF map type. We'll need to
> define a new map type for user-space -> kernel, as none of the helpers
> exported for BPF_MAP_TYPE_RINGBUF will apply to a user-space producer
> ringbuffer, and we'll want to add one or more helper functions that would
> not apply for a kernel-producer ringbuffer.
>
> This patch therefore adds a new BPF_MAP_TYPE_USER_RINGBUF map type
> definition. The map type is useless in its current form, as there is no way
> to access or use it for anything until we add more BPF helpers. A follow-on
> patch will therefore add a new helper function that allows BPF programs to
> run callbacks on samples that are published to the ringbuffer.
>
> Signed-off-by: David Vernet <void@manifault.com>
> ---
>  include/linux/bpf_types.h      |  1 +
>  include/uapi/linux/bpf.h       |  1 +
>  kernel/bpf/ringbuf.c           | 70 +++++++++++++++++++++++++++++-----
>  kernel/bpf/verifier.c          |  3 ++
>  tools/include/uapi/linux/bpf.h |  1 +
>  tools/lib/bpf/libbpf.c         |  1 +
>  6 files changed, 68 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
> index 2b9112b80171..2c6a4f2562a7 100644
> --- a/include/linux/bpf_types.h
> +++ b/include/linux/bpf_types.h
> @@ -126,6 +126,7 @@ BPF_MAP_TYPE(BPF_MAP_TYPE_STRUCT_OPS, bpf_struct_ops_map_ops)
>  #endif
>  BPF_MAP_TYPE(BPF_MAP_TYPE_RINGBUF, ringbuf_map_ops)
>  BPF_MAP_TYPE(BPF_MAP_TYPE_BLOOM_FILTER, bloom_filter_map_ops)
> +BPF_MAP_TYPE(BPF_MAP_TYPE_USER_RINGBUF, user_ringbuf_map_ops)
>
>  BPF_LINK_TYPE(BPF_LINK_TYPE_RAW_TRACEPOINT, raw_tracepoint)
>  BPF_LINK_TYPE(BPF_LINK_TYPE_TRACING, tracing)
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 7bf9ba1329be..a341f877b230 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -909,6 +909,7 @@ enum bpf_map_type {
>         BPF_MAP_TYPE_INODE_STORAGE,
>         BPF_MAP_TYPE_TASK_STORAGE,
>         BPF_MAP_TYPE_BLOOM_FILTER,
> +       BPF_MAP_TYPE_USER_RINGBUF,
>  };
>
>  /* Note that tracing related programs such as
> diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
> index ded4faeca192..29e2de42df15 100644
> --- a/kernel/bpf/ringbuf.c
> +++ b/kernel/bpf/ringbuf.c
> @@ -38,12 +38,32 @@ struct bpf_ringbuf {
>         struct page **pages;
>         int nr_pages;
>         spinlock_t spinlock ____cacheline_aligned_in_smp;
> -       /* Consumer and producer counters are put into separate pages to allow
> -        * mapping consumer page as r/w, but restrict producer page to r/o.
> -        * This protects producer position from being modified by user-space
> -        * application and ruining in-kernel position tracking.
> +       /* Consumer and producer counters are put into separate pages to
> +        * allow each position to be mapped with different permissions.
> +        * This prevents a user-space application from modifying the
> +        * position and ruining in-kernel tracking. The permissions of the
> +        * pages depend on who is producing samples: user-space or the
> +        * kernel.
> +        *
> +        * Kernel-producer
> +        * ---------------
> +        * The producer position and data pages are mapped as r/o in
> +        * userspace. For this approach, bits in the header of samples are
> +        * used to signal to user-space, and to other producers, whether a
> +        * sample is currently being written.
> +        *
> +        * User-space producer
> +        * -------------------
> +        * Only the page containing the consumer position, and whether the
> +        * ringbuffer is currently being consumed via a 'busy' bit, are
> +        * mapped r/o in user-space. Sample headers may not be used to
> +        * communicate any information between kernel consumers, as a
> +        * user-space application could modify its contents at any time.
>          */
> -       unsigned long consumer_pos __aligned(PAGE_SIZE);
> +       struct {
> +               unsigned long consumer_pos;
> +               atomic_t busy;

one more thing, why does busy have to be exposed into user-space
mapped memory at all? Can't it be just a private variable in
bpf_ringbuf?

> +       } __aligned(PAGE_SIZE);
>         unsigned long producer_pos __aligned(PAGE_SIZE);
>         char data[] __aligned(PAGE_SIZE);
>  };

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