Re: [PATCH bpf-next v1 5/8] bpf: Add bpf_dynptr_clone

[Shmulik Ladkani <shmulik.ladkani@gmail.com>]

On Wed,  7 Sep 2022 17:02:51 -0700 Joanne Koong <joannelkoong@gmail.com> wrote:

> Add a new helper, bpf_dynptr_clone, which clones a dynptr.
> 
> The cloned dynptr will point to the same data as its parent dynptr,
> with the same type, offset, size and read-only properties.

[...]

> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 4ca07cf500d2..16973fa4d073 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5508,6 +5508,29 @@ union bpf_attr {
>   *	Return
>   *		The offset of the dynptr on success, -EINVAL if the dynptr is
>   *		invalid.
> + *
> + * long bpf_dynptr_clone(struct bpf_dynptr *ptr, struct bpf_dynptr *clone)
> + *	Description
> + *		Clone an initialized dynptr *ptr*. After this call, both *ptr*
> + *		and *clone* will point to the same underlying data.
> + *

How about adding 'off' and 'len' parameters so that a view ("slice") of
the dynptr can be created in a single call?

Otherwise, for a simple slice creation, ebpf user needs to:
  bpf_dynptr_clone(orig, clone)
  bpf_dynptr_advance(clone, off)
  trim_len = bpf_dynptr_get_size(clone) - len
  bpf_dynptr_trim(clone, trim_len)

This fits the usecase described here:
  https://lore.kernel.org/bpf/20220830231349.46c49c50@blondie/