Support BPF dynamic pointers

[dthaler]

Describe the feature you'd like supported

Linux supports memory allocation, access, and release via helper functions.

 struct bpf_dynptr_kern {
 void *data;
 u32 size;
 u32 offset;
 } __aligned(8);

 /* Dynamically allocate memory */
 bpf_dynptr_alloc(size, 0, &ptr);

 /* Get a dynptr to __sk_buff data */
 bpf_dynptr_from_skb(skb, &skb_ptr);

 /* Get a dynptr to xdp_md data */
 bpf_dynptr_from_xdp(ctx, &xdp_ptr);

 /* Advance the ptr into skb->data */
 bpf_dynptr_advance(&skb_ptr, sizeof(struct ethhdr));

 /* Write into the memory */
 bpf_dynptr_write(&ptr, 0, buf, sizeof(buf));

 /* Release the memory allocation */
 bpf_dynptr_put(&ptr);

References:

• BPF: Dynamic pointers [LWN.net]
• http://vger.kernel.org/bpfconf2022_material/lsfmmbpf2022-dynptr.pdf

Support is blocked on vbpf/prevail#273

Proposed solution

Support the same helper functions that Linux supports

Additional context

No response