Re: [Patch bpf-next v2 3/5] bpf: compute data_end dynamically with JIT code

From: Lorenz Bauer <lmb@cloudflare.com>
To: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	duanxiongchun@bytedance.com, wangdongdong.6@bytedance.com,
	jiang.wang@bytedance.com, Cong Wang <cong.wang@bytedance.com>,
	Jakub Sitnicki <jakub@cloudflare.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [Patch bpf-next v2 3/5] bpf: compute data_end dynamically with JIT code
Date: Fri, 12 Feb 2021 10:55:52 +0000	[thread overview]
Message-ID: <CACAyw98ABpLzjjw9zMdttgRWvim=oOqAZrDaXrLQ49eYhVsbJQ@mail.gmail.com> (raw)
In-Reply-To: <20210210022136.146528-4-xiyou.wangcong@gmail.com>

On Wed, 10 Feb 2021 at 02:22, Cong Wang <xiyou.wangcong@gmail.com> wrote:
>
> From: Cong Wang <cong.wang@bytedance.com>
>
> Currently, we compute ->data_end with a compile-time constant
> offset of skb. But as Jakub pointed out, we can actually compute
> it in eBPF JIT code at run-time, so that we can competely get
> rid of ->data_end. This is similar to skb_shinfo(skb) computation
> in bpf_convert_shinfo_access().
>
> Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Lorenz Bauer <lmb@cloudflare.com>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>

...

> @@ -9520,6 +9510,29 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
>         return insn - insn_buf;
>  }
>
> +static struct bpf_insn *bpf_convert_data_end_access(const struct bpf_insn *si,
> +                                                   struct bpf_insn *insn)

Is it worth adding a reference to this function in skb_headlen(),
since we're basically open coding that function here?

> +{
> +       /* si->dst_reg = skb->data */
> +       *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data),
> +                             si->dst_reg, si->src_reg,
> +                             offsetof(struct sk_buff, data));
> +       /* AX = skb->len */
> +       *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, len),
> +                             BPF_REG_AX, si->src_reg,
> +                             offsetof(struct sk_buff, len));
> +       /* si->dst_reg = skb->data + skb->len */
> +       *insn++ = BPF_ALU64_REG(BPF_ADD, si->dst_reg, BPF_REG_AX);
> +       /* AX = skb->data_len */
> +       *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, data_len),
> +                             BPF_REG_AX, si->src_reg,
> +                             offsetof(struct sk_buff, data_len));
> +       /* si->dst_reg = skb->data + skb->len - skb->data_len */
> +       *insn++ = BPF_ALU64_REG(BPF_SUB, si->dst_reg, BPF_REG_AX);
> +
> +       return insn;
> +}
> +
>  static u32 sk_skb_convert_ctx_access(enum bpf_access_type type,
>                                      const struct bpf_insn *si,
>                                      struct bpf_insn *insn_buf,
> @@ -9530,12 +9543,7 @@ static u32 sk_skb_convert_ctx_access(enum bpf_access_type type,
>
>         switch (si->off) {
>         case offsetof(struct __sk_buff, data_end):
> -               off  = si->off;
> -               off -= offsetof(struct __sk_buff, data_end);
> -               off += offsetof(struct sk_buff, cb);
> -               off += offsetof(struct tcp_skb_cb, bpf.data_end);
> -               *insn++ = BPF_LDX_MEM(BPF_SIZEOF(void *), si->dst_reg,
> -                                     si->src_reg, off);
> +               insn = bpf_convert_data_end_access(si, insn);

This generates a new warning:

../net/core/filter.c: In function ‘sk_skb_convert_ctx_access’:
../net/core/filter.c:9542:6: warning: unused variable ‘off’ [-Wunused-variable]
 9542 |  int off;
      |      ^~~

--
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

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