On Thu, Apr 28, 2022 at 2:12 PM Joanne Koong <joannelkoong@xxxxxxxxx> 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@xxxxxxxxx>
> ---
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;
> +}
> +
[...]
