Re: [PATCH bpf-next v3 3/6] bpf: Dynptr support for ring buffers

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

On Thu, Apr 28, 2022 at 2:12 PM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> Currently, our only way of writing dynamically-sized data into a ring
> buffer is through bpf_ringbuf_output but this incurs an extra memcpy
> cost. bpf_ringbuf_reserve + bpf_ringbuf_commit avoids this extra
> memcpy, but it can only safely support reservation sizes that are
> statically known since the verifier cannot guarantee that the bpf
> program won’t access memory outside the reserved space.
>
> The bpf_dynptr abstraction allows for dynamically-sized ring buffer
> reservations without the extra memcpy.
>
> There are 3 new APIs:
>
> long bpf_ringbuf_reserve_dynptr(void *ringbuf, u32 size, u64 flags, struct bpf_dynptr *ptr);
> void bpf_ringbuf_submit_dynptr(struct bpf_dynptr *ptr, u64 flags);
> void bpf_ringbuf_discard_dynptr(struct bpf_dynptr *ptr, u64 flags);
>
> These closely follow the functionalities of the original ringbuf APIs.
> For example, all ringbuffer dynptrs that have been reserved must be
> either submitted or discarded before the program exits.
>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---

Looks great! Modulo those four underscores, they are super confusing...

>  include/linux/bpf.h            | 10 ++++-
>  include/uapi/linux/bpf.h       | 35 +++++++++++++++++
>  kernel/bpf/helpers.c           |  6 +++
>  kernel/bpf/ringbuf.c           | 71 ++++++++++++++++++++++++++++++++++
>  kernel/bpf/verifier.c          | 18 +++++++--
>  tools/include/uapi/linux/bpf.h | 35 +++++++++++++++++
>  6 files changed, 171 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 757440406962..10efbec99e93 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -394,7 +394,10 @@ enum bpf_type_flag {
>         /* DYNPTR points to dynamically allocated memory. */
>         DYNPTR_TYPE_MALLOC      = BIT(8 + BPF_BASE_TYPE_BITS),
>
> -       __BPF_TYPE_LAST_FLAG    = DYNPTR_TYPE_MALLOC,
> +       /* DYNPTR points to a ringbuf record. */
> +       DYNPTR_TYPE_RINGBUF     = BIT(9 + BPF_BASE_TYPE_BITS),
> +
> +       __BPF_TYPE_LAST_FLAG    = DYNPTR_TYPE_RINGBUF,

it's getting a bit old to have to update __BPF_TYPE_LAST_FLAG all the
time, maybe let's do this:

__BPF_TYPE_FLAG_MAX,
__BPF_TYPE_LAST_FLAG = __BPF_TYPE_FLAG_MAX - 1,

and never touch it again?

>  };
>

[...]

> + *
> + * void bpf_ringbuf_discard_dynptr(struct bpf_dynptr *ptr, u64 flags)
> + *     Description
> + *             Discard reserved ring buffer sample through the dynptr
> + *             interface. This is a no-op if the dynptr is invalid/null.
> + *
> + *             For more information on *flags*, please see
> + *             'bpf_ringbuf_discard'.
> + *     Return
> + *             Nothing. Always succeeds.
>   */

let's also add bpf_dynptr_is_null() (or bpf_dynptr_is_valid(), not
sure which one is more appropriate, probably just null one), so we can
check in code whether some reservation was successful without knowing
bpf_ringbuf_reserve_dynptr()'s return value


>  #define __BPF_FUNC_MAPPER(FN)          \
>         FN(unspec),                     \

[...]

> +BPF_CALL_4(bpf_ringbuf_reserve_dynptr, struct bpf_map *, map, u32, size, u64, flags,
> +          struct bpf_dynptr_kern *, ptr)
> +{
> +       void *sample;
> +       int err;
> +
> +       err = bpf_dynptr_check_size(size);
> +       if (err) {
> +               bpf_dynptr_set_null(ptr);
> +               return err;
> +       }
> +
> +       sample = (void __force *)____bpf_ringbuf_reserve(map, size, flags);

I was so confused by these four underscored for a bit... Is this
what's defined inside BPF_CALL_4 (and thus makes it ungreppable). Can
you instead just open-code container_of and __bpf_ringbuf_reserve()
directly to make it a bit easier to follow? And flags check as well.
It will so much easier to understand what's going on.

> +
> +       if (!sample) {
> +               bpf_dynptr_set_null(ptr);
> +               return -EINVAL;
> +       }
> +
> +       bpf_dynptr_init(ptr, sample, BPF_DYNPTR_TYPE_RINGBUF, 0, size);
> +
> +       return 0;
> +}
> +
> +const struct bpf_func_proto bpf_ringbuf_reserve_dynptr_proto = {
> +       .func           = bpf_ringbuf_reserve_dynptr,
> +       .ret_type       = RET_INTEGER,
> +       .arg1_type      = ARG_CONST_MAP_PTR,
> +       .arg2_type      = ARG_ANYTHING,
> +       .arg3_type      = ARG_ANYTHING,
> +       .arg4_type      = ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_RINGBUF | MEM_UNINIT,
> +};
> +
> +BPF_CALL_2(bpf_ringbuf_submit_dynptr, struct bpf_dynptr_kern *, ptr, u64, flags)
> +{
> +       if (!ptr->data)
> +               return 0;
> +
> +       ____bpf_ringbuf_submit(ptr->data, flags);

this just calls bpf_ringbuf_commit(), let's do it here explicitly as well

> +
> +       bpf_dynptr_set_null(ptr);
> +
> +       return 0;
> +}
> +
> +const struct bpf_func_proto bpf_ringbuf_submit_dynptr_proto = {
> +       .func           = bpf_ringbuf_submit_dynptr,
> +       .ret_type       = RET_VOID,
> +       .arg1_type      = ARG_PTR_TO_DYNPTR | DYNPTR_TYPE_RINGBUF | OBJ_RELEASE,
> +       .arg2_type      = ARG_ANYTHING,
> +};
> +
> +BPF_CALL_2(bpf_ringbuf_discard_dynptr, struct bpf_dynptr_kern *, ptr, u64, flags)
> +{
> +       if (!ptr->data)
> +               return 0;
> +
> +       ____bpf_ringbuf_discard(ptr->data, flags);
> +

ditto


> +       bpf_dynptr_set_null(ptr);
> +
> +       return 0;
> +}
> +

[...]