Re: [PATCH v1 bpf-next 0/4] Support bpf_kptr_xchg into local kptr

[Martin KaFai Lau <martin.lau@linux.dev>]

On 7/27/24 8:01 PM, Amery Hung wrote:
> This series allows stashing kptr into local kptr. Currently, kptrs are
> only allowed to be stashed into map value with bpf_kptr_xchg(). A
> motivating use case of this series is to enable adding referenced kptr to
> bpf_rbtree or bpf_list by using allocated object as graph node and the
> storage of referenced kptr. For example, a bpf qdisc [0] enqueuing a
> referenced kptr to a struct sk_buff* to a bpf_list serving as a fifo:
> 
>      struct skb_node {
>              struct sk_buff __kptr *skb;
>              struct bpf_list_node node;
>      };
> 
>      private(A) struct bpf_spin_lock fifo_lock;
>      private(A) struct bpf_list_head fifo __contains(skb_node, node);
> 
>      /* In Qdisc_ops.enqueue */
>      struct skb_node *skbn;
> 
>      skbn = bpf_obj_new(typeof(*skbn));
>      if (!skbn)
>          goto drop;
> 
>      /* skb is a referenced kptr to struct sk_buff acquired earilier
>       * but not shown in this code snippet.
>       */
>      skb = bpf_kptr_xchg(&skbn->skb, skb);
>      if (skb)
>          /* should not happen; do something below releasing skb to
>           * satisfy the verifier */
>      	...
>      
>      bpf_spin_lock(&fifo_lock);
>      bpf_list_push_back(&fifo, &skbn->node);
>      bpf_spin_unlock(&fifo_lock);
> 
> The implementation first searches for BPF_KPTR when generating program
> BTF. Then, we teach the verifier that the detination argument of
> bpf_kptr_xchg() can be local kptr, and use the btf_record in program BTF
> to check against the source argument.
> 
> This series is mostly developed by Dave, who kindly helped and sent me
> the patchset. The selftests in bpf qdisc (WIP) relies on this series to
> work.

The set lgtm. With the doc fix in the bpf_kptr_xchg, you can carry my Ack.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>